Evolution of the Aleutian Arc: A Dynamic Model Different from Strict Plate Tectonics

Posted July 2002
Revised January-July 2007 and March-May 2010, December 2010
with additional explanations,
and with removal of extraneous and outdated material

by

James N. Murdock
611 Green Valley Dr. SE
Albuquerque, NM 87123
USA

jmsmurdock

@ gmail.com


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Abstract

This web site is meant to give a general concept of the evolution of the structure, stratigraphy, and landforms of the eastern Aleutians from 250 Ma to the present. (This region is a zone of convergence of the Pacific and North American plates.) The evolution is depicted by 17 cartoons (called snapshots) that show an elementary animation of the changing major features. Each of the snapshots is briefly justified or explained. They were constructed as if the reported geologic structures of the arc defined the amount of convergence of the two plates. As the megathrust has not been identified at the trench, the snapshots show a presently operating interplate blind thrust (decollement) with its front about 50 km landward of the trench. As such, the model will accommodate only a small fraction of the usually indicated amount of convergence. Indeed, during the post mid Miocene, the model shows roughly ten kilometers of convergence (having occurred in several brief sub-episodes with one in progress today), whereas the conventional plate model predicts several hundred kilometers (having occurred continuously). The small value is approximately consistent with the 5-6 km of the post-Miocene displacement between the Pacific and North American plates reported on the nearby Fairweather transform fault, albeit the timing of the displacement within this interval is controversial. As briefly described in the Appendix, this uncertainty motivated me to carefully examine the report of a critical test that was conducted at another convergent margin. Whereas the test was initially reported as inconclusive, I found a decimal error in the calculations: The amount of convergence appears to be at least an order of magnitude smaller than predicted by the conventional plate model, similar to what I have found for the Aleutian region.

A thumbnail overview of the snapshots may be obtained by clicking here (236 KB). A table compares the interpretations of advocates of the conventional model with mine. An Appendix gives an overview of how I developed the major concepts.


INTRODUCTION

Advocates of strict Plate Tectonics (SPT) typically assume that the present-day observations (including the geodesy) are representative of a long-term process. (By long term, I mean millions of years.) Furthermore, the advocates typically assume that the amount of convergence of the plates is determined by an amount of calculated sea floor spreading, and they build a tectonic model of an arc based on this assumption. In contrast, I assume that the timing and amount of convergence of the plates might be indicated by the geology of an arc, except in its earliest stages of development. I have studied primarily the Aleutians. This is a convergent zone of the Pacific and North American plates.

I consider the evolution of two segments of the Aleutians: The the Amchitka-Amlia area of the central Aleutians and Cook Inlet-Kodiak Island and its forearc and backarc of the eastern Aleutian arc, Figure 1. While the eastern Aleutians appears to have first formed near the beginning of the Mesozoic, the central Aleutians appears to have first formed near the beginning of the Cenozoic. I postulate that today's central Aleutians and today's eastern Aleutians represent two different stages in the growth of an arc, with the central Aleutians representing the oceanic stage of development. The eastern Aleutians, with the great Alaska-Aleutian batholith of the backarc, represents a more continental stage. I argue that the geology of the central Aleutians provides an insight into the evolution of the eastern Aleutians during its early stages.

The central and eastern Aleutians are perhaps the most thoroughly investigated regions of a tectonic arc. In addition, the eastern Aleutians is a representative area for the hypothetical "megathrust" of SPT. (By megathrust I mean an inter-plate thrust fault with many hundreds of kilometers of displacement that supposedly intersects the seafloor in a trench area.) Works that contributed to the construction of the model include reports by others on

Thus, the model of the evolution is based mainly on scientific descriptions by other investigators. However, the seed is a seismic refraction model of the crust-mantle interface beneath the forearc and ridge of the central Aleutians reported by me (Murdock, 1967, 1969). It argues two parallel linear structural troughs, one beneath and following the ridge crest, and another one between it and the trench (Fig. A2). They are separated by a thrust fault (the RTB thrust of Fig. A2) that dips steeply towards the volcanoes. Although the model was not constructed with Plate Tectonic concepts in mind, in the context of the plate model, the thrust fault would bound the continental plate. As such, the edge of the plate is about 100 km landward of the trench, not near the trench as is usually portrayed by advocates of the conventional model. (More details of the model are given in the Appendix, together with a brief description of the models of others who have conducted major seismic refraction and reflection experiments in the area.)

