Response: I am earnestly persuaded from Scripture that not only is Christian apologetics a legitimate enterprise but it is also an urgent imperative and non-negotiable duty for every generation of those belonging to the Lord Jesus. The apostle Paul points out in 2 Cor. 10, for example, that believers are in a very real war with the forces of darkness and that “destroying speculations and every lofty thing raised up against the knowledge of God” is a genuine part of our responsibility in this struggle. Paul himself is an example in his aggressive confrontations against the Judaizers who were advocating a substitute ‘gospel’. Both he and the apostle John took an uncompromising stand against the Gnostic errors that were being introduced in their day. Jude admonished his readers to “contend earnestly for the faith which was once for all delivered to the saints.” The apostle Peter in the third chapter of his second letter warns his readers of a devastating heresy of the last days, a prediction that indeed has been fulfilled during these past two centuries. This heresy has largely paralyzed the church in our day. One can hardly find a believer anywhere in Western society today, for example, who is willing to quote God’s Word in a public venue and to assert its divine authority. The primary reason for this shocking erosion in the confidence in Scripture is, at least in my assessment, is precisely the heresy about which Peter wrote. This heresy, which represents a frontal assault on the truthfulness of Scripture, has gone effectively unchallenged, at least in any serious way, now for more than two hundred years.

Just what is this heresy, and why has it been so difficult to refute? The deception, in Peter’s own words, is that “all continues just as it was from the beginning of creation” (2 Peter 3:4). It is clear from the context that his error has to do with the manner in which the physical history of the earth is to be interpreted. Specifically, it involves a willful denial of the Flood cataclysm. If the reality of the Genesis Flood is the central issue relating to the viability of this heresy, why is it then that the church has been so impotent in demonstrating its reality to the skeptics? After all, the water-laid sedimentary rocks almost everywhere around us commonly contain fossils, with frequent evidence that animals were buried while still alive. Moreover, such thick sequences of sedimentary rocks blanketing such vast areas of the normally high-standing continents should obviously be testifying that some drastically different circumstances must have prevailed in the past compared to what we observe occurring today. So, again, why have Christians been so impotent in defending the reality of this world-destroying event so prominent in Scripture?

The answer, in my assessment, is fairly simple. The reason is that until not so long ago Christians were utterly at a loss for a mechanism that could produce the staggering amount of geological change evident in the fossil-bearing portion of the rock record within the short span of time provided in the Genesis account. Without a credible mechanism, they were at a distinct disadvantage in the contest to provide the best explanation for the rock record. As decade after decade and generation after generation passed without an effective answer to the challenge laid down by James Hutton in 1795 and refined by Charles Lyell through a large part of the nineteenth century, the tendency for Christian believers was to withdraw, to retreat more and more, and to allow the uniformitarian side to win by default.

However, in what I believe was God’s providence, about fifty years ago there was a major discovery within the mainstream earth science community. The discovery was that the earth’s interior was not as strong and rigid as seismologists had been claiming and that, in fact, the solid earth was a much more dynamic entity than even most specialists had ever imagined. The decade of the 1960’s witnessed a genuine paradigm shift like Thomas Kuhn had just described in his 1962 book, The Structure of Scientific Revolutions, within the earth science community. By 1970 the vast majority in that community were persuaded that the new concept of plate tectonics, which included the formation of new ocean floor along the mid-ocean ridge system and subduction of older ocean floor into the deep ocean trenches, was basically correct. This new framework unleashed a huge amount of new effort to explore and seek to understand the world’s seafloor. Already by 1970 the conclusion seemed to be secure that all of seafloor crustal basement rocks were basaltic in composition, that this basalt had been generated at a mid-ocean ridge via partial melting of mantle rock below as part of the seafloor spreading process, and that all of crustal basement rocks of today’s ocean floor were no older than Mesozoic in age. Among the implications were that the entire Atlantic ocean floor had formed since the early Mesozoic and that both North America and South America had moved westward away from Europe and Africa by some three thousand miles since that point in the geological record.

Response: First of all, lateral motion of continents is possible only because there is this weak layer—generally referred to as the asthenosphere—beneath the continental plates whose presence results in a mechanical decoupling of an individual plate from the stronger mantle below. Because the strength of mantle rock varies inversely with temperature in an exponential way, the asthenospheric layer, which is typically several hundred degrees hotter than a lithospheric plate, is normally on the order of a thousand times weaker. (Because rock strength also depends on pressure and increases with pressure, below a depth about 300 km rock strength increases despite increasing temperature.) It is the striking contrast in strength between lithosphere and asthenosphere that results in this almost complete mechanical decoupling of the plates from the deeper mantle below. This decoupling occurs even when the base of the continent plate is not smooth and even when it has a ‘keel’ of strong and also probably buoyant rock (buoyant perhaps because it is depleted in iron). Again, this remarkable mechanical decoupling is because of the presence of such a weak asthenospheric layer.

Next, it useful to point out that continental lithosphere is usually comprised of an upper layer, typically about 35 km thick, of buoyant continental crust with an average density of about 2800 kg/m³ and a mean composition similar to that of granite and normally a thicker lower layer of ultramafic mantle rock with a density of about 3300 kg/m³. Most mountains are generally of continental crustal (i.e., granitic) composition and are generally isostatically compensated by means of a crustal root that may extend as deep as 70 km. It is normally the lower ultramafic layer of mantle rock that gives continental lithosphere most of its strength.

So then, what are the forces that cause a continental plate to move laterally? There are several different forces that come into play. First, a plate can have both a continental part and an ocean part. (The difference is that the continental portion has the layer of buoyant continental crust while the oceanic portion does not.) If an oceanic portion happens to be subducting, then the subducted part of the plate, being negatively buoyant relative to the mantle into which it is sinking, exerts what is referred to as a ‘slab pull’ force on the remainder of the plate. That force tends to drag the plate toward the subduction zone. On the other hand, for a plate that contains both a continental part and an oceanic part, the oceanic part can have a mid-ocean ridge as part of its boundary. If that is the case, there tends to be what is called a ‘ridge push’ force which acts along that boundary. The ridge push force can be shown to arise from the topographic elevation of the ridge. The other important class of force is the collisional interaction with other plates at their mutual boundaries. Generally speaking, the drag forces on the bases of the plates seem to be small and insignificant in comparison to these just mentioned, because of the extreme weakness of the asthenosphere. This general picture of the relative importance of the various forces is supported by various lines of observational evidence as well as computational modeling of these processes.

