Rigid Plate Tectonics Research Articles

Deciphering paleogeography from orogenic architecture: Constructing orogens in a future supercontinent as thought experiment

Orogens that form at convergent plate boundaries typically consist of accreted rock units that form an incomplete archive of subducted oceanic and continental lithosphere, as well as of deformed lithosphere of the former upper plate. Reading the construction of orogenic architecture forms the key to decipher the pre-orogenic paleogeographic distribution of oceans and continents, as well as bathymetric and topographic features that existed thereon such as igneous plateaus, seamounts, microcontinents, or magmatic arcs. Current classification schemes of orogens divide between settings associated with termination of subduction [continent-continent collision, continent-ocean collision (obduction)] and with ongoing subduction (accretionary orogenesis), alongside intraplate orogens. Perceived diagnostic features for such classifications, particularly of collisional orogenesis, hinge on dynamic interpretations linking downgoing plate paleogeography to upper plate deformation, plate motion changes, or magmatism. Here, we show, however, that Mesozoic-Cenozoic orogens that undergo collision almost all defy these proposed diagnostic features and behave as accretionary orogens instead. To reconstruct paleogeography of subducted and upper plates, we therefore propose an alternative approach to navigating through orogenic architecture: subducted plate units comprise nappes (or mélanges) with Ocean Plate Stratigraphy (OPS) and Continental Plate Stratigraphy (CPS) stripped from their now-subducted or otherwise underthrust lower crustal and mantle lithospheric underpinnings. Upper plate deformation and paleogeography respond to the competition between absolute motions of the upper plate and the subducting slab. Our navigation approach through orogenic architecture aims to avoid <i>a priori</i> dynamic interpretations that link downgoing plate paleogeography to deformation or magmatic responses in the upper plate, to provide an independent basis for geodynamic analysis. From our analysis we identify ‘rules of orogenesis9 that link the rules of rigid plate tectonics with the reality of plate deformation. We use these rules for a thought experiment, in which we predict orogenic architecture that will result from subducting the present-day Indian Ocean and colliding the Somali, Madagascar, and Indian margins using a published continental drift scenario for a future supercontinent as basis. We illustrate that our inferred rules (of thumb) generate orogenic architecture that is analogous to elements of modern orogens, unlocking the well-known modern geography as inspiration for developing testable hypotheses that aid interpreting paleogeography from orogens that formed since the birth of plate tectonics.

Lithosphere tearing at STEP faults: response to edges of subduction zones

Slab edges are a relatively common feature in plate tectonics. Two prominent examples are the northern end of the Tonga subduction zone and the southern end of the New Hebrides subduction zone. Near such horizontal terminations of subduction trenches, ongoing tearing of oceanic lithosphere is a geometric consequence. We refer to such kinks in the plate boundary as a Subduction-Transform Edge Propagator, or STEP. Other STEPs are the north and south ends of the Lesser Antilles trench, the north end of the South Sandwich trench, the south end of the Vrancea trench, and both ends of the Calabria trench. Volcanism near STEPs is distinct from typical arc volcanism. In some cases, slab edges appear to coincide with mantle plumes. Using 3D mechanical models, we establish that STEP faults are stable plate tectonic features in most circumstances. In the (probably rare) cases that the resistance to fault propagation is high, slab break-off will occur. Relative motion along the transform segment of the plate boundary often is non-uniform, and the STEP is not a transform plate boundary in the (rigid) plate tectonics sense of the phrase. STEP propagation may result in substantial deformation, rotation, topography and sedimentary basins, with a very specific time-space evolution. Surface velocities are substantially affected by nearby STEPs.

PLACA: a white box for plate reconstruction and best-fit pole determination

PLACA was developed as a general purpose computer program for the earth scientist that has to manipulate the rigid plate tectonics theory, from the representation of plate reconstructions to the determination of best-fit poles using magnetic anomalies, fracture zones or volcanic alignments. In the “forward” mode PLACA has the features of most available demonstration software plus the ability of simulating mid-oceanic ridges (MOR) as dynamic segments that move with a fraction of the relative motion between two plates. The movement can be illustrated by the generation of flow lines, plate trajectories and/or pseudo-magnetic anomalies on the ocean floor. The evolution of simple triple junctions of the R–R–R type can also be studied. In the “modifying” mode, best-fit poles can be computed either visually or by a systematic search routine. Five methods are available to evaluate the residuals between predicted and observed reconstructed points. Once the best-fit pole is obtained, several statistical tests can be done to evaluate the confidence on the fit. Surprisingly, the use of high-resolution geophysical data that are now available in the oceanic domain doesn’t always imply that the reconstruction poles are better constrained, as shown by an application example. Other examples presented document a discussion on the precision of plate reconstructions and suggest a few words of caution for the general user of PLACA.

Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

A plate tectonic model for the Cenozoic development of the region of SE Asia and the SW Pacific is presented and its implications are discussed. The model is accompanied by computer animations in a variety of formats, which can be viewed on most desktop computers. GPS measurements and present seismicity illustrate the high rates of motions and tectonic complexity of the region, but provide little help in long-term reconstruction. Plate boundaries shifted rapidly in the Cenozoic. During convergence of the major plates, there were numerous important episodes of extension, forming ocean basins and causing subsidence within continental regions, probably driven by subduction. Within eastern Indonesia, New Guinea and the Melanesian arcs, there are multiple Cenozoic sutures, with very short histories compared to most well-known older orogenic belts. They preserve a record of major changes in tectonics, including subduction polarity reversals, elimination of volcanic arcs, changing plate boundaries and extension within an overall contractional setting. Rapid tectonic changes have occurred within periods of less than 5 Ma. Many events would be overlooked or ignored in older orogenic belts, even when evidence is preserved, because high resolution dating is required to identify them, and the inference of almost simultaneous contraction and extension seems contradictory. There were three important periods in regional development: at about 45, 25 and 5 Ma. At these times, plate boundaries and motions changed, probably because of major collision events. The 45 Ma plate reorganisation may be related to India–Asia collision, although some important intra-Pacific events, such as voluminous Eocene boninite magmatism, seem to be older and require other causes. Indentation of Asia by India modified the Asian continent, but there is little indication that India has been the driving force of tectonics in most of SE Asia. The most important Cenozoic plate boundary reorganisation was at about 25 Ma. The New Guinea passive margin collided with the East Philippines–Halmahera–South Caroline Arc system. The Australian margin, in the Bird's Head region, also began to collide with the SE Asian margin in Sulawesi. The Ontong Java Plateau collided with the Melanesian Arc. These collisions caused a major change in the character of plate boundaries between about 25 and 20 Ma. Since 25 Ma, tectonic events east of Eurasia were driven by motion of the Pacific Plate. Further, west, the movement of Australia northwards caused rotations of blocks and accretion of microcontinental fragments to SE Asia. Plate motions and boundaries changed again at about 5 Ma, for uncertain reasons, possibly as a consequence of Pacific Plate motion changes, arc–continent collision in Taiwan, or other boundary changes at the Pacific margin, for example in the Philippines. Areas to the west and east of New Guinea, the Banda Sea and Woodlark Basin, illustrate the speed of change, the unexpected interplay of convergence and extension, and the importance of subduction as the engine of change. Subduction has been the principal driving mechanism for tectonic change, although its manifestations are varied. They include collision-related phenomena, partitioning of oblique convergence, and effects of hinge roll-back and pull forces of subducting slabs. Magmatism is not always associated with subduction, depending on the movement of subduction hinge, and there may be important extension of the upper plate both perpendicular and parallel to the length of subduction zones. Strike-slip faulting is observably very important within the Pacific–Australia–Eurasia convergent setting, yet appears in few tectonic models. Long-term strike-slip deformation can explain some of the complexities of areas such as New Guinea, including magmatism and its absence, and thermo-chronological data showing very young and rapid cooling of the mobile belt and fold belt. The inadequacies of the tectonic model reflect in part the difficulties of applying rigid plate tectonics, when there is clear evidence of changing shapes of fragments. Geological knowledge of the region is still inadequate and significant improvements to regional data sets, such as palaeomagnetic data and isotopic ages, are required. New tomographic techniques offer an important means of testing this and other reconstructions. However, valuable insights could also be obtained from simple data sets, such as sediment volumes, if more information that is complete were available in the public domain. Two-dimensional plate tectonic cartoons of small areas are no longer adequate descriptions or tools for understanding. It is essential to test plate tectonic models by using animation techniques with reconstructions drawn at short time intervals, which expose flaws in models, show major gaps in knowledge and help identify truly regional events. Observations of present-day tectonics, and all geological evidence, indicate that the model presented here is over-simplified. Improvements in this, or new models, will inevitably be more complex than the reconstructions described here.