As also discussed further in the Appendix, I correlate the trough beneath the ridge of the central Aleutian model (Fig. A2) to the basin beneath Cook Inlet-Shelikof Strait (Fig. 2) of the eastern Aleutians. The basin is bounded on its oceanward side by a fault zone that dips steeply toward the volcanoes, similar to the ridge trough of the central Aleutians.

As the basin of Cook Inlet appears to correlate to the trough beneath the ridge of the central Aleutians, the area of Kodiak Island (and by inference, the Kenai Peninsula) of the eastern Aleutians (Fig. 1) appears to correlate to the linear trough area between the ridge and the trench of the model of the central Aleutians. Although in the central Aleutians this latter trough is a sedimentary basin, in the eastern Aleutians the layers of the proposed former trough have been greatly deformed and are now being uplifted, producing a mountain range on Kodiak Island and the Kenai Peninsula. How the great linear trough of the forearc might have formed and been transformed from a sedimentary basin into a mountain range is the primary issue this presentation addresses.

A sequence of 17 cartoons called "snapshots", each briefly justified, depicts the evolution of the general structures, sedimentary columns and landforms of the Kodiak Island-Cook Inlet area of the eastern Aleutians (Fig. 1), based on the fundamental assumptions and interpretations outlined above. The sequence is an elementary animation of the evolving features.

INDICATIONS OF DISPLACEMENTS OF THE PLATES THAT ARE MUCH SMALLER THAN THOSE OF STRICT PLATE TECTONICS

As mentioned above, I am arguing that the cumulative amount of convergence of the plates might be indicated by the structures of the arc, except in its earliest stages of development. In construction of the snapshots, is it realistic to consider displacements smaller than those of SPT? This is a critical issue.

Small Displacements Measured on the Transform Fault

The Fairweather transform fault that bounds the Pacific and North American plates in southeastern Alaska shows coseismic displacements (Plafker and coworkers, 1978), and the earthquake mechanisms of the fault are used to construct movements of the plate models (e.g., as shown by fig.40 of DeMets et al., 1990). One might expect the fault to show serial amounts of offsets increasing to an indeterminable amount, perhaps hundreds of kilometers (the plate model of DeMets and coworkers indicates 50 km/m.y.). However, Plafker and coworkers (1978) reported that they could show only 5-6 km of post-Miocene offset, and they postulated that most of the remaining amount of the SPT model (hundreds of kilometers) occurred on a fault zone offshore (sometimes called the "Transition Zone" fault) whose existence was not clearly demonstrated. (Here I urge a careful reading of the 1978 paper of Plafker and coworkers because later, Plafker, 1987 p. 260, postulated that the offsets are much younger, perhaps no older than the Sangamon stage of the Pleistocene, i.e., about 130,000 yrs.)

Small Convergence Indicated by the Absence of Incontrovertible Evidence of the Megathrust in the Aleutians

If the megathrust does not exist, the amount of convergence of the plates very likely must be compensated in the region landward of the trench. Production of the structures of this region will accommodate only a small fraction of amount of convergence of the plates proposed by SPT even during the latest Cenozoic. Hence, the megathrust, with underthrusting at the trench, is a critical concept of SPT. On the other hand, the non-existence of it is critical to my arguments.

To have a concept of how a subduction zone might appear, I looked at the report of an ancient subduction zone described by Connelly (1978) on Kodiak Island (Fig. 1). (It is sometimes referred to as a paleosubduction zone or fossil subduction zone.) Connelly depicted it dipping steeply toward the volcanoes with its outcrop being 5-10 or more kilometers wide. (As he correlates the complex to a Beniof zone at a trench, he proposes, p. 766, that it might have been rotated from near horizontal to its present steep inclination by underplating, a hypothetical process of SPT.) He (p. 766) reported it as a "chaotic assemblage" of rocks with tectonic inclusions ranging from millimeters to kilometers in size. He reported the rocks being intensely sheared, resulting in the loss of most stratal continuity. (Connelly stated that a similar zone, the McHugh Complex, has been reported on the Kenai Peninsula. He refers to the belt formed by the two as the Uyak-McHugh Complex, as shall I, although I am considering it only on Kodiak Island.) While Connelly did not display a scaled cross section, a general geologic section (von Huene et al., 1979) shows the oceanward part of the complex dipping 40-50 degrees toward the volcanoes. The complex is bounded on the on the northwest by the Border Ranges fault that dips more steeply toward the volcanoes. (When I refer to the Uyak-McHugh Complex, I mean to include this fault.)