CPT Q. #3 – 101 Q&A on Catastrophic Plate Tectonics

Response: A crucially important issue here is the strength contrast between the lithosphere at the earth’s surface and the rock layer lying immediately beneath it. This zone, discovered to be very weak relative to the lithosphere above it, is known as the asthenosphere (from Greek asthenēs ‘weak’ + sphere). The British geologist Joseph Barrell in 1914, in connection with his studies of post-glacial rebound, first introduced the idea of a strong outer layer (which he named the lithosphere) overlying a much weaker layer (which he named the asthenosphere).¹ He realized such a weak zone in which lateral flow of rock could occur was required for isostatic compensation to take place. Seismologists, beginning notably from their analyses of the large 1960 Chilean earthquake, have identified that low seismic wave speeds characterize this region. They now refer this portion of the upper mantle as the ‘low velocity zone’. Since seismic wave speeds depend on the shear strength, or rigidity, of the rock, the strikingly lower seismic wave speeds at asthenospheric depths imply significantly lower rock strength in that region. Just how weak is the asthenosphere relative to the lithosphere? Various lines of observational evidence indicate that thicker oceanic lithosphere has an inelastic or viscous strength on the order of 1022-1023 Pa-s, while the asthenosphere has a viscosity on the order of 1018-1019 Pa-s. In other words, the lithosphere typically is at least a thousand times, and more typically ten thousand times, stronger than the asthenosphere.

Why is the asthenosphere so weak relative to the overlying lithosphere? The primary reason is its high temperature. Rock strength depends very strongly (exponentially) on temperature, and the difference in the strength due to temperature alone is huge. Indeed, since the temperature of the asthenosphere is not that far below the melting (solidus) temperature of its lowest melting point minerals, it is not that surprising that its strength is so far below that of the overlying lithosphere. Laboratory experiments show that the viscous strength η of silicate minerals² obeys an Arrhenius law of the form:

where E* is the mineral’s activation energy, V* is its activation volume, p is the pressure, T is the absolute temperature, R = 8.3145 J/mol-K is the ideal gas constant, and η0 is the reference viscosity at reference temperature T0. For the upper mantle mineral olivine, E* is about 500 kJ/mol and V* is about 4 x 10-6 m3/mol.

However, yet another factor contributing to asthenospheric weakness is the likely presence of water and carbon dioxide, at 100 ppm or so levels, within the lattices of the minerals of the asthenosphere rocks.^3,4 Laboratory experiments show that the presence of these volatiles leads to a further dramatic reduction in rock strength. To summarize thus far, the layer of rock that underlies the lithospheric plates is dramatically weaker than the plates themselves. Hence, lithospheric plates should be readily able to penetrate into the layer of rock beneath them. Moreover, given the extreme contrast in rock strength between the asthenosphere and lithosphere, the drag forces on the base of the plates also should be small compared to the plate strength.

A useful tool that can be brought to bear on the mechanics of subduction is numerical simulation. Many numerical simulations of these mechanics over the last 30 years [ed. as of ~2012], including my own, clearly demonstrate that subduction is a robust and viable physical process. To understand the basic mechanics, several points are important to grasp. First, rocks not only can and do display reversible elastic deformation when subjected to stresses (as do most solids) but also can and do undergo inelastic non-reversible changes in shape. (Inelastic deformation is a standard and important topic included in most every graduate level mechanical engineering curriculum.) In subduction not only does the slab itself bend inelastically, it can also stretch or compress inelastically in its downward journey. But not only does the slab deform inelastically, but the mantle rock into which it sinks also deforms inelastically to accommodate the downgoing slab. The numerical methods that simulate these mechanics typically guarantee perfect conservation of mass and energy while enforcing perfect consistency of forces acting on each parcel of material throughout the computational domain. These methods also usually allow the material strength to vary from cell to cell as a function of temperature and also, in many cases, the local stress conditions. These methods are highly developed and are routinely applied in a broad spectrum of engineering applications, from the mechanical designs of turbine blades to anti-tank projectiles to nuclear weapons. The methods work. Applied to the earth and to the deformation that occurs as rock rises and sinks in the mantle as a result of differences in its buoyancy, the methods show clearly that subduction can and does take place when physically realistic values for densities, temperatures, and various material parameters are applied. Simulations confirm that the large contrast in strength between the asthenosphere and the lithosphere leads to traction forces on the base of the lithosphere which are relatively small. This in turn implies that not much pushing or pulling is required to move a plate over the underlying mantle. It also means that the stresses within the horizontal portion of a plate are generally small and well below the stress levels needed to fracture the plate.

These basic conclusions apply both to the case of uniformitarian plate tectonics (UPT) and to the regime of catastrophic plate tectonics (CPT). In the case of UPT, stress weakening in the rock deformation law is either omitted or switched off, and therefore the rocks remain strong and the deformation rates remain at the levels we observe in the present. However, when stress weakening is included (as it ought to be), the potential for runaway exists and CPT can occur. In the CPT regime, the strength contrast between lithosphere and asthenosphere remains; lithosphere subducts and behaves in largely the same way as in the non-CPT case, except that velocities are dramatically higher and the time scale is dramatically shorter.

Response: There are thousands of papers documenting the reality of accretionary wedges at subduction zones in the standard literature. Let me pick just one published 16 August 2009 in Nature Geoscience by M. Strasser et al. entitled: “Origin and evolution of a splay fault in the Nankai accretionary wedge” (available here [ed. link updated from]). This paper deals with the faulting history within the accretionary wedge that lies within the Nankai Trough, where the Philippine Sea plate is actively subducting below southwest Japan and repeatedly producing destructive earthquakes and tsunamis. Figure 1 from this paper is reproduced below. Panel a, which is a shaded relief map of the region, shows the corrugated upper surface of the wedge, spanned by the red line segment, between the relatively smooth sediment covered top of the Philippine Plate to the southeast and the smooth floor of the Kumano Basin to the northwest. Panel b shows the result of a seismic transect along this line segment. This figure shows a portion of the Philippine Plate that is actively subducting (as indicated by 13 major earthquakes during the last 1300 years, 11 of which are estimated to have been greater than magnitude 8.0) beneath a thick wedge of accreted sediments which are highly contorted and sliced by numerous internal faults, including the megasplay fault shown in red on the left in panel b. Some of these earthquakes generated tsunamis greater than 25 m high. (For more details, see here) It is believed that slip on the megasplay fault during the large earthquakes is responsible for the large-amplitude tsunamis. Sediments recovered in the three drill cores that were an important part of this study suggest that splay fault activity has varied strongly through time, with alternating high-activity periods during which splay-fault thrusting accommodated a large part of the plate convergence. One possible cause the authors cite for this variation is roughness changes on the incoming plate (such as the presence ridges or seamounts).