Magnetic and tectonic studies of the dueling propagating spreading centers at 20°40′S on the East Pacific Rise: Evidence for crustal rotations

We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20°40′S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/V Moana Wave during which continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly‐2‐aged crust. A three‐dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The results indicate consistent asymmetric spreading faster to the east, discontinuous high magnetizations in the discordant zone associated with the discontinuity, and southward migration of the feature at a rate of 90–100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre‐Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges.

Constraints on rift propagation history at the cobb offset, juan de Fuca Ridge, from numerical modeling of tectonic fabric

Rift propagation has been proposed as the mechanism for the reorganization of the Juan de Fuca Ridge during the Cenozoic. In particular, magnetic anomalies in the Cobb Offset region of this ridge have been interpreted in terms of a complex “dueling propagator” history, in which rift propagation has occurred alternately in opposing directions on adjacent ridge segments. Although the overall propagation history has been inferred from the magnetic data, these data are limited in their resolution for the detailed history by the length of time during which magnetic polarity remains constant. We thus examine whether more detailed resolution within polarity chrons can be obtained using seafloor tectonic fabric from acoustic imagery. By modeling lineations as isochrons, synthetic fabric can be generated for a given rift propagation history and the fit of the model to the data can be compared numerically. For the Cobb, we used a propagation history model with a given timing and direction of propagation, and generated synthetic fabric for different propagation rates. The fabric is best fit for propagation rates between 400 and 900 mm/yr. Slower ( < 300 mm/yr) propagation rates yield significantly poorer fits, whereas faster ( > 900 mm/yr) propagation rates yield slightly poorer fits. The fabric results are thus in accord with the interpretation, based on magnetics alone, that the rates of rift propagation significantly exceeded the spreading rates. An interesting feature of the model is that the complex tectonic fabric is reasonably well fit on this scale ( ~ 1 km) by simple rift propagation models which do not invoke shear zone behavior and thus deviate from rigid plate tectonics.

Formation of curved seafloor fabric by changes in rift propagation velocity and spreading rate: Application to the 95.5°W Galapagos propagator

The geometry of oceanic spreading centers can change by rift propagation, in which a new ridge segment propagates and preempts a preexisting one. As the locus of relative motion between the two plates shifts, the dying and growing ridge segments are joined by a boundary, usually a transform, which migrates with time, causing seafloor to be transferred from one plate to the other. As a result, originally ridge‐parallel isochrons in a zone between the growing and failed rifts can be reoriented to trends oblique to both the ridge and spreading direction. An attractive feature of this model is that the reorientation occurs by strictly rigid plate tectonics, since relative motion occurs only between points on different plates. Recent surveys of the Galapagos propagating rift system at 95° W show seafloor fabric curving away from the growing rift and toward the failed rift. These observations have been interpreted as requiring a zone of distributed simple shear between the two rifts. Such behavior would represent a local deviation from rigid plate tectonics, since relative motion would be distributed over a finite region not part of either of the two plates. Here we explore possible rigid plate models for the formation of curved seafloor lineaments by rift propagation. We find that such fabric can be generated by changes either in the velocity (rate or direction) of rift propagation, or in the spreading rate during rift propagation. The curvature of various tectonic elements is diagnostic of the different possibilities. The geometry observed at the Galapagos can result from either rift propagation acceleration, or a spreading rate decrease, during the last few hundred thousand years. The reverse curvature could result from either deceleration of rift propagation or an increase in spreading rate. Propagator acceleration and spreading rate slowing, which produce similar structural trends, could in principle be distinguished using spreading rate data. At Galapagos, however, since either would have occurred within the present period of normal polarity, both are consistent with the magnetic anomaly data. We thus conclude that the data interpreted as requiring a shear zone are equally consistent with two distinct models based on rigid plate tectonics. It may, however, be possible to discriminate between rigid and shear models using fabric observations at other propagators. The shear model requires that the sense of curvature at Galapagos be a ubiquitous feature of active propagators. In contrast, the rigid plate tectonic model allows for fabric with a variety of orientations, including reverse curvature formed by either propagation deceleration or increasing spreading rate and linear fabric formed by constant propagation and spreading velocities.