Three sets of holes (DSDP 180-182) have been drilled in the trench region of the eastern Aleutians (von Huene et al., 1973) by the Deep Sea Drilling Project, see Fig. 3. Sites 181 and 182 were drilled to several hundred meters on the landward trench wall, with Site 182 being landward of Site 181 which was within about 10 km of the trench floor. Recovery at Site 182 was poor. Site 180 was drilled and cored to 470 m in the trench. This site was about 16 km seaward of the landward trench wall. The net result of observations from these three holes might be best summarized by a conclusion of von Huene and Kulm (1973, p. 976):

"If intense underthrusting is the principal tectonic mechanism under the continental shelf, there must be a high degree of decoupling across a relatively narrow zone, with very little transmission of compressional stress to the upper crust"
Nothing analogous to the broad and chaotically deformed Uyak-McHugh Complex was reported in the area of this experiment.

Furthermore, marine seismic reflections in the trench region of the eastern Aleutians also show no evidence of the disruptions similar to the paleosubduction zone, or indeed that one might expect of any major thrust fault zone. In analyzing a single-channel seismic reflection experiment conducted at the trench, von Huene and Kulm (1973), though favoring underthrusting, state (p. 974, 975):

"Folds and faults that indicate underthrusting will probably not be detected until better techniques are developed; even the better than average single-channel seismic records do not reveal them."
Later, using multichannel seismic recording and processing, Fisher and von Huene (1980) seem to have looked for the megathrust again in the same general area, but they did not report it. Indeed, they reported that reflectors can be traced from beneath the trench to 30 km inland. They cross the area of the megathrust without a major disruption, see the section with the investigators' comment. Subsequently, even by using the most advanced seismic data reduction techniques, Davis and von Huene (1987) could show only subtle features (they interpret as) supporting the megathrust in the trench region off of Kodiak Island.

Davis and von Huene appealed to high pore pressures to explain what they state are apparent extremely low compressive stresses. The model presented herewith is in accordance with their observation of indications of extremely low compressive stresses in the trench region, but it shows a more elementary cause of them.

The megathrust has not been incontrovertably identified in the central Aleutians either. There the trench fill ranges up to four kilometers thick, with 2-3 km being typical (Scholl et al., 1982, 1987). Nevertheless, this thick fill supposedly is either being underthrust and/or accreted without profound deformation of the layers of the fill at the base of the landward wall, e.g., see figure 21 of Scholl and coworkers (1987). Another stringent requirement is the source of the sediments. Because SPT requires rapid disposal of the sediments of the trench (either by the underthrusting or by the accretion), they must be very young (Scholl et al., 1982, speculated 0.5 Ma). Therefore, for a 2-4 km thickness, the sedimentation rate must have been very large. To address the source of these sediments, Scholl and coworkers (1982) proposed westward transport of them by turbidity currents from the eastern Aleutian trench, more than 1000 km away. (I have not seen a discussion of what effect, if any, prevailing ocean currents might have.) An alternate interpretation would be a nearby source for the sediments (the islands of the arc), with the trench fill being buried beneath the landward wall, not underthrust. This would suggest that the age of the fill is much older than required by SPT, perhaps as old as Miocene.

CHRONOLOGY OF THE MODEL THAT AGREES WITH OTHERS

Although the beginning and ending times of episodes of convergence for most of the Mesozoic must be speculative, for the Late Cretaceous and Cenozoic of the model, in my earlier studies of the forearc (Murdock, 1999a,b), I estimated their times. This was done primarily by studying reports of the structure, stratigraphy, and sedimentology of the forearc (oceanward of Kodiak Island) together with reports of the magmatic activity of the backarc. I reported three episodes of convergence. The first one began in the latest Cretaceous, with a precursor 20 m.y. earlier, and ended in the mid Paleocene. The next one began in the Late Eocene, possibly with a precursor in the mid Eocene, and lasted perhaps until the early part of the Miocene. The last one began in the Late Miocene and is ongoing today. The times of these episodes agree very well with the intervals of deformation proposed by Fisher, Detterman, and Magoon (1987) in their general description of the stratigraphy of Cook Inlet and the backarc. They also show three intervals. One began in the Late Cretaceous and ended in the early Paleocene. A tentative one began in the latest Eocene and ended in the earliest Miocene. The last one began in the mid Miocene and is ongoing today. Thus, the two semi-independent analyses agree remarkably well and suggest that the deformation across the arc has been roughly synchronous.