Another example of observational support for the reality of subducted sediment and seafloor volcanic cones accumulating in subduction zones is the classic example of the Franciscan terrane along some 650 km of the California coast mostly north of San Francisco.^1,2,3,4 These Franciscan rocks represent the trench complex of a Mesozoic, east-dipping subduction zone beneath the western coast of North America. During that time period the Farallon Plate was being subducted beneath the overriding North American Plate. Simultaneous with the formation of these subduction zone rocks was the development of a volcanic arc system that produced the Sierra Nevada batholith. The rock units in today’s Franciscan terrane are derived from sediments deposited in an accretionary wedge as well as from slices of the underlying oceanic crust and mantle. Rock ages range from uppermost Jurassic through Cretaceous. The sediments of the accretionary wedge are medium- to fine-grained detrital rocks (graywackes, micrograywackes, and dark shales) derived from the Sierran volcanic-plutonic arc. They were deposited into the trench as turbidites. Not only were substantial volumes of these sediments were subducted atop the downgoing oceanic plate, in this subduction process they were also metamorphosed to high metamorphic grade!

How deeply were these sediments subducted to reach this degree of metamorphism? The petrology and geochemistry of the minerals in the metamorphosed graywackes and off-scraped basalts yield the answer. For example, the albite present in the original sedimentary and igneous rock has been converted into jadeitic pyroxene plus quartz. This reaction can only occur in a high pressure, low temperature environment. Laboratory experiments indicate temperatures of about 200°C and pressures of about 0.6-0.7 GPa. This corresponds to depths of about 20 to 30 km in a subduction zone. Another metamorphic mineral that constrains the pressure-temperature conditions of the metamorphism is glaucophane. Glaucophane (meaning ‘blue appearing’ in Greek) belongs to the amphibole family of minerals and has the chemical formula Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂. Its name comes from its typical blue color. Rocks containing significant amounts of glaucophane are generally referred to as blueschists. The mix of metamorphic rock types commonly associated with blueschist is referred to as the ‘blueschist facies.’ Blueschist facies rocks generally form under pressures of greater than 0.6 GPa, equivalent to depth of burial in excess of 15-18 km, and at temperatures of between 200 to 500 °C. This is a ‘low temperature, high pressure’ prograde metamorphic path and is also known as the Franciscan facies series, after this Franciscan complex of California where these rocks are so magnificently exposed. Similar blueschists in similar tectonic settings also occur in Greece, Turkey, Japan, New Zealand and New Caledonia.

A major issue associated with the occurrence of these blueschist facies rocks, however, is how they manage return to near the earth’s surface after reaching the pressure conditions needed for their formation. Minerals such as jadeite and glaucophane revert to lower pressure phases as they rise to the surface if their temperatures are much above 200°C. The standard geological community freely acknowledges this is an unsolved problem and has recognized this reality for the past forty years [ed. as of ~2012]. Whatever the process, they acknowledge that it must be rapid on order to preserve the high-pressure, low temperature minerals. Most acknowledge that the positive buoyancy of the metamorphosed rocks must be a key factor. Jadeitic pyroxene is less dense and more viscous than olivine that occurs in the mantle. This difference in buoyancy allows the subducted crustal rock to rise. They also marvel at the fact that many of the Franciscan tectonic blocks, some at least 10 km in lateral dimension, have been returned to the surface relatively intact (that is, stratigraphically coherent) although locally very contorted.

One primary difficulty for the mainstream community is how quickly these high-pressure, low-temperature metamorphic minerals must return to the surface in order to escape conductive heating at the depths where the metamorphism takes place. The key issue here is the time scale, and in the uniformitarian framework, the typical time span inferred between subduction and exhumation implies too much heating at depth. This difficulty, of course, disappears in the CPT framework because the subduction and exhumation velocities are orders of magnitude higher and the time scale is orders of magnitude shorter.

A second major issue is the volume of the sediments that are able to be entrained by the downgoing oceanic plate. The higher the speed of downgoing plate, the higher are the shear stresses that act on potentially entrained sediments. The greater these shear stresses are, the thicker in general will be the layer of subducted sediment. Furthermore, the larger the volume of entrained sediment carried to depth, the more readily and faster that volume—because of its buoyancy— will be able to rise again to the surface. Whereas in UPT the subduction and exhumation of huge volumes of blueschist facies rocks observed in the Franciscan complex are nearly impossible to account for, within the CPT framework they are to be expected. For these reasons, the blueschist facies terranes from many places in the world represent powerful support for the reality of CPT.

Why have I devoted so much space to discussion of the Franciscan rocks? First, this complex represents a place on earth where one can actually observe first-hand a sequence of rocks some 100 km wide that accumulated in an ocean trench as an accretionary wedge by subduction processes in the past. As such, these rocks testify powerfully that past subduction is genuine and unambiguous. Second, these rocks reveal clearly that buoyant sediments can be and have been subducted to significant depths, been metamorphosed, and then, because of their buoyancy, rapidly returned to the surface. Third, these rocks indicate that processes different in some way from those acting presently were involved. The simplest way to account for the large volumes of sediment involved as well as the rapidity they returned to the surface is dramatically higher plate speeds. In other words, these rocks testify strongly that past subduction was distinctly different in some ways from what is currently occurring today. By way of summary, these field examples of subduction, while understandable in the framework of conventional plate tectonics, fit more consistently within the CPT framework in that CPT can more readily account for such features as blueschist facies rocks in older accretionary complexes.

Finally, in regard to whether or not oceanic plateaus have been scraped off by subduction, there are numerous papers that document that a significant portion of western North America is the result of what is sometimes referred to as ‘collage tectonics’. Indeed, the recognition that many modern and ancient continental margins are comprised of diverse crustal fragments, each with a distinctive stratigraphy and separated by tectonic contacts, initially occurred for the Pacific coast part of North American in the 1970’s. Over 200 distinct terranes have been now been identified in this portion of North America, which includes most of Alaska, much of British Columbia, and major portions of California, Oregon, and Washington. This region is now understood to be a collage of oceanic plateaus, oceanic volcanic arcs, and continental fragments, brought together over large distances (in many cases more than a thousand km) by plate motions and plate convergence. Other continental regions including eastern Siberia, much of China, south-central and southwestern Asia, central Europe, eastern Australia, and southeastern New England, are also now recognized to be the amalgamation of diverse crustal fragments that have been brought together by plate convergence and subduction. Because this literature is so vast, I simply provide below as an example a summary description of the collage tectonics aspects of British Columbia written as a student assignment in 2005 and posted here [ed. archived copy; the original page is gone]. I have shortened it slightly and omitted the list of references:

TECTONIC EVOLUTION OF NORTHERN BRITISH COLUMBIA by Chad Seigel

Introduction: Northern British Columbia is mountainous area composed of various faults and orogenic belts which are part of a larger geographic area known as the Canadian Cordillera. This area is composed of ancient cratonic basement, and a collage of terranes which later accreted to this basement as shown in the figures below. This report will focus on a transect cut through northern British Columbia from east to west, starting on the BC/Alberta border, running along the 60th parallel and ending on the Alaskan coast. Although much more complex, the five morphological belts and the ancient cratonic basement which are located along this transect will be briefly summarized in terms of the terranes of which each belt is composed and tectonic origin and time of accretion.

