Overthrust Plate Research Articles

On the Forces Driving Plate Tectonics: Inferences from Absolute Plate Velocities and Intraplate Stress

Summary Absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries. Plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere. Intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress. The absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar. While the global pattern of absolute velocity is probably close to those predicted by such models, the absolute motions do not therefore provide a strong test of the driving mechanism. Comparison of predicted intraplate stress with the long-wavelength features of the global stress field, however, as determined by in situ measurements, earthquake mechanisms, and stressinduced geological structures, does prove to be a powerful test of possible driving forces. All absolute velocity models have several interesting properties. Lithosphere in the equatorial half of the Earth is moving significantly faster than lithosphere in the polar half. Some connection to the Earth's rotation is implied since this statement is demonstrably untrue for co-ordinate poles much different from the geographic pole. Subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate, confounding attempts to explain the dip angles of Benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate. Ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere; such rapid migration may be a necessary but is not a sufficient condition for ridge jumps. Force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere. From the rms absolute velocities of individual plates, there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere. The predicted intraplate deviatoric stress cannot match the well-determined stress fields in North America and Europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system. The mid-plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities.

Aspects of Southern Canadia cordilleran thermal history from fussion track and coal rank data in the lewis and other overthrust plates : Kirk G. Osadetz, Insttiture of Sedimentary and Petroleum Geology, 3303 33rd St. N.W., calgary, alberta T2L 2A7, CANADA; Barry P. Kohn and Shimon Feinstein, Department of Geology and Mineralogy, Ben Gurion University of the Negev, P.O. Box 653,

Aspects of Southern Canadia cordilleran thermal history from fussion track and coal rank data in the lewis and other overthrust plates : Kirk G. Osadetz, Insttiture of Sedimentary and Petroleum Geology, 3303 33rd St. N.W., calgary, alberta T2L 2A7, CANADA; Barry P. Kohn and Shimon Feinstein, Department of Geology and Mineralogy, Ben Gurion University of the Negev, P.O. Box 653,

Tectonics and heat sources for granulite metamorphism of supracrustal-bearing terranes

There are generally three important processes in the granulite-facies metamorphism of supracrustal rocks: (1) transport from the surface to 15–30+ km, (2) heating to 700–800+ °C, and (3) transport back to the surface and re-exposure at the top of a crust of “normal” thickness (35–40 km). Mantle-derived magmatism, and/or magmatic and/or metasomatic fluid flow, can transport the necessary heat for granulite metamorphism, but many granulite terranes do not contain sufficient volumes of mafic intrusive rocks, or evidence of extensive fluid flow, to readily explain heating by this mechanism. One possible process for the formation of large granulite terranes involves continental collision and crustal doubling by simple-shear thrusting. Heating of the underthrust supracrustal rocks would occur by thermal relaxation, but, if granulite-facies temperatures are to be attained at mid-crustal levels, this mechanism necessarily produces extensive melting of the lower crustal plate, suggesting that granulite terranes formed in this way should contain substantial syn- or post-metamorphic crustally-derived intrusions. In this situation, subsequent uplift and re-exposure at the surface is a natural consequence of erosion and isostatic readjustment. An alternative process which may allow the heating of supracrustal rocks to granulite-facies temperatures is imbricate thrusting in which thin slivers or “flakes” of supracrustal rocks are thrust down to the lowermost crust. This mechanism requires a separate tectonic event, such as a second thrusting event, to transport the granulite-facies rocks back to the surface. Other possible heat sources for supracrustal granulite terranes include preheating of the overthrust crust by arc or pre-orogenic magmatism, and shear heating on thrust planes. Models indicate that pre-heating of the overthrust plate has only minor thermal effects upon the thermal history of the underthrust crust, although shear heating may be significant. Heat sources for granulite metamorphism of the Archean Limpopo Belt of South Africa are considered in terms of possible tectonic mechanisms. The metamorphic grade and presence of abundant supracrustal rocks in this terrane demonstrate that it was once at the Earth's surface, buried to ∼ 8.5 kbar (∼ 28 km) and heated to ∼ 850°C, then re-exposed at the surface. There are no recognizable mantle-derived igneous rocks of suitable age, and no evidence for extensive fluid flow that could have provided a magmatic heat source for the metamorphism. Shear heating is unlikely because the pattern of metamorphic isograds is not related to faults or shear zones. Imbricate thrusting of thin silvers of supracrustal rocks into the lowermost crust to allow heating without magmatism would require a separate tectonic event to transport the granulites to the surface; there is no evidence for such a later event. The most satisfactory and self-consistent model for Limpopo metamorphism involves thermal relaxation of a perturbed temperature profile produced by Tibetan-style continental collision of the Zimbabwe and Kaapvaal Cratons, with consequent crustal doubling to at least 65 km thick, and the generation of crustal melts which advected heat into the zone of granulite metamorphism. There is no evidence for significant pre-collisional heating of either crustal plate; both probably had typical “shield-type” pre-collisional geothermal gradients. Crustally-derived magmas, therefore, must have been produced during thermal and structural equilibration of the Limpopo collision. Abundant granitoid rocks of similar age to the metamorphism occur both as migmatites and as discrete plutons.