THE SNAPSHOTS OF THE EVOLUTION

The 17 snapshots are displayed in this section. Two of them are fundamental. They are today's structure of the central Aleutians (Snapshot 3) and today's structure of the eastern Aleutians (Snapshot 17). As stated above, I assume that the central Aleutians and the eastern Aleutians represent two different stages in the evolution. The first snapshot is an extrapolation back to the early Cenozoic from today's structure of the central Aleutians. From the reported history of the geology of the eastern Aleutians (e.g., Kirschner and Lyon, 1973), I propose that this snapshot represents the major structure of the eastern Aleutians during the latest Paleozoic and/or early Mesozoic (I have not attempted to show the evolution before this time. It might not be largely different from the SPT model). As mentioned previously, the last snapshot represents the eastern Aleutians today.

The 17 snapshots will be loaded and a thumbnail overview of the evolution will be displayed by clicking here (236 KB).

In the snapshots, the edge of the North American plate has remained fixed in space and the plates have converged to this surface, arbitrarily. As might have been observed from the thumbnail presentation, there is not a substantial difference in the displacements of the two plates of the model or an order of magnitude difference in the apparent deformation of the ridge and the forearc. Whereas this might be realistic for most of the Mesozoic and Cenozoic of the eastern Aleutians, the latter probably is not realistic for the episode of convergence that occurred at the boundary between the two eras. Here the Cretaceous exposure of the forearc has been described as intensely faulted, as discussed further in the next section.

Each full-sized snapshot (presented below) has an associated caption that gives a brief overview of the main features of the display. In addition, each caption contains one or more links to copies of geologic columns by others, with my annotations, usually also with supporting information that relates to the time of the snapshot. References are provided.

The times shown on the full-sized snapshots and discussed in the text correlate to the geologic time scale: Decade of North American Geology, 1983 Geologic Time Scale (Palmer, 1983). Some of the others, e.g. Fisher and coworkers, appear to have used an earlier version (van Eysinga, 1975) of the scale.

The snapshots might be best-viewed on the browser Internet Explorer Version 4 or later. The snapshots are best-viewed full screen at 800 x 600 monitor resolution, with a setting of more than 256 colors, if available.

Access to the individual full-sized snapshots and their captions will be achieved by clicking here .


DISCUSSION OF SOME IMPORTANT FEATURES NOT SHOWN ON THE SNAPSHOTS

Deformation of the Cretaceous Kodiak Formation of the Forearc

The Kodiak Formation (green color on the snapshots) of Kodiak Island is part of the Chugach terrane that extends for 2100 km around the Gulf of Alaska (Plafker, 1987; Plafker et al., 1994). It is described as 60-100 km wide, and it is commonly referred to as a slate belt. Cross sections of the eastern Aleutians (e.g. Plafker et al., 1982; von Huene et al., 1979; Sample and Moore, 1987) show it to be intensely faulted and folded. Sample and Moore (p. 7) stated "it consists of about 80% coherent landward-dipping thrust packets" on Kodiak Island. I have not attempted to show this type of deformation that reportedly occurs at intervals as small as 5-10 meters (Sample and Moore, p. 12). (Most of the short-wave length deformation on Kodiak Island occurred before solidification of the granodioritic or tonalitic plutons--batholiths, dikes, and sills--in the late Paleocene as the plutons reportedly are not deformed in the manner of the surrounding rocks.) Modeling the short-wave length deformation might increase the amount of convergence of the plates of the model at the Cretaceous-Tertiary (K-T) boundary by tens of kilometers.