Ancient Continental Margin: The ancient continental margin of North America was formed during late Proterozoic when continental rifting tore the former supercontinent Rodinia apart creating ancestral North America (Laurentia). This resulted in the formation of a passive continental margin much like the present day east coast of North America which persisted until middle Devonian. During this time a thick miogeoclinal deposit of sediment which was eroded from the Canadian Shield to the east was deposited on the continental shelf of the proto Pacific Ocean known as Panthalassa. This supracrustal wedge increases in thickness from east to west where it is approximately 5 km thick at the edge of the Foreland Belt.

Foreland Belt: Moving from east to west along the transect the Foreland Belt is encountered. Although this belt is the next in the sequence of orogenic belts it was the last to form. During the break up of Pangea during the Jurassic the North American continent began to move westward like a giant bulldozer accreting terranes which lay just off the coast of the continent. In doing so these terranes were squeezed in a giant vice between the subducting oceanic lithosphere off the coast of North America, and the wedge shaped North American craton. These terranes were squeezed upwards and downwards which resulted in the detachment of the thick miogeoclinal sequences deposited during the late Proterozoic and Paleozoic from the cratonic basement. These sequences were upthrust onto the edge of the North American craton forming the Foreland Belt.

Omenica Belt: Next along the transect is the Omenica Belt. The Omenica Belt is the region of overlap between the Intermontaine Belt to the west and the Foreland Belt to the east. During the late Devonian, it is thought that an oceanic trench was created at the continent ocean boundary caused by the enormous weight of the old dense oceanic crust combined with the weight of sediments deposited on the continental shelf. Stratigraphic and U-Pb age constraints indicate that this episode of magmatism was Devonian-Mississippian and has been interpreted as subduction related magmatism at a convergent margin. Today, nowhere on Earth is there evidence of subduction being initiated, and the cause of change from a passive, intra-plate margin to a convergent, inter-plate margin is uncertain. This newly formed convergent boundary created magmatic island arcs which formed on the edge of the North American craton not far from the continental margin. These arcs are the Slide Mountain, Cassiar, and Yukon-Tanana Terranes which make up the Omenica Belt. These two arcs with their associated back arc basins have gone through numerous metamorphic events throughout their history, and today represent the once deeply buried roots of this ancient magmatic arc assemblage.

Intermontaine Belt: The Intermontaine Belt is composed of the Cache Creek, Nisling, and Stikine terranes. The Cache Creek and Nisling terranes were formed in the western Pacific during the Permian to Middle Triassic. Sedimentary rocks of the Cache Creek Terrane contain a particular assemblage of fossils that are found in Asia and not in ancient North American rocks. This suggests that the Cache Creek Terrane likely originated far from North America and may have existed on the other side of the Pacific Ocean. The Stikine Terrane is a Carboniferous to Early Jurassic island arc which was formed in the east Pacific. The Stikine Terrane is believed to have evolved in the east Pacific of the Northern Hemisphere and moved northward to dock with ancestral North America sometime during the Middle Jurassic. During the Early to Middle Jurassic the two terranes joined to form the Intermontaine Belt. After this time arc related magmatic activity continued into the Tertiary. Late Triassic through Tertiary plutons intrude structurally imbricated Stikine and Cache Creek terranes in the Atlin-Bennett Lake area of northern British Columbia.

Coast Belt: The Coast Belt is the suture zone between the Intermontaine Belt and the Insular Belt and is composed of plutonic and metamorphic rock. During the mid Cretaceous, the exotic Insular superterrane collided with North America, further deforming the Intermontaine terranes. The deformation compressed the upper crust of the Nisling and Stikine terranes by more than 160 km, approximately the width of the Coast Belt.

Insular Belt: Westernmost is the Insular Belt which is composed of the Wrangel, Alexander, Chugach, and Yukatat terranes. The Wrangel and Alexander terranes are island arcs having formed in the Pacific during the Devonian. These arcs amalgamated during the Carboniferous and collided with the continent during the Cretaceous. It is uncertain exactly when the Insular Islands finally arrived on North American shores. Some evidence suggests they persisted as an offshore volcanic chain for some time, much like the islands of Japan do today. In any event, the final collision between the islands and the continent did not occur until mid-Cretaceous time. The Chugach and Yukatat terranes are composed of sedimentary rock of Tertiary age. Products of erosion were deposited in Pacific Ocean floor and carried northward on the oceanic plate to be accreted as major components of Chugach and younger accretionary terranes southern Alaska.

Summary: The collage of terranes and faults that make up the continental crust of northern British Columbia are extremely complex and have been simplified here for clarity. Much is currently known about the complex geology of this area and many questions still remain unanswered. With new theories and geoscience techniques more questions and answers will be brought to light, once again changing our understanding of the tectonic evolution of northern British Columbia. (End of article)

Although these striking evidences for long distance transport and assembly of these terranes in a collage-like manner is accepted within the UPT framework by the mainstream earth sciences community, it is a major stretch to imagine that the slow and gradual plate motions we observe today could have achieved such incredible tectonic feats. It is dramatically more reasonable, however, that these sorts of amalgamations could and did take place if the plate velocities were orders of magnitude higher than they are today during a brief but intense cataclysm.

CPT Q. #5 – 101 Q&A on Catastrophic Plate Tectonics

Response: There are hundreds of articles in the peer-reviewed literature that document mélange structure in subduction zone accretionary wedges. I have already described the complex and contorted structure observed both by drilling and seismic profiling of the accretionary wedge that lies within the Nankai Trough where the Philippine Plate is actively subducting beneath southwestern Japan. Similarly, I have already described the classic example of a fossil subduction zone, the Franciscan terrane along the central and northern California coast, which displays strongly contorted sediments and ocean crustal rocks that have been subducted to depths of 20-30 km, metamorphosed to blueschist grade, and then amazingly have risen up the subduction channel back to the surface. One of many other examples I could point to is in Guatemala, similar in many ways to the Franciscan, and described in the recent paper by Marroni, et al. [ed. as of ~2012], “Deformation history of the eclogite- and jadeitite-bearing mélange from North Motagua Fault Zone, Guatemala: insights in the processes of a fossil subduction channel,”¹ whose abstract I reproduce as follows:

In Guatemala, along the northern side of the Motagua Valley, a mélange consisting of blocks of eclogite and jadeitite set in a metaserpentinitic and metasedimentary matrix crops out. The metasedimentary rocks display a complex deformation history that includes four tectonic phases, from D1 to D4. The D1 phase occurs only as a relic and is characterized by a mineral assemblage developed under pressure temperature (P-T) conditions of 1.00-1.25 GPa and 206-263°C. The D2 phase, characterized by isoclinal folds, schistosity and mineral/stretching lineation, developed at P-T conditions of 0.70-1.20 GPa and 279-409°C. The following D3 and D4 phases show deformations developed at shallower structural levels. Whereas the D1 phase can be interpreted as the result of underplating of slices of oceanic lithosphere during an intraoceanic subduction, the following phases have been acquired by the mélange during its progressive exhumation through different mechanisms. The deformations related to the D2 and D3 phases can be regarded as acquired by extrusion of the mélange within a subduction channel during a stage of oblique subduction. In addition, the structural evidences indicate that the coupling and mixing of different blocks occurred during the D2 phase, as a result of flow reverse and upward trajectory in the subduction channel. By contrast, the D4 phase can be interpreted as related to extension at shallow structural levels. In this framework, the exhumation-related structures in the mélange indicate that this process, probably long-lived, developed through different mechanisms, active in the subduction channel through time.