Microplate and shear zone models for oceanic spreading center reorganizations

The reorganization of oceanic spreading centers is often accomplished through the process of rift propagation. We first examine general rift propagation geometries for the simple two‐plate case in which the propagating rift instantaneously reaches full spreading rate while the other rift dies instantaneously, such that the propagating and dying rifts never overlap. A velocity space representation provides a framework for describing the geometry of rift propagation and its various tectonic elements, and shows how both the failed rift and isochrons between the two rifts are reoriented as a consequence of purely rigid plate tectonics. We then discuss the more complex geometries that result when both growing and dying rifts spread simultaneously during propagation, such that a technically independent overlap region, like the Easter or Juan Fernandez plates along the East Pacific Rise, is produced. We examine the effect of the propagation rate and rise time of spreading on the evolution of overlap regions between dual oceanic spreading centers. The tectonics and evolution of such regions can be modeled by assuming that they act either as rigid microplates or as zones of distributed shear. Rigid plate models are suggested by the reasonable success of Euler vectors derived from inversions of plate motion data (spreading rates, transform azimuths, and slip vectors) both in describing available data and in predicting subsequently acquired data. On the other hand, recent structural data showing fabric oblique to the surrounding spreading centers might suggest shearing, rather than rigid behavior, in the interior of the Easter microplate. We use schematic evolutionary simulations for an Easter‐style geometry to examine whether plate motion data and orientations of preexisting and present structures can be used to discriminate between rigid and shear models. In a microplate model, rigid plate tectonics requires that relative motion occur only along the boundaries of the overlap region, while structures within the overlap zone rotate rigidly. In contrast, in a shear model, relative motions at the boundaries differ from those predicted by rigid plate tectonics, and the angular relationships of structures within the overlap region are altered by internal deformation. Thus inversion of relative motion data yields different results for rigid and shear behavior. Similarly, under favorable circumstances, structural trends could be diagnostic of the different cases. We find that much of the Easter structural data suggestive of shearing are also consistent with rigid plate tectonics. We conclude that microplate models offer useful testable hypotheses for describing overlap systems and that shearing need be invoked only to explain features not describable by rigid plate tectonics. Furthermore, the approaches used here to study current overlap regions should be useful in the analysis of similar regions formed during past oceanic plate boundary reorganizations.

Statistical tests for closure of plate motion circuits

The standard method of testing whether data describing the relative motion of three plates meeting at a triple junction are consistent with rigid plate tectonics uses the condition of plate circuit closure: the Euler vectors or angular velocities describing the relative motion of each plate pair must sum to zero. Deviations from closure suggest that the data are systematically in error, or that the plate boundaries have been identified incorrectly, or that the plates are non‐rigid. We examine two methods, one based on a chi‐square test and the other based on an F‐ratio test, of testing whether plate motion circuits close. We find the latter test is useful, but the former is of limited value because of the common practice of systematically overestimating errors in plate motion data. We apply the F‐ratio test to Minster and Jordan's data for the boundaries dividing the Pacific, Cocos, and Nazca plates, which prove consistent with plate circuit closure. Application of the test to their data for the boundaries dividing the Australian, Antarctic, and African plates shows nonclosure even after deletion of data from the Carlsberg Ridge, which recently has been suggested to divide the African plate from an Indo‐Arabian plate. This nonclosure suggests systematic errors in the data or internal deformation of one or more of the plates meeting at the Indian Ocean Triple Junction.

Tectonics of the Easter plate

We present a new model for the Easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges. We use the distribution of earthquakes, eleven new focal mechanisms, and existing bathymetric and magnetic data to describe the tectonics of this area. Both the Easter-Nazca and Easter-Pacific Euler poles are sufficiently close to the Easter plate to cause rapid changes in rates and directions of motion along the boundaries. The east and west boundaries are propagating and dying ridges; the southwest boundary is a slow-spreading ridge and the northern boundary is a complex zone of convergent and transform motion. The Easter plate may reflect the tectonics of rift propagation on a large scale, where rigid plate tectonics requires boundary reorientation. We use simple schematic models to illustrate the general features and processes which occur at plates resulting from large-scale rift propagation.

Continental rifting and the implications for plate tectonic reconstructions

Previous plate tectonic reconstructions have tried to recreate the pre‐rifting (Pangea) configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries. Such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting, that the continental boundaries are isochrons, and that the continents rift without distortion. This paper proposes a model for continental breakup with distortion, where ocean basins are formed by propagating rifts. This model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents. As rifts propagate through a continent, the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading. The propagating rift model implies that large amounts of continental extension (up to 150 km) occur and that the continental boundaries are not isochrons. This extension due to rifting results in ‘apparent’ overlap when the continents are returned to their original configuration. Because of the gradational nature of this extension, the best representation of the pre‐rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation. The overlap is equivalent to the amount of extension that has taken place on both continental edges. Reconstructions based on this model are presented for the Gulf of California, the Gulf of Aden, the Norwegian‐Greenland Sea, and the South Atlantic Ocean. They are able to resolve the discrepancies that occur when rigid plate tectonics are applied. This model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons.