Thickness of the Uyak-McHugh Complex

The illustrations show the Uyak-McHugh Complex of the eastern Aleutians as a single fault, when in reality the steeply dipping zone of tectonic melange crops out in a band ten or more kilometers wide on Kodiak Island and wider on the Kenai Peninsula. I regard the main fault of the zone to bound it on its oceanward side, i.e., the side where new melange was produced. This is the thrust fault shown by Connelly in his fig. 1, i.e., the Uganik thrust.

Long-Term Uplift of Kodiak Island of the Forearc

The illustrations do not adequately show the long-term uplift (long wavelength deformation) that is required to expose the Paleocene plutons and to form the post-Paleocene mountains of Kodiak Island. The mountains appear to have been produced by erosion of an uplifted and previously leveled surface (e.g., as discussed by von Huene and coworkers, 1987, p. 207, for the post-Miocene).

The Contact Fault and Ghost Rocks Formation of the Forearc

As stated in the caption of Snapshot 9, the Kodiak Formation is cut by the Contact fault. Although sometimes shown by advocates of SPT as a thrust fault, Sample and Moore (1987, p. 15, 16) described it as a strike slip fault with at least 15 km of right lateral displacement. It occurs about 10 km inland from, and parallel to, the southeastern shore of Kodiak Island. The Ghost Rocks Formation, apparently faulted into place, crops out between the fault and the ocean. Whereas the formation was mapped by Nilsen and Moore (1979) as Tertiary, it has been intruded by a pluton dated as 62-63 Ma (Armentrout in Moore et al., 1983, their table 2), suggesting that it could be as old as Cretaceous, the age of the Kodiak Formation. Therefore, because Nilsen and Moore (1979) reported that the clastic rocks of the Ghost Rocks Formation are similar to those of the Kodiak Formation, for the purposes of the model I provisionally regard the two as basically equivalent, and I have not included the Ghost Rocks Formation as a separate entity, although in contrast to the Kodiak Formation, it does include volcanic flows.

Sources of the Igneous Rocks of the Arc

I have not addressed the sources that produce the volcanoes and the igneous intrusives of the arc. Other than to note that the patterns of earthquake hypocenters suggest (caption of Ss 4) that the active RTB thrust might be a channel for the magmas of the volcanoes of the ridge trough, and to suggest that the volcanoes of the backarc are related to the giant batholith, I do not have a model for the sources. (Could the proposed backarc upwarp be a contributing factor in the production of the composite batholith--composed of rocks ranging from hornblende gabbro and diorite to granite, Miller, 1994--and its associated volcanoes?)

Perhaps noteworthy here, in the caption of Snapshot 4 I speculate that the RTB thrust would not be a conduit for magmas when it is locked, although it might be a conduit for the volcanoes of the ridge trough when it is unlocked, as stated above. The pattern of earthquake hypocenters (discussed in the Appendix) and the model suggest that the RTB thrust is locked eastward beginning between the Shumagin Islands and Kodiak Island. In the locked region, I speculate that the volcanoes are related entirely to the batholith of the backarc. Interestingly, Fournelle et al. (1994, pg. 730) have noted that the lavas and pyroclastics are dominantly basalts and basaltic andesites from Mount Veniaminof (120 km northeast of the Shumagins) westward, but to the east they are dominantly siliceous.

Transverse Folds Behind the Shelf Break of the Forearc

As shown by Snapshot 17, the folding that produces the shelf break structure of the eastern Aleutian forearc also produces a longitudinal trough landward of the shelf break. This trough is modified by transverse folds and uplifts to form two basins (Albatross and Stevenson), each about 150 km long parallel to the southeastern seaboard Kodiak Island. They themselves are modified by transverse folds, forming pairs of sub-basins 70-80 km long. Perhaps the transverse folds might be produced by differential displacements related to rough patches ("asperities") on the decollement (Murdock, 1999b). Perhaps they temporarily lock it but allow adjacent smooth areas to move. This concept is based on the asperities modeled by others (Johnson et al., 1996) beneath the region of the basins of the shelf.