Aspects of Cordilleran Geothermal History from Fission-Track Analysis in the Lewis and Coleman Overthrust Plates, Southwestern Alberta and Southeastern British Columbia

Apatite fission-track data from pre-Tertiary rocks in the southern Canadian Front Range and Foothills are younger than their stratigraphic ages, indicating thermal setting. Limited confined track-length distribution data indicate rapid cooling from the total annealing zone at about the age recorded. There appears to be a consistent difference between apatite ages of the Lewis plate (64-80 Ma) and the underlying Coleman plate (46-59 Ma). Zircon single-grain fission-track age distributions in both overthrust plates show a long tail of scattered ages but with a distinctive signal, indicating thermal resetting during the Cretaceous or Tertiary. The age of the thermal zircon fission-track resetting signal overlaps or is only slightly older than corrected apatite fission-track ages, in their respective thrust plates. Between 70 and 80 Ma, Lewis plate samples experienced temperatures higher than those required for total apatite annealing and were subsequently cooled vary rapidly through apatite fission-track annealing to apatite fission-track stability temperatures. The Coleman plate data suggest an analogous thermal history but occurring 15-20 Ma later, during the early Tertiary. Oligocene sediments in the Flathead graben have apatite fission-track ages and track lengths like that of the Lewis plate, the known source of these sediments, indicating no significant postorogenic thermal events.

Drumduan Group of East Nelson, New Zealand: Plant-bearing Jurassic arc rocks metamorphosed during terrane interaction

Abstract The Drumduan Group, consisting of the Marybank and Botanical Hill formations, forms a fault-bounded block northeast of Nelson City, and comprises bedded pyroclastic and cpiclastic (including sedimentary) debris of probable terrestrial deposition. Lawsonitc is widespread, but not uniformly present. Plant macroibssils, including Mataia podocarpoides, Rissikia talbragarensis, Taenioptens thomsoniana, T. sp. cf. T. crassinervis, Cladophlehis sp. and Podozamites sp., in the Marybank Formation indicate a Jurassic age. The group is separated from the Brook Street Volcanics (Permian) by the Delaware Fault Zone, along which arc slivers of Wakapuaka Phyllonite (pumpcllyitc-actinolite facies). In the west the group shows widespread hydrothermal alteration related to the adjacent Tasman Intrusives, but the Brook Street rocks are not similarly altered and are not metamorphosed beyond prehnite-pumpellyite facies. We infer that during the Rangitata Orogeny the Drumduan Group, the Tasman Intrusives, and the underlying basement rocks, including the Pepin Group, were underthrust below the Brook Street Volcanics along the Delaware Fault Zone. The resulting depth of burial and confining nature of the overthrust plate on one side, and the Tasman Intrusives on the other, were responsible for the development of metamorphic rocks characterised by a relatively high P/T ratio and by the crystallisation of lawsonitc, locally in abundance. Fluid pressures were probably nowhere greater than lithostatic pressure. The Delaware Fault Zone appears to form a major suture between the rocks of east and west Nelson. The Drumduan Group, the Tasman Intrusives, and the Pepin Group are assigned to the Tuhua Orogen.