While mélange formation is expected in places like the Nankai Trough today under the slow rates of convergence assumed in UPT because the sediments are inherently soft, many of the other striking features of the fossil accretionary wedge deposits, such as large volumes of blueschist rocks returned to the surface, are not readily explained in the framework of UPT, but instead seem to require the catastrophic conditions associated with CPT.

CPT Q. #6 – 101 Q&A on Catastrophic Plate Tectonics

Response: While plates in general display considerable strength, there is impressive observational evidence that they can and do rift, or split apart, under the right circumstances. A good example is the rifting of the Arabian block from Africa with the formation of the Red Sea and the Gulf of Aden. Just what the forces were that initiated the rifting may not be so clear, but the reality that a once coherent plate has split into two pieces and that the two pieces are presently moving away from each other is documented by many lines of evidence, including GPS measurements. Another example of continental rifting is the separation of Baja California from Mexico and the subsequent migration of this block several hundred kilometers to the northwest. The cause in this case seems to be clearer, given that according to present GPS measurements the motion of this block is essentially identical to that of the Pacific Plate. The implication is that some of the western portion of the North American Plate has overridden the ridge that earlier formed the southeastern boundary of the Pacific Plate and that the forces associated with the divergence at this ridge were sufficient to cause the rifting away of this sliver of North American Plate. Careful numerical simulation indicates that lithospheric plates can also fail in compression. The most common circumstance is when a slab of oceanic lithosphere becomes sufficiently thick through cooling, it begins to founder under its own weight, initially producing a broad depression above it and then fracturing and sinking into the weaker and less dense mantle beneath it.

In regard to the issue of a lithospheric plate moving relative to the asthenospheric layer beneath it, I pointed out earlier that the case is compelling that this layer is on the order of a thousand to ten thousand times weaker than normal lithosphere and that the drag forces exerted by the asthenosphere on the base of the lithosphere tend to be extremely small.

Response: Because rock at sufficient depth under stress does indeed begin to deform inelastically or plastically, the boundary below a few km depth between a subducting plate and the overriding plate is never vertical but instead is inclined, typically, at an angle of 30-45 degrees. Inelastic deformation of the edge of the overriding plate readily allows this to occur. The fact that plates are subducting today means that one plate diving beneath another plate not only is possible; it is an undeniable reality. As shown in the caption is a figure from the NASA website here showing velocities of more than 900 GPS stations worldwide. The velocity discontinuities at mid-ocean ridges and also subduction zones is to me indisputable evidence that both seafloor spreading and subduction are occurring on our earth today.

Response: According to GPS measurements, subduction is taking place today into the arc-shaped Aleutian Trench, the arc-shaped Sumatra Trench, and many other trenches that are far from straight lines. The error in the belief that subduction can occur only along a straight line is a failure to recognize that a subducting plate deforms as it plunges into the mantle. Many papers in the peer-reviewed literature provide strong seismic evidence that subducted slabs tear and deform dramatically in their journey downward into the mantle.

One location where slab tear is almost a certainty is at the Aleutian-Kamchatka corner. In the paper by Davaille and Lees, “Thermal modeling of subducted plates: tear and hotspot at the Kamchatka corner,”¹ the authors ask:

“How can the Pacific Plate, which is subducting at an oblique angle in the western Aleutians, physically connect to the relatively steeply dipping Kamchatka slab to the west?”

They continue:

“The surficial manifestation of the connection is the massive Bering transform zone (TZ), extending from Attu Island westward towards Kamchatka (see figure below). In Kamchatka the margin between the Pacific Plate and North America takes a sharp turn southwards, towards the Kurile Trench and Japan. How does the Pacific Plate accommodate this sharp apparent bend? Does the Pacific plate drape over the corner as a tablecloth folds around a table corner, or is the Pacific Plate torn at the corner along the TZ to accommodate the deformation? In this paper we explore evidence and implications for the latter hypothesis.”

The authors do make a strong case that there is a tear in the Pacific Plate along the Bering Fracture Zone.

The extreme deformation that subducted slabs undergo is beautifully illustrated in a recent [ed. 2008] paper by Sigloch and Nolet entitled: “Two-stage subduction history under North America inferred from multiple-frequency tomography.”² Figure 2 from this paper, reproduced below, shows via seismic tomography the present shape of the Farallon Plate, which has subducted beneath the western coast of North America since the earliest Jurassic and continues to do so as the modern Juan de Fuca Plate along the coasts of Oregon and Washington. The authors infer from the seismic data as well as from numerous surface observations that this plate underwent a large amount of tearing in its complicated history.

Studies on the way slabs deform after they subduct reveal that they can and do undergo extreme deformations, including tears. Most of the studies utilize seismology to provide actual images of these deforming masses of rock inside the earth. The fact that subducted slabs can and do deform means that geometrical constraints that would apply to perfectly rigid slabs do not apply to the real ones. This conclusion is valid not only in the case of the uniformitarian framework but also in the case of CPT.

Response: More than half of the world’s active volcanoes above sea level today encircle the Pacific Ocean to form what is frequently referred to as the circum-Pacific “Ring of Fire.” This horseshoe shaped belt, some 40,000 km long, is associated almost exclusively with nearby ocean trenches, as indicated in the map below produced by the USGS. Almost without exception, the volcanoes are on the side of the trench beneath which the subduction is occurring.

Response: Trenches themselves represent huge mass deficiencies, where, instead of rock, there is water. The free-air gravity anomalies observed over trenches are typically in the range of -100 to -300 milligals (one milligal is 10-5 m/s2), depending on the amount of sediment fill. One can verify that mass deficiencies corresponding to those associated with trenches do indeed produce these sorts of free-air gravity signatures. The formula for the gravity anomaly ∆g produced by an infinitely long line of excess mass γ per unit length at depth b below the surface and observed directly above the line is given by the definite integral from -∞ to ∞ of the integrand Gγdx/(x2 + b2), which is equal to πGγ/b, where G = is the universal gravitational constant. Approximating the cross-sectional area of a trench as a triangle of height h and width 2h/3, with its center h/3 below the surface, yields the formula ∆g = πGh∆ρ, where ∆ρ is the density contrast between what is filling the trench and normal crustal rock. For a trench depth h of 6000m and a ∆ρ of , corresponding to the trench being filled with water, we get a resulting gravity anomaly of -214 milligal. The extra density of the slab immediately beneath the trench, because of its cold average temperature, can readily be shown to be negligible in comparison.