Plate kinematics of the Denali fault system

Two segments of the Denali fault system, the McKinley strand west of the Delta River and the Dalton–Shakwak fault east of the Delta River, have nearly perfect small circle geometries. This geometry permits interpretation of the right-lateral slip along these faults in terms of rigid plate tectonics. Their poles of rotation are in the Gulf of Alaska at 59.63°N, 147.38°W and 50.38°N, 154.02°W respectively. A model in which there has been simultaneous motion on both faults since 38 Ma ago predicts a third fault at their juncture which must act as a plate boundary with northwesterly thrust motion in this time interval. The Broxson Gulch thrust, which extends from near the Susitna River to its termination at the Delta River, meets these requirements. Paleozoic and Mesozoic volcanics, as well as Oligocene or younger strata, are thrust beneath sillimanite schists along this fault, and major pre-Tertiary fold structures are truncated by it. Given the direction of tectonic transport on all three faults and a displacement of 38 km on the McKinley strand, the minimum displacements on the Broxson Gulch and the Denali (Dalton–Shakwak) faults in the last 38 Ma are approximately 54 and 90 km respectively. The previously correlated Maclaren and Ruby Range metamorphic belts, however, indicate 300–400 km offset since about 55 Ma ago. Our results require that about 300 km of this be taken up west of the Maclaren belt, either on the McKinley strand or on thrust segments similar to the Broxson Gulch, or both. Our results further indicate that the arcuate shape of these segments of the Denali fault system are intrinsic properties of the faults themselves and that oroclinal bending need not be invoked to explain them.

Gorda plate motions from magnetic anomaly analysis

Significant variations of spreading rates and directions in space and time along the Gorda ridge can be determined by analysis of magnetic anomalies. Converted to motion vectors, they suggest the existence of at least two and possibly three Gorda plate segments. Assuming rigid plate tectonics, present motion between the northern and two southern segments should be NW-SE right-lateral at around 3 cm/yr and any fault between them should intersect the ridge near 42°N. A linear bathymetric trough may represent at least part of such a proposed fault zone, named here the Gorda fault zone. At the continental margin, the northern Gorda plate segment has probably continued to converge obliquely for at least 5 m.y. However, the southern Gorda plate segments may have recently ceased to subduct and now be moving almost parallel to the slope. This seems to be in accord with seismological and geological evidence near and north of Cape Mendocino. In the future, rapidly decreasing spreading on the southern Gorda ridge may effectively lock the southern Gorda plate segment to the Pacific plate so that the present Cape Mendocino triple junction will transfer northwards to a point on the margin near 41°30′N.

Propagating rifts on midocean ridges

Spreading center jumps identified west of the Galapagos Islands near 95°W occur in a pattern consistent with the propagating rift hypothesis. A new rift is gradually breaking through the Cocos plate. Each successive jump is slightly longer than the preceding jump. The new spreading center grows at a new azimuth toward the west as the old one dies. The jumps are a manifestation of rift propagation. We extend the analysis of propagating rifts to the case of continuous propagation and predict patterns of magnetic anomalies and bathymetry consistent with the observed patterns. In particular, we correctly predict the trends of fossil spreading centers and V patterns of magnetic anomaly offsets required by the propagating rift hypothesis. Similar V patterns have been observed on many other spreading centers and have been interpreted in various ways. The propagating rift hypothesis appears to offer a simple explanation, consistent with rigid plate tectonics, for each of these patterns. This hypothesis may also have important implications for continental rifting.

North American-Eurasian Plate Boundary in northeast Asia

The intracontinental portion of the boundary between the North American and Eurasian plates can be identified ort the basis of seismicity, recent tectonics, and earthquake focal mechanisms. The simplest plate geometry that can explain these data involves a North American-Eurasian boundary that extends from the Nansen ridge through a broad zone of deformation in northeast Asia to the Sea of Okhotsk and thence southward through Sakhalin and Hokkaido to a triple junction in the Kuril-Japan trench. Such a configuration can account quantitatively for the slip vectors derived from earthquake mechanisms in Sakhalin and Hokkaido. On the basis of new slip vector data the North American-Eurasian angular velocity vector is revised only slightly from previous determinations. The intracontinental plate boundary is diffuse and may be controlled by ancient plate sutures. Deformation within about 10° of the rotation pole, which lies very near the boundary, cannot be modeled by rigid plate tectonics. These characteristics of intracontinental plate boundaries are related to the greater thickness and heterogeneity of continental lithosphere and to the influence of continents on the plate tectonic driving forces.