Fluids and thrusting

Recent studies of the structure of the continental crust by COCORP and BIRPS have shown that major thrust structures in the lower crust are ubiquitous. Plate-tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs (the Himalayas or the Andes) involve thrusting of continental materials or ocean crust beneath continents, and local overthrusting may accompany transcurrent motion. In all such cases massive fluid flow must occur. As first stressed by M.K. Hubbert and W.W. Rubey, thrusting mechanisms require that fluid pressures are close to, or greater than, lithostatic pressures. When thrusting occurs, compaction of the underplate must occur rapidly, expelling pore water. Crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base. If the overthrust plate is thick and hot at the base, retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients. In such a region veins need not occur but leaching phenomena may dominate. If an underthrust plate is hydrated, and/or contains sedimentary aquifers, large fluid volumes are expelled through shear zones into the overthrust plate. The scale of fluid-release processes can be large. Thus, in thin-skinned tectonics where the overthrust plate is 10–15 km thick, induced fluid release can easily reach 4 · 10 9 g m −2 of thrust surface. The flow of such fluids will be controlled by the thrust surface and lithology, both of which will influence hydraulic fracture mechanisms and spacing of fractures. The chemistry of thrust-derived fluids may be highly variable depending on lithology and the time constants of thrusting. Vertical thermal and redox environments will be similarly dependent. In the subduction process, impressive quantities of fluids must pass back up the thrust surface, and such fluids have recently been directly observed during drilling. Dewatering of spilites, serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and, at great depth, flux mantle melting. Extreme metasomatism in blueschist belts must result from such fluids. A case of particular interest involves the cessation of subduction when a high-level slab reaches thermal equilibrium. In general, flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high T- P with locally derived fluids at low water/rock ratios during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution, and at shallower crustal levels, there may be incursion of oxidizing near-surface fluid reservoirs into the faults. These fluids may have extremely low δ 18O-values, where mountain ranges form on rebound faults, and high-altitude depleted fluids penetrate down the rebound structure. Extensive mineralization may be associated with such thrust-derived fluids. Examples which will be discussed include the Mother Lode Au deposits of California, U.S.A., and the large U deposits of Lagoa Real, Brazil.

True polar wander and plate‐driving forces

A net torque on the global lithosphere can be exerted both by “ridge push” and “trench pull” forces. A net ridge push torque is present if there is an asymmetry in the age distribution of seafloor about one or more ocean ridges. A net torque due to trench forces is present if the forces per trench length acting on the subducting and overthrust plates are not equal in magnitude. The combined net torque on the lithosphere contributed by ridge and trench forces must be balanced by an opposing torque, most likely due to basal shear tractions associated with a global rotation of the lithosphere relative to the underlying mantle. Such a rotation should be identifiable as true polar wander. The lack of significant rotation of the lithosphere with respect to the magnetic dipole axis during the Cenozoic, therefore, places a strong constraint on the nature of torques acting on the lithosphere. We suggest that the most likely explanation for negligible true polar wander in the Cenozoic is that the net ridge and trench torques nearly cancel one another. The net ridge and trench torque vectors are in nearly opposite directions and have been so through the Cenozoic, so the condition of near cancellation amounts to a restriction on the magnitude of the force imbalance along trench boundaries. Such a near cancellation of net torques and the resulting negligible true polar wander are likely to be consequences of present plate geometries and need not be representative of earlier eras.

Subduction of aseismic ridges beneath the Caribbean Plate: Implications for the tectonics and seismic potential of the northeastern Caribbean