However, the higher density of subducted slabs often does produce a discernible gravity signature behind the trenches. This can be seen from a visual inspection of the free-air gravity anomaly map of the world’s ocean floors shown below. Positive gravity anomalies shown in orange are evident behind the Tonga-Kermadec Trench east of Australia and behind the Izu-Bonin and Marianas Trenches south of Japan. Again, this gravity signature is the result of the subducted slabs greater density, because of their lower temperature, relative to the surrounding mantle rock. These principles apply equally well to both UPT and CPT.

CPT Q. #11 – 101 Q&A on Catastrophic Plate Tectonics

Response: Earthquakes that occur well behind the trench location tend to be deep-focus earthquakes which occur at depths between 300 and 700 km beneath the earth’s surface. Because subducted lithosphere should not exhibit brittle behavior at such depths, the mechanism responsible for these deep earthquakes has stirred controversy since their actual depth was first verified more than 70 years ago [ed. as of ~2012]. Because mineralogical phase changes occur in the lower part of the upper mantle where these earthquakes are most frequently observed, a possible and leading candidate mechanism has been the catastrophic transformation of metastable olivine into the higher density spinel phase. Because of the low temperatures in the core of the subduction slab, this phase transition likely may not always spontaneously occur as the slab passes through the depth where the phase transition otherwise ought to take place. When this is the case, metastable olivine is transported to greater depths and has the potential to transform rapidly to the spinel phase, provided there is some process to initiate this transformation. However, a simple volumetric implosion of the low density olivine phase to produce the higher density spinel phase does not match the pattern of earthquake waves these earthquakes radiate—a pattern which typically implies a large amount of shear deformation.

However, about 20 years ago [ed. in 1990] H. W. Green and P. C. Burnley in “The failure mechanism for deep-focus earthquakes,” Geological Society, London, Special Publications 54, 133-141, 1990, described the mechanism now generally thought to account for these deep focus earthquakes. In the abstract of this paper they summarize their findings:

Experimental deformation of Mg₂GeO₄ olivine at pressures between 1 and 2 GPa in the spinel stability field has led to discovery of a faulting instability that develops at the kinetically-controlled threshold of transformation. Very fine-grained olivine and spinel are found in fault zones. Deformation at lower temperatures is ductile; transformation is inhibited and specimens are very strong. Deformation at higher temperatures also is ductile but transformation is rapid and specimens are much weaker. Detailed examination of the microstructures of specimens deformed in the faulting regime lead to an anticrack theory of faulting that explains the experimental data and provides a fundamentally new mechanism for deep-focus earthquakes. The new mechanism is analogous to the Griffith theory of fracture; nucleation and growth of spinel under stress produces spinel-filled microanticracks normal to the maximum compressive stress that link up to produce faulting. The friction paradox for deep earthquakes is resolved because this faulting process provides a fine-grained, superplastic, ‘lubricant’ for faults. The temperature distribution within subducting slabs of lithosphere requires that the conditions of instability are reached as a natural consequence of subduction; metastable olivine in the interior of deep slabs warms to a critical temperature where faulting ensues in the presence of a shear stress.

To summarize, Green and Burnley used the germanium analog mineral, Mg₂GeO₄, instead of silicate olivine, (Mg,Fe)₂SiO₄, to investigate the mechanics of this phase transition in the laboratory in a large enough volume to be able to observe and characterize the actual faulting process. The germanium analog is softer and changes to the spinel structure at much lower pressure than the silicate mineral. Their experiment appears to elucidate how this phase transition can unfold extremely rapidly and also generate large-scale shear motions within the core of a subducting slab.

Another observation that points to the likelihood of the mechanism involving the rapid transformation of olivine to spinel and possibly other lower density phases such as pyroxene transform to their higher density phases is that deep focus earthquakes cease abruptly below a depth of about 680-700 km, which represents the boundary between the upper and lower mantle. This is the depth at which the major upper mantle phases are converted to the yet higher density phases perovskite and magnesiowuestite. Hence, whatever the mechanism is, it shuts down when these transitions between upper mantle mineral phases no longer can occur.

These observations and conclusions apply equally to both UPT and CPT.

Response: The paper by Sigloch et al., “Two-stage subduction history under North America inferred from multiple-frequency tomography,”¹ mentioned above in my response to question 8, provides a dramatic example of the ability of current generation seismic tomography methods to convincingly reveal the 3D structure of subducted slabs. Figure 2 from this paper, reproduced below, shows the present shape of the Farallon Plate which, not only subducted beneath the western coast of North America since the earliest Jurassic in the past, but continues to do so as the modern Juan de Fuca Plate along the coasts of Oregon and Washington.

The paper by Miller et al., “Imaging changes in … the subducting Pacific Plate along the Izu-Bonin-Mariana arc,”² shows the geometry of the portion of the Pacific Plate that is currently subducting in the Izu-Bonin Trench south of Japan. Two figures from their paper, shown below, strongly suggest that the slab is in the process of tearing as it subducts.

These same authors, Miller et al., in another paper³ provide a 3D image, displayed below, of the subducted slab beneath Japan which gives rise to so many large earthquakes in that region.

These papers provide a sample of the work currently being done which is providing an expanding catalog of 3D seismic imaging of subducted slabs from many places around the world [ed. as of ~2012].

Response: This question is motivated by the sentence from the abstract of the 2004 paper by Booker et al., “Low electrical resistivity associated with plunging of the Nazca flat slab beneath Argentina,”¹, that reads:

“But between 28° and 33° S the subducted Nazca Plate appears to be anomalously buoyant, as it levels out at about 100 km depth and extends nearly horizontally under the continent.”

The full abstract reads,

Beneath much of the Andes, oceanic lithosphere descends eastward into the mantle at an angle of about 30°. A partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material. This wedge is the ultimate source for magma erupted at the active volcanoes that characterize the Andean margin. But between 28° and 33° S the subducted Nazca Plate appears to be anomalously buoyant, as it levels out at about 100 km depth and extends nearly horizontally under the continent. Above this ‘flat slab’, volcanic activity in the main Andean Cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge. But it is unknown where slab volatiles go once this happens, and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward. Here we present results from a magnetotelluric profile in central Argentina, from which we infer enhanced electrical conductivity along the eastern side of the plunging slab, indicative of the presence of partial melt. This conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth, or that melt is supplied from the 410 km discontinuity, consistent with the transition-zone ‘water-filter’ model of Bercovici and Karato.