An Exception, Pronounced Deformation at the Trench

Lewis and coworkers (1988) reported a seismic reflection experiment in the trench and forearc region off of the Shumagin Islands (Fig. 1). The area of the experiment begins about 300 km southwest of Kodiak Island and thus is not in the region covered by the snapshots. It extends about 500 km southwest to offshore Unimak Island. In some instances the reflection sections that the experimenters presented clearly display pronounced compression-related features at the foot of the landward wall (including the floor of the trench), and these features may extend landward for 20 or more kilometers (their fig. 9 perhaps is the clearest example). Lewis and coworkers interpreted the deformation as being produced by convergence of the plates. However, the topography of the seafloor suggests massive slides to me, albeit the inclination of the slope measured over 50 km is only several degrees, generally with one or two ramps of much steeper seaward inclination superimposed. On the other hand, the front of the blind thrust might crop out in the Shumagin trench region. In this situation, it could be mistaken for the megathrust.

SOME FUNDAMENTAL DIFFERENCES BETWEEN THE MODEL AND CONCEPTS PROPOSED BY OTHERS

Whereas the intervals of deformation proposed by advocates of SPT and the time span of episodes of convergence proposed by me may approximately agree, I am proposing that the episodes of convergence might be composed of several brief sub-episodes or pulses of convergence (Murdock, 1999b). Although this concept cannot be verified for earlier episodes of convergence, it is almost certainly valid for the post mid Miocene episode. It is important to note that these pulses show only of the order of a total of 10 km of convergence during the entire post mid Miocene episode. In contrast, based on the amounts of interpreted seafloor spreading, advocates of SPT appear to propose hundreds of kilometers of continuous convergence during this time.

From reports of the local structure, sedimentology and stratigraphy, especially from the report of Nilsen and Moore (1979), I argue that the rocks of the forearc were deposited and deformed in situ in the regional sense. In contrast, based primarily on paleomagnetic inclinations, advocates of SPT propose the concept of "terranes"--vast stretches of land, up to a hundred or more kilometers wide and a thousand or more kilometers in length--that have been transported up to thousands of kilometers and accreted to the Aleutians. Plafker (e.g., 1987) has described this concept. However, in coastal and Baja California, Dickinson and Butler (1998) have demonstrated that the geology appears to be a more reliable indicator of probable differential transport than the unadjusted paleomagnetic data. Although Fisher, Detterman, and Magoon (1987, p. 224) appear to accept the concept of terranes, they discussed a similar conflict with the local geology. For instance, two terranes, that (according to the paleomagnetic data) supposedly were separated by thousands of kilometers during the Paleocene, seem have been deformed synchronously during this time.

Fisher, Detterman and Magoon (1987, p. 224) proposed that the layers of the basin beneath lower Cook Inlet (Fig. 2) were warped into the basin-wide syncline during the latest Cretaceous. While I agree with their general analyses for the Cretaceous, I believe a precursor to the "latest Cretaceous" produced warping, perhaps even significant warping (Murdock, 1999a), near 84 Ma. Also, in contrast to the concepts of Fisher and coworkers noted above, the model I present speculates that episodes of production of the ridge trough might have occurred in the pre-Cretaceous of the Mesozoic as well.

Connelly (1978) stated that the age of the Uyak-McHugh Complex on Kodiak Island is uncertain and determining its age is "problematical". Nevertheless, by using the age of the youngest fossils (mid Cretaceous) in the rocks of the tectonic melange, together with his estimate of the age of regional tectonic activity, he suggested Late Cretaceous for the age of its "emplacement", seemingly by accretion at a trench (his fig. 2). As the ages of the (scarce) fossils reportedly range up to Permian, I believe that the age of the tectonic melange is undetermined but that its oldest elements are probably younger than the Permian fusulinids Connelly discusses. The model infers production of the melange beginning when the once continuous plate first ruptured, i.e., near the beginning of the Mesozoic. In addition, also in contrast to the concept of Connelly, the model infers production of the melange at the steeply dipping boundary of the continental plate, landward of the trench--not at the trench with subsequent upward rotation by underplating to explain its present steep inclination toward the volcanoes.

PROJECTED TECTONICS OF THE ARC

The Forearc

With additional convergence of the plates, the model suggests that the front of the decollement will travel oceanward, producing shorter wavelength deformation as the front travels beneath progressively thinner sediments. An example of this stage of evolution might be the Nankai Trough region offshore of Japan, where, in contrast to today's Aleutians (with exception of the Shumagins), clear evidence of compression is present in the landward trench wall. The decollement has been documented by drilling beneath the trench there (Moore, Karig, et al., 1991; Moore and Shipley, 1993). Moore and Shipley (p. 73) describe it as a 19-m thick zone of intense brecciation. (Further investigations are now, 2010, underway, the NanTroSEIZE Project.)  The deformation indicative of the decollement appears gradually to become non-existent in the direction of propagation, a few kilometers seaward of the drill hole of the illustration. I regard the decollement to be the equivalent of the front of the blind thrust of the Aleutian model.   