Normal seafloor entering the Puerto Rico and northern Lesser Antillean trenches in the northeastern Caribbean is interrupted by a series of aseismic ridges on the North and South American plates. These topographic features lie close to the expected trend of fracture zones created about 80–110 m.y. ago when this seafloor was formed at the Mid‐Atlantic Ridge. The northernmost of the ridges that interact with the Lesser Antillean subduction zone, the Barracuda Ridge, intersects the arc in a region of high seismic activity. Some of this seismicity including a large shock in 1974, occurs within the overthrust plate and may be related to the deformation of the Caribbean plate as it overrides the ridge. A large bathymetric high, the Main Ridge, is oriented obliquely to the Puerto Rico trench and intersects the subduction zone north of the Virgin Islands in another cluster of seismic activity along the inner wall of the trench. Data from a seismic network in the northeastern Caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity. Magnetic anomalies, bathymetric trends, and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the Main and Barracuda ridges are parts of a formerly continuous aseismic ridge, a segment of which has recently been overridden by the Caribbean plate. Reconstruction of mid‐Miocene to Recent plate motions also suggest that at least two aseismic ridges, and possibly fragments of the Bahama Platform, have interacted with the subduction zone in the northeastern Caribbean. The introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long. These ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes. They also leave a geologic imprint on segments of the arc with which they have interacted. A 50‐km landward jump of the locus of island arc volcanism occurred in Late Miocene time along the northern half of the Lesser Antilles. We postulate that the subduction of a segment of seafloor of anomolously thick crust, being more buoyant than adjacent seafloor, resulted in a marked shoaling in the dip of the descending slab and, therefore, a shift of the locus of volcanism. In the region near western Puerto Rico and eastern Hispanolia, Plio‐Pleistocene interaction with a similar feature, in this case a part of the Bahama Platform, about 3–4 m.y. ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone. That segment of the Bahama Platform interferred with the subduction process and was subsequently sutured onto the Caribbean plate when the boundary jumped about 60 km to the northeast. The maximum size of historic shallow earthquakes along the Lesser Antillean arc varies from about 7.0–7.5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8.0–8.5 along the northern part of the arc where the dip is shallow. The interaction of anomalous seafloor, as along the northern Lesser Antilles, can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones. Major seismic gaps and regions of high seismic potential currently exist along the northern Lesser Antilles and to the north of Puerto Rico. Both gaps are bounded by anomalous features on the downgoing plate. The intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes. A great shock in 1787 may have ruptured an existing seismic gap north of Puerto Rico between 65° and 67°W. Thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes.

Possible seismic reflections from the downgoing Pacific Plate, 275 kilometers arcward from the Eastern Aleutian Trench

Multichannel seismic reflection data collected near the southern Kenai Peninsula, Alaska, show deep events that could stem either from a linear diffractor or from a curved reflector. Whichever is the cause, it must be at a minimum depth of 12 km. If a linear diffractor causes the most obvious events, it strikes at a 60° angle across the surface trend of Mesozoic rocks. Thus the diffractor could be associated with a transverse tectonic boundary that, on the basis of other evidence, is thought to strike through the area of this study. Alternatively, these deep events could be reflections from rocks associated either with a detached piece of oceanic crust or with the downgoing Pacific plate. These associated rocks could be porous and overpressured sedimentary rocks that are squeezed between indurated or metamorphosed sedimentary rocks in the overthrust plate and igneous rocks in the downgoing oceanic crust.

Anelastic response of the Earth to a dip slip earthquake

The deformation of the earth's surface immediately following an earthquake is well modeled by a dislocation in an elastic half space. Some time after the event, however, the elastic stresses begin to relax due to flow in the more fluid regions of the upper mantle (the asthenosphere). This relaxation causes further slow displacements observable at the earth's surface. This paper examines the pattern and timing of these postseismic motions for a vertical dip slip fault and a 30° dip thrust fault. A variety of rheological models are studied. These models encompass variations in asthenosphere thickness, mesosphere viscosity, fault depth, and, most important, power law flow. Both Newtonian (n = 1) and non‐Newtonian (n = 3) Maxwell rheologies are considered for the asthenosphere and mesosphere. The results for a vertical dip slip fault show that a peripheral upwarp develops within a distance of a few lithosphere thicknesses from the rupture zone on the downthrown side of the fault (a comparable downwarp develops on the upthrown side). These peripheral warps grow to a maximum height of about 5% of the total fault slip some time after the earthquake event and then collapse at much greater times. The vertical motions for a 30° dip thrust fault show that after the earthquake a general uplift occurs in the region near the rupture zone. Horizontal displacements propagate asymmetrically away from the rupture zone, the largest displacements occurring in the overthrust plate. The principal result of the comparison of n = 1 and n = 3 rheological laws is that the basic pattern of uplifts and horizontal motions is very similar: the major difference is the time dependence of the development of the pattern. In spite of the complex geometrical structure of the subduction zone the displacements ui(1)(t) for Newtonian rheology are nearly the same as ui(3)(t′) for the non‐Newtonian rheology if t′ ∼ t3. Thus although the qualitative pattern of the displacements is not diagnostic of power law flow, the great difference in time dependence allows a clear discrimination of the two flow laws.