Note that beyond that flat zone the slab “finally rolls over to descend steeply into the mantle.”

Flat subduction has been shown by seismic studies to be occurring today in several places in the world, two separate segments in fact beneath South America including the one just described. A seismic investigation of the northern segment beneath Peru is contained in a 1992 Virginia Tech master’s thesis by E. O. Norabuena entitled “Velocity structure of the subducting Nazca Plate beneath central Peru as inferred from travel time anomalies”².The findings of this study are summarized in the figure below:

Note that like the similar zone beneath Chile and Argentina, the slab after moving nearly horizontally, in this case for only about 300 km, then plunges downward at a steep angle. This thesis also includes a map showing the tectonic setting, shown below.

There is also a strong case that the Farallon Plate that subducted beneath the western coast of North America. This was first proposed more than 20 years ago [ed. in 1988] by P. Bird in “Formation of the Rocky Mountains…”³ The abstract of this paper is as follows:

One hypothesis for the information of the Rocky Mountain structures in late Cretaceous through Eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the North American lithosphere. The horizontal components of the motion of this plate are known from paleomagnetism, and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism. New techniques of finite-element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the North American Plate. A model that has a realistic temperature-dependent rheology and a simple plane-layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present. Successful prediction of this model include (i) the location, amount, and direction of horizontal shortening that has been inferred from Laramide structures; (ii) massive transport of lower crust from southwest to northeast; (iii) the location and timing of the subsequent extension in metamorphic core complexes and the Rio Grande rift; and (iv) the total area eventually involved in Basin-and-Range style extension.

In a broad sense, this model has predicted the belt of Laramide structures, the transport of crust from the coastal region to the continental interior, the subsequent extension in metamorphic core complexes and the Rio Grande rift, and the geographic region of late Tertiary Basin-and-Range extension. Its principal defects are that (i) many events are predicted about 5 million to 10 million years too late and (ii) the wave of crustal thickening does not travel far enough to the east. Reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects.

The correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the Laramide orogeny is probably correct. The Rocky Mountain thrust and reverse faults formed in an environment of east-west to northeast-southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the North American crust. Nonuniform crustal thickening by simple-shear transport also caused relative uplifts; therefore, this model is consistent with both of the range-forming mechanisms that have been inferred. A new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional Basin-and-Range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away.

This landmark paper has been widely referenced in subsequent work on the geology of the western United States. The inference of a period of flat subduction by the Farallon Plate beneath western North America is prominent in the much more recent paper by Sigloch, McQuarrie, and Nolet (previously mentioned in my response above to question 8 that presents the 3D seismic tomography image of the strongly contorted Farallon Plate. The author’s interpretation of the 3D tomographic image in terms of the subduction history of the Farallon Plate is provided in the figure below, reproduced from their paper:

Of course, the interpretation in this article as well as in the previous one is in terms of the uniformitarian time scale, which is to be rejected in regard to as far as absolute dates are concerned. In summary, the case for flat subduction of slabs is compelling, not only in the present but also in the past. Numerical models show that it is mechanically plausible. The main driving force for moving the slab is the slab pull arising from the negative buoyancy of the cold dense material that comprises the slab. These conclusions apply equally for UPT and CPT.

CPT Q. #14 – 101 Q&A on Catastrophic Plate Tectonics

Response: This question is motivated by a statement in a paper by P. Molnar entitled: “Continental tectonics in the aftermath of plate tectonics”¹:

…the buoyancy of thick continental crust keeps it afloat. If continental lithosphere were strong enough to maintain its integrity at a subduction zone, the buoyant continental crust would not only resist being subducted, but the subducting plate would abruptly grind to a halt when the continental “passenger” reached the trench.

The paragraph containing this quote is the first paragraph in a section entitled “Differences between continents and oceans” and focuses on the contrast in buoyancy. The next paragraph focuses on their contrast in strength. It reads:

The strength of the continental lithosphere also contrasts with that of the oceanic lithosphere. The strongest part of the oceanic lithosphere seems to lie in the mantle, between 20 and 60 km depth, between a brittle upper part and above its increasingly ductile lower part, which grades downward into the asthenosphere (Fig. 3). In the same depth range where oceanic lithosphere is strongest, however, continental lithosphere consists of crust, not mantle. At temperatures typical of the lower crust (400-700 °C), the minerals comprising the crust appear to be much weaker than olivine, the strong mineral that comprises most of the upper mantle. Consequently, continental lithosphere could be much weaker than oceanic lithosphere. Oceanic lithosphere behaves as a virtually rigid plate because of its strong core, but, as the late C. Goetze noted in the mid-1970s, continental lithosphere might consist of three layers: a brittle upper-crustal layer, a weak lower crust and a stronger uppermost mantle, which, nevertheless, would not be as strong as the strongest part of the oceanic lithosphere. This jam-sandwich-like rheological profile (Fig. 3) is also suggested by the frequent occurrence of earthquakes (brittle deformation) in the upper crust, their nearly complete absence in the (presumably weak, ductile) lower crust, and their occasional presence in the underlying upper mantle.

The point of this paragraph is that in regard to overall strength continental lithosphere contrasts strongly with the oceanic lithosphere. Indeed, the strength profile of continental lithosphere has frequently been referred to as a “jam sandwich” because of the weakness of the warm lower crust. So the original quote, taken in its context, does not suggest or imply that continental lithosphere is strong enough to maintain its integrity at a subduction zone and therefore that subduction should abruptly grind to a halt. It is just the opposite. The author in the following paragraph is providing reasons why continental lithosphere is weak and deformable and why this grinding to a halt state of affairs does not generally take place. This observation applies equally to UPT and CPT.

CPT Q. #15 – 101 Q&A on Catastrophic Plate Tectonics

Response: This perception is simply not true. Detailed estimates of plate convergence and divergence are provided in a 2003 paper by Peter Bird, “An updated digital model of plate boundaries,”¹. Below is a table that summarizes the results (see ibid. p. 46).

Table 3. Collective Properties of Plate Boundaries by Class

The total length of convergent boundaries is 91,762 km, while the total length of divergent boundaries is 94,810. If we neglect the continental convergent and rift boundaries and consider only convergent and divergent boundaries in the ocean basins, the total length of subduction zones and other convergent segments is 68,759 km, while the total length of ocean ridges is 67,338 km, again very similar.

The current rate of area increase along the oceanic ridges is 0.095 m2/s, which is very close to the current rate of area loss along convergent boundaries in the oceans, 0.094 m2/s. While there is no logical or geometrical requirement for the total lengths of convergent and divergent boundaries to be identical, they are amazingly similar.