The Ridge Trough

I have tried to maintain the model of the trough as an isoclinal fold in accordance with the cross section of lower Cook Inlet of Figure 2. The model suggests that if isoclinal folding cannot be maintained, the trough will be destroyed. The cross section of upper Cook Inlet of Figure 2 might show an initial stage of the destruction. However, assuming the Great Valley of California corresponds to a ridge trough of the Aleutian model, its northward geographic projection, the Klamath Mountains, might be an example of a more advanced stage of the destruction.

The Backarc

The model speculates the entire continental plate of the backarc being upwarped during convergence of the plates. The model suggests that additional convergence with upwarp, if real, will eventually expose the lower crust/upper mantle adjacent to the composite batholith. Should this occur, I would expect a basaltic regime in the backarc. An example might be the Modoc Plateau of northern California, again assuming the Aleutian model for the Sierra Nevada region.

SUMMARY

Foundation of the Fundamental Interpretations

Throughout the paper, I have tried to give the reader an insight into the foundation of the interpretations. The associated table gives an overview how well-based I regard major features of the model.

Tectonics of the Model

Two different dynamic displays of the summary of the tectonics of the model are available. One is implemented with Java Script and is intended for Internet Explorer Version 4 or later users only. The letter keys of the keyboard are used to control the dynamic display of it. The second one is for other browsers that accept frames, and its dynamic display is controlled by the mouse, in the manner of the previous displays of the snapshots. If you are using Internet Explorer V. 4 or later and wish to exercise keyboard control, please close all other programs and click here. Otherwise, if you are using another browser that accepts frames, or wish to exercise mouse control, please click here.

Contrasts: Conventional Interpretations and Those of This Paper

The associated table gives an overview of the contrasting interpretations of virtually the same data set. The differences relate to the fundamental assumptions: As stated previously, advocates of SPT assume that the amount of convergence of the plates is dictated by a calculated amount of reported sea floor spreading, whereas I am assuming that the amount of convergence of the plates might be indicated by the geology of the arc.

IMPLICATIONS OF THE MODEL BEYOND THE ALEUTIANS

Two competing hypotheses provide mechanisms to compensate for sea-floor spreading: Strict Plate Tectonics, by far the dominant hypothesis, and Expansion of the Earth, with the leading advocate of the latter having been Carey (e.g., 1988). The model presented here suggests that elements of both hypotheses might apply, as is infrequently considered: Without doubt plates or something similar to them have converged, on the other hand, the amount of convergence seems to have been only a very small fraction of that advocated by SPT, especially in the post mid-Miocene. If the reported rates of sea-floor spreading are correct (e.g., as reviewed by DeMets et al. 1990) and operate long term, and if the model presented here is representative, some other mechanism must have compensated for the huge remainder. One candidate would be episodic expansion of the Earth. In this scenario, convergence of the plates would be essentially the sole compensator during only very brief intervals (during an orogeny?), and during the other times expansion of the Earth would be the primary compensator. An alternative would be that the reported long-term spreading rates are somehow in error, again, assuming the model applies widely.

Although perhaps only by chance, the most pronounced amount of convergence during the interval of the model seems to have occurred within a few million years of the K-T boundary, the time of the K-T Event. Also, the central Aleutian arc seems to have originated in the late Mesozoic-early Cenozoic, just as the eastern Aleutian arc might have originated in the late Paleozoic-early Mesozoic, which was another time of mass extinction of life, albeit the relationship of this latter extinction to an impact is not known, in contrast to the usually accepted cause of extinction at the K-T boundary.

Quantitative models of the compensation for sea-floor spreading and precise timing of the episodes of convergence in the Aleutians and elsewhere might lead to a better understanding of what drives the dynamics of the Earth, whether it is internal to the Earth, external to it, or both.

REFERENCES

APPENDIX: The Seismic Refraction Experiment and Development of the Fundamental Concepts

ACKNOWLEDGMENTS