The earthquake cycle in subduction zones

Lithospheric plates are subducted episodically, a few meters at a time, during earthquakes. These earthquakes recur over intervals ranging from decades to centuries. Within weeks to perhaps years after such an earthquake, elastic stresses induced in the asthenosphere relax, allowing displacement and stress to propagate away from the subduction zone into the adjacent plates. Finite element modelling of this process has shown that the propagation is asymmetric, the subducted slab serves to buffer the subducting plate against extensive motion. We have constructed a simple analytic model which incorporates the effect of he subducted slab in a fundamental way. The slab's effect is modelled by a Maxwell viscoelastic element which is attached to the subducting plate. Both plates obey Elsasser's displacement propagation equations. The elastic response of the Maxwell element at first restrains the motion of the subducted plate. The plate eventually moves on the timescale of stress relaxation in the Mesosphere (hundreds to thousands of years).The model predicts that the trench moves toward the subducting plate at an average speed of one half of the convergence rate. A strong extensional pulse is propagated into the overthrust plate shortly after the earthquake, whereas the stress pulse is small in the subducting plate. Although this extension changes into compression before the next earthquake in the cycle, the period of strong extension following the earthquake may be responsible for extensional tectonic features in the back‐arc region.

Tectonic stress in the plates

The state of stress in the lithosphere provides strong constraints on the forces acting on the plates. The directions of principal stresses in the plates as indicated by midplate earthquake mechanisms, in situ stress measurements, and stress‐sensitive geological features are used to test plate tectonic driving force models, under the premises that enough data exist in selected areas to define a regionally consistent stress field and that for most such areas the dominant forces producing such stresses are plate tectonic in origin. Force models include buoyancy forces at ridges, subduction zones, and continental convergence zones and variously parameterized viscous shear between the lithosphere and the asthenosphere. A linear finite element method, based on the wave front solution technique, is used to predict the intraplate stress for each force model. Several long‐wavelength patterns for the orientation of horizontal principal deviatoric stresses are observable in the stress data. Maximum compressive stresses trend E‐W to NE‐SW for much of stable North America and E‐W to NW‐SE for continental South America. In western Europe the maximum compressive stresses trend NW‐SE, while in Asia the trend is more nearly N‐S, especially near the Himalayan front. In the Indian plate the trend varies from nearly N‐S in continental India to more nearly E‐W in Australia. Horizontal stresses are variable in Africa but tend to indicate a NW‐SE trend for the maximum compressive stress in west Africa and an E‐W trend for the minimum compressive stress in east Africa. Oceanic lithosphere away from plate boundaries is generally in a state of deviatoric compression, although few focal mechanisms can be constrained to define the orientation of the principal stresses.Comparison of stress orientations predicted for a wide range of driving force models to these regional stress observations provides a powerful test of the models. Ridge pushing forces are required in all models that match the stress orientation field. The net pulling force of subducted lithosphere, if such a force acts approximately symmetrically about the plate boundary, is at most a few times larger than other forces acting on the plates. Resistive forces associated with trench thrust faults and motion of the slab with respect to the mantle must therefore nearly balance the large gravitational potential of the slab. The upper limit on the ratio of net slab to ridge forces may be increased by less than a factor of 2 if net slab forces are reduced for the fastest moving plates by assuming that the resistance to subduction increases with convergence rate. Forces acting to resist further continental convergence along the Alpine‐Himalayan belt are important for models of the intraplate stress field in Europe, Asia, and the Indian plate. Inclusion of continental convergence zone forces, however, does not affect the upper bound on the ratio of net slab to ridge forces. A variety of possible lithosphere‐asthenosphere interactions have been tested. Resistive viscous drag forces acting on the base of the lithosphere improve the fit between calculated and observed stresses in the Nazca and South American plates as long as the drag coefficient is nonzero beneath oceanic lithosphere. The calculated intraplate stress field is not very sensitive to an increased drag coefficient beneath old oceanic lithosphere compared to young oceanic or continental lithosphere. Increasing the drag coefficient beneath continents compared to oceanic lithosphere by a factor of 5 or 10 has little effect on the overall fit of calculated stresses to observed stresses. Models in which viscous drag forces drive, rather than resist, plate motions are in poor agreement with intraplate stress data, although this lack of fit may depend on the oversimplified model of mantle flow patterns that has been assumed. A model of the driving mechanism in which slab forces act only on the plate with subducted lithosphere and where viscous drag forces are assumed to balance the torque on each plate produced by ridge and trench forces predicts stresses in good agreement with the data for most continental regions. The fit between calculated and observed stresses for this model is relatively poor for most oceanic regions. This model suggests, however, that it is probably an oversimplification to assume that the net force exerted by the slab acts symmetrically about the plate boundary, though some pulling force on the overthrust plate appears necessary. While the role of drag forces in the driving mechanism remains poorly constrained, the finite element technique that has been developed may be applied in the future to any specific and realizable flow pattern that is proposed for the mantle.