Response: The first of these sites is the north end of Explorer Ridge, located off the coast of British Columbia just south of Queen Charlotte Island and northwest of Vancouver Island, where it forms a triple junction with the Queen Charlotte Fault and the north end of the Cascadia Trench. The location is indicated in the figure below by the black arrow. As far as can be determined from observation, both the Explorer Ridge and the Cascadia Trench terminate at this triple junction. There is nothing which seems to indicate that the ridge continues toward the continent beyond the trench. New plate produced by spreading at the end of the ridge appears to be accommodated by oblique subduction into the trench. So this site does not appear to correspond to a case of a ridge itself subducting into a trench.

The second site, at 20.5 °N, 107 °W, is near the northern end of the East Pacific Rise along the western coast of Mexico just below the mouth of the Gulf of California. The region is shown in the map below.¹

One notes that the location 20.5 °N, 107 °W is in the middle of the small Rivera Plate and not on a ridge, although it is near the Acapulco Trench. So it is not clear what feature was intended here.

The third site is along the western coast of southern Chile, where the Chile Rise, an oceanic ridge between the Nazca and Antarctic Plates, is indeed being subducted into the Peru-Chile Trench. A PowerPoint geology class presentation of this remarkable feature is available here (or here). Below are some of these slides.

Why is it possible for this portion of the Chile Rise to subduct? The main reason is that the plates on either side of the ridge, the Antarctic Plate and The Nazca Plate, are sufficiently strong to carry this ridge segment along as they move into the trench. What happens to the rock in the ridge as it migrates into the subduction zone? To the extent that the Antarctic and Nazca Plates continue to diverge from each other, which their strength away from the ridge would cause them to tend to do, there will be flow of warm rock from below to fill the resulting gap. But this flow is relative to the movement of the gap into the trench and beneath the South American Plate. So whether the net motion is actually upward or downward depends on which of the two vertical components of motion is greater. This conclusion is the same for both UPT and CPT.

Response: The Franciscan terrane in California, as was mentioned in my response to question 4 above, is a classic example of a fossil subduction zone complex that has been in the geology literature for the past 40 years [ed. as of ~2012]. With only a short drive north of San Francisco, one can inspect these rocks first hand. Similar fossil subduction zones containing high grade metamorphic blueschist rocks have been described in Greece, Turkey, Japan, New Zealand, and New Caledonia east of Australia. Another such example mentioned in my response to question 5 above is the North Motagua Fault Zone in Guatemala. There are dozens of other spectacular examples well documented in the standard literature.

Response: These features are evident in the schematic diagram below.

Laboratory experiments conducted now for almost 40 years [ed. as of ~2012] have shed some extremely important light on these questions. The experiments involve the use of molten wax to investigate on a laboratory scale the mechanics of how a medium consisting of a brittle upper layer and a ductile lower layer deforms when pulled apart. The classic paper that launched this approach is by Oldenburg and Brune, “Ridge transform fault spreading pattern in freezing wax.”¹

The abstract of this paper reads as follows:

A laboratory experiment shows that ridge-ridge transform faults, inactive fracture zones, and other features characteristic of spreading oceanic ridges can be produced in a variety of paraffins. Although the resultant pattern depends upon the temperature of the wax and the ratio of spreading rate to surface cooling, the characteristic orthogonal ridge transform fault system is a preferred mode of separation. Symmetric spreading occurs under conditions of no tensile strength across the ridge, and the stability of transform faults is a consequence of their lack of shear strength. The experiment also shows that properties characteristic of oceanic ridges occur under conditions of passive convection where upwelling of material at the ridge crest is a result only of hydrostatic forces in the fluid; that is, the plate separation is caused not by large convective forces beneath the ridge but rather by tensile forces in the plate.

The figure below shows the apparatus and basic results of the experiment.

What this experiment and many similar ones since reveal is that the essential physics responsible for the segmented ridge/transform fault geometry is the presence of a strong brittle upper layer and a much weaker ductile lower layer, with surface cooling causing the brittle layer to thicken with time, yet with sufficient spreading motion to keep the divergent zone weak by replenishment with hot ductile material from below. To the extent that this experiment represents an analog to ridge tectonics on the earth, the authors conclude that “spreading ridges may be formed under the influence of tensile stresses only, and forces from an active convection cell located beneath the ridge axis are not required.” Let me here emphasize that subsequent observation strongly confirms that this generally is the case for the earth—that mid-ocean ridges are mainly the product of the divergent motion of the plates on either side and not a result of the upwelling limb of a convection cell below. In other words, the spreading ridges are largely passive features—the result of plate divergence. (This is very much contrary to many popular characterizations of plate tectonics concepts.) A second major conclusion of the authors of this paper is that “the stability of transform faults is a consequence of their lack of shear strength.”

To summarize up to this point, the picture provided in this 1972 experiment with molten wax comes close to describing the essential physics responsible for symmetric spreading at ridges and for the fracture zones that offset segments of ridge at approximately 90° angles, both within the framework of uniformitarian plate tectonics (UPT) and catastrophic plate tectonics (CPT). The very weak asthenosphere reaches almost to the surface at a spreading ridge. Partial melting of asthenospheric rock generates the basaltic magma that fills the gap as the oceanic plates diverge along the ridge axis. The circulation of sea water within this hot rock cools it rapidly and causes a strong and brittle surface layer to develop away from the ridge axis. However, the slip along the active portion of the transform faults keeps these active faults weak. Hence, there does seem to be a solid correspondence between the crucial aspects of the molten wax experiment and the mid-ocean ridge/transform fault environment.

Response: Usually when I have used this wording I have had in view the numerical simulations of the supercontinent breakup that I have undertaken over the years. These calculations are initialized with a zone of cold material around much of the perimeter of the supercontinent to initialize motion within the spherical shell domain. This results in a large component of spherical harmonic degree two flow inside the domain. The 3D seismic tomography images below of the earth’s actual lower mantle density distribution that prominently displays this degree-two pattern. Blue corresponds to higher density, presumably cold, mantle rock, while red corresponds to low density, presumably hot, mantle rock.

The type of flow characterized by downwelling mostly around a great circle, to conserve mass, leads to upwellings on either side of the circle, as suggested by the red features in the images above. The presumed upwelling in the middle of the eastern hemisphere beneath Africa was near the center of the original supercontinent. If this interpretation is correct, then this upwelling flow would have had the tendency to cause the supercontinent to pull apart. Below are some snapshots from one of the computer simulations that shows the dynamics that result from the initial temperature perturbation as conservation of mass and energy is enforced and forces are balanced everywhere throughout the domain and velocity, pressure, and temperature are recomputed at each grid point time step after time step in the calculation. Although the motions are driven solely by gravity that acts in the radial direction, there are significant horizontal forces and velocities, especially near the boundaries, that naturally arise in order to conserve mass.

In summary, the downwelling flow around the perimeter of the continental region leads to upwelling flow beneath that region which results in an overall tendency to pull the supercontinent apart.