Heat flow and elevation of the marginal basins of the western Pacific

High heat flow and shallow crustal depths have been observed in many marginal basins landward of the trench-island arc system in the western Pacific. All the basins in the north-western Pacific where adequate heat-flow data exist, except the deep west Philippine basin, show a high mean heat flow. This is in marked contrast to the southwestern Pacific, where only the Fiji plateau and Lau basin have high heat flow. All active midocean ridges show a uniform relationship between heat flow and elevation: high and shallow where the ridge is young, and uniform and deep where the oceanic crust is old. In the absence of magnetic data in the marginal basins, this relationship has been used to determine whether or not these basins have the same thermal and dynamic structure as the midocean ridges. The heat flow and elevation data from the southwestern Pacific basins are compatible with the ridge data, whereas heat flow and elevation data from the Japan abyssal plain, Parece Vela basin, and the Shikoku basin in the northwestern Pacific have too high a heat flow for their mean depth. Convincing evidence has been presented that the marginal basins of the northwestern Pacific owe their origin and development to large-scale crustal extension landward of the island arc system. However, the intrusion of oceanic crust associated with this extension cannot alone adequately account for the heat flow and elevation data. Three other factors are discussed that, in addition to the intrusion process, may account for the observed distribution of heat flow and crustal depth: (1) continuous incipient intrusion, which raises the ‘effective’ thermal conductivity of the crust and hence the heat flow in these basins; (2) development of a thinner lithospheric plate landward of the island arc system of the northwestern Pacific; and (3) dynamic forces acting on the upper mantle landward of the downgoing slab, decreasing the observed elevation of the marginal basins. There are problems with all three of these factors, but a model using only the second is preferred, because it is simple to develop and is compatible with rigid plate tectonics.

Heat flow, bathymetry, and the Mid-Atlantic Ridge at 43°N

A detailed profile of heat-flow values across the mid-Atlantic ridge near 43°N is typical of mid-oceanic ridge areas, except for low values (about 1.0 μcal/cm2 sec, or 42 mw/m2) 100–300 km east of the ridge crest. The mid-Atlantic ridge south of 43°N has a heat-flow distribution and bathymetric profile consistent with the hypothesis of rigid-plate tectonics with a spreading rate of less than 1.0 cm/yr for the last 40 m.y. and in conflict with the spreading-rate history deduced from magnetic anomalies. The ridge north of 43°N is abnormally flat, is unusually elevated in a region of low heat flow to the west, and has an abnormally low axial magnetic anomaly. These features cannot be explained by a simple model of rigid-plate motion.

Global Distribution of Eulerian Poles

THE root of present day rigid plate tectonics is Euler's fixed axis theorem, whereby the displacement of any fraction of the total surface of a sphere which does not carry every point on it into itself is a rigid rotation about an axis which passes through the centre of the sphere (Eulerian axis), and pierces the surface of the sphere in two fixed antipodal points (Eulerian poles).

Tectonics of the Mendocino Triple Junction

Interpretation of reflection profiles and of the magnetic anomaly pattern over the Gorda Basin and Escarpment gives broad agreement with the triple junction model of McKenzie and Morgan (1969). However, the basin has undergone internal deformation, a local departure from rigid plate tectonics, and the escarpment has had a component of underthrusting by the Gorda block. Faults in the Gorda Basin which disturb young turbidites parallel the trends of magnetic anomalies, suggesting deformation of the oceanic crust along lines of primary weakness. The northeast trends of the faults give a constraint on first-motion solutions for earthquakes within the basin and suggest left-lateral slip on the faults. Analysis of the geometry and timing of the Gorda Basin deformation based on the magnetic pattern gives an average gross tectonic strain rate of 10 −14 /sec. These observations give a measure of the mechanics of deformation of oceanic lithosphere very close to a spreading rise crest.

Regional and Global Fault Slip Rates from Seismicity

Rates of fault slip calculated from the seismicity of the world's chief seismic regions agree well with the predictions of “rigid plate tectonics”. Global fault activity seems to have decreased fivefold in the past seventy years, indicating that earthquakes are coupled on a global scale within decades. This article also discusses the possible recurrence of a period of high activity.