Large thrust earthquakes and tsunamis: Implications for the development of fore arc basins

Variations of the subduction regime around the Pacific are reflected in changes of both the lengths of rupture zones and the source areas for tsunamis associated with large shallow earthquakes. Crustal deformation occurring during the earthquake and related tsunamic activity play important roles in the development and maintenance of topographic features in the fore arc region. A regional comparison of the average length of upper slope basins and terraces with the maximum length of earthquake rupture zones shows that longer basins and terraces characteristically occur in regions with larger rupture zones. Examples from Japan, Alaska, and the Aleutians clearly show how these topographic features reflect the size and spatial distribution of seismic‐tsunamic source areas. In many areas this relationship may be explained by the coseismic reactivation of structural units within the fore arc region. The two principal areas of coseismic reactivation are the trench slope break and the frontal arc region. Thus upper slope basins, deep sea terraces, and other bathymetric features may serve as indicators of the tectonic regime and of seismic‐tsunamic risk along convergent margins. Variations of the dimensions of rupture zones also appear to be influenced by the blocklike behavior of the overthrust plate. In many instances the dimensions of upper slope basins and terraces are equivalent to those of the crustal blocks. Regions with basin or terrace lengths greater than 100 km have histories of large ruptures involving adjacent segments. Regions with basin or terrace dimensions less than 100 km tend to rupture independently of adjacent segments.

On the Character of Thrust Faults With Particular Reference to the Basal Tongues

ABSTRACT Some detailed observations made along thrust faults are reported in this paper. The following areas were visited: Canton Glarus, Switzerland; Las Vegas area, Nevada; Glacier Park area, Montana; southern Canadian Rockies west of Calgary, Alberta. The following observations seem to have universal validity: 1. The contact is usually sharp and unimpressive in view of the great amount of displacement. 2. Structures which have been named 'tongues' seem to be common. They are features where material from the overridden sequence is seemingly injected as a tongue into the base of the overthrust plate. 3. Secondary (or splay) thrusts are common. 4. Coalescence of tongues may produce pseudo-boudins. 5. Minor folding and faulting can usually be observed in both the thrust plate and the underlying rocks. The intensity of such deformations is normally comparatively weak, at least in view of the large displacements these thrust plates have undergone, and such internal distortions are strongly affected by the gross lithology of the rocks involved. 6. Late deformations, particularly by normal faulting, are present on many thrust plates. They should be recognized for what they are: post-thrusting features completely unrelated to the emplacment of the thrust plates. This paper is an observational note only and no attempt is made to give a complete explanation of the structural significance of the various features. Particular emphasis is placed on the basal tongue and the associated pseudo-boudins structures. Both have received little attention in the literature to date and both are at present not fully understood.

Structural deformation of the Idaho-Wyoming overthrust belt (U.S.A.), as determined by Triassic paleomagnetism

A paleomagnetic study of Triassic redbeds from fourteen sites within the Idaho-Wyoming overthrust belt and three sites from the adjacent stable foreland was undertaken in an attempt to decipher the origin of the arcuate shape of the thrust-fold belt in north-western Wyoming. This belt rises from beneath the Snake River lava plain in eastern Idaho and trends east-southeast into Wyoming. South of Jackson, Wyoming, the belt swings 45 to 70 degrees to trend almost due south into west-central Wyoming. A total of 193 samples were paleomagnetically analyzed using spinner and cryogenic magnetometers. Thermal demagnetization up to 681° C reveals that the natural remanent magnetization consists characteristically of two directional components. A small secondary (chemical) magnetization is revealed at temperatures below 500° C, whereas a proportionally large characteristic magnetization appears to be primary and conforms to the expected Triassic latitudes of the North American craton. The site-mean directions have Fisher precision k ranging from 14 to 149, with confidence cones of 95%-probability ranging between 4.8° and 16.2°. The characteristic (primary) directions of magnetization indicate that the structural bend in the overthrust belt originated through tectonic rotations of the thrust sheets in the horizontal plane, as the paleomagnetic declinations show a systematic change corresponding to the structural configuration mentioned above. We propose that as overthrusting progressed from west to east in the Late Mesozoic and Early Tertiary, the thrust sheets experienced an increasing resistance to movement due to the proximity of the ancient arcuate foreland margin. The buttressing effect of the foreland hinge on the overthrust plates caused the frontal edges of the thrusts to assume the configuration of the foreland margin resulting in horizontal rotations of the thrust sheets. The easternmost Prospect—Cliff Creek—Jackson thrust sheet came into actual contact with the ancestral Teton—Gros Ventre Precambrian block on the north and the Game Hill reverse fault on the east; as a result the northern sections of the thrust sheet rotated by an amount of up to 60 degrees in a counter-clockwise sense and the southern and eastern sections rotated in a clockwise direction by an amount of up to 30 degrees.

Radiometric ages from Ryukyu arc region and an 40Ar/ 39Ar age from biotite dacite on Okinawa

A biotite dacite that intrudes metamorphic rocks on Okinawa in the Ryukyu island arc has been dated at 12 m.y. by the 40Ar/ 39Ar method. The details of this age measurement and a compilation of radiometric ages for the Ryukyu island arc and adjacent regions are presented. These data suggest that from 65 to 12 m.y. ago the magmatic axis of the Ryukyu arc was confined to a very narrow zone along the arc. In Kyushu and Shikoku, the southern Japanese islands, intrusive and volcanic igneous rocks dated as 21−12 m.y. occur over a much wider zone than in the Ryukyu arc. The apparent difference in width of the magmatic zones may be due to different absolute motions of the overthrust plates of those two regions of subduction. The dissimilarity of available radiometric ages for the Ryukyu arc and for Taiwan suggest different histories for the development of these two features. The occurrence of active volcanoes in association with the Okinawa Trough, northwest of the Ryukyu island arc, may indicate that the trough itself developed in the last 12 m.y.

Geophysical Profile Bearing on the Origin of the Jotun Nappe in the Norwegian Caledonides

The Jotun nappe is composed of relatively mafic allochthonous rocks that may have been emplaced by: (1) overthrusting of a rigid plate from a great distance, (2) igneous intrusion, (3) formation of recumbent folds from a near or distant source. A source area for the Jotun nappe has never been demonstrated. A gravity profile shows a positive Bouguer gravity anomaly of about 34 mgal over the Jotun nappe. Gravity interpretation yields models with maximum thickness from 8 to 15 km for a trough-shaped area of the Jotun nappe. The gravity model can be interpreted in terms of an overthrust plate or of locally derived folded nappes similar to the Swiss Alps, but locally derived folded nappes seem to be most consistent with available geological and geophysical data.

Radial Movements in the Western Wyoming Salient of the Cordilleran Overthrust Belt

Research Article| June 01, 1969 Radial Movements in the Western Wyoming Salient of the Cordilleran Overthrust Belt GARY W CROSBY GARY W CROSBY University of Montana, Missoula, Montana Search for other works by this author on: GSW Google Scholar GSA Bulletin (1969) 80 (6): 1061–1078. https://doi.org/10.1130/0016-7606(1969)80[1061:RMITWW]2.0.CO;2 Article history received: 26 Jan 1967 rev-recd: 23 Oct 1968 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GARY W CROSBY; Radial Movements in the Western Wyoming Salient of the Cordilleran Overthrust Belt. GSA Bulletin 1969;; 80 (6): 1061–1078. doi: https://doi.org/10.1130/0016-7606(1969)80[1061:RMITWW]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Sinuous overthrust belts at geosyncline-craton boundaries form a series of salients (convex cratonward) and re-entrants. Structural patterns suggest movements diverging across salients and converging across re-entrants with mass movement toward the craton. Orientation data on bedding, cleavage, thrust surfaces, striations, fold hinges, bedding-cleavage intersections, and striated subvertical fractures were collected at 22 stations on the western Wyoming salient and were plotted stereographically. For each structural element, π-circles, lineation girdles, or planes normal to point maxima were determined statistically, and azimuths of contained horizontal lines were averaged to get the station's mean movement direction. A map plot of the 22 mean movement directions indicates about 70° of divergence along the salient from Idaho, through Wyoming, and into Utah.The statistical center of divergence is in northern Oneida County, Idaho, where uplift with erosion has beveled the rocks to approximately Precambrian age material. A total Paleozoic and Triassic-Jurassic isopach shows maximum thickness in a local basin in the miogeosynclinal trough to have been near the subsequent divergence center of Cretaceous and early Tertiary overthrust movements. The apex of uplift roughly coincides with the center of diverging movement and the estimated pre-existing depocenter; maximum uplift was greater than 45,000 ft. Present near isostatic balance, subsequent to 45,000+ ft of uplift, suggests isostasy was, in part, the mechanism of uplift and, in accord with isostatic principle, maximum uplift was in the area of greatest depression. Inasmuch as overthrust plates on the salient apparently do not involve crystalline basement rock, origin of the salient is best explained as a radial gravitational-gliding response of the sedimentary sequence to maximum local uplift at the center of diverging movement. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evidence of pleistocene events in the structure of the continental shelf off the northeastern United States

In 1958 and 1959 a seismic reflection reconnaissance was made of three regions of the continental shelf off northeastern United States: the outer shelf off New Jersey and Long Island, the shelf south of Martha's Vineyard, and Georges Bank. Broadband high resolution techniques, including the first use of arrays of underwater spark sources, permitted the delineation of reflectors as little as 4–6 m apart to depths of 100–300 m beneath the seafloor. Profiles of the outer shelf off the New Jersey coast and Long Island suggest that the major events of the Pleistocene age can be traced in a complex series of five sedimentary sequences separated by well developed unconformities. These sedimentary sequences and the erosional surfaces separating them appear to be the products of fluvial and marine processes during regressions and transgressions of the sea. Thirty kilometers south of Martha's Vineyard folded and overthrust plates of layered sediments found along a lobate front are apparently the result of ice pressure. The shelf edge south of Martha's Vineyard was built up and out apparently without enduring the heavy erosion found off New Jersey. A strong reflector that post-dates the time when the layered sediments were disturbed south of Martha's Vineyard is overlain by more than 200 m of sediments at this shelf edge. At the northern edge of Georges Bank both the layered base-rock and the more recent sediments are cut by filled channels which drained north towards the Gulf of Maine. The southern surface of the bank is built up more than 70 m by several successive deposits. Within these deposits there are filled channels. This and other evidence suggests that glaciers overrode the northern part of the bank, and that these successive deposits are a result of glacial depositions, modified by marine reworking during successive regressions and transgressions of the sea in Pleistocene time.

Structural features of the Virgin Spring area, Death Valley, California

A flat thrust fault of middle or later Tertiary age, believed to have followed roughly the contact of later pre-Cambrian sediments with earlier pre-Cambrian metamorphic rocks, is well exposed throughout the Virgin Spring area, about 10 miles square, in the Black Mountains east of Death Valley. On this thrust later pre-Cambrian, Cambrian, and Tertiary rocks have moved relatively westward for an unknown distance. The rocks of the overthrust plate are broken into innumerable blocks and slices, which are thrust over one another to form an extremely complex mosaic. This assemblage of blocks is named the Amargosa chaos, and the flat fault upon which the chaos lies is named the Amargosa thrust. The chaos is divided into the Virgin Spring, Calico, and Jubilee facies, each characterized by certain kinds of rock. The Amargosa thrust and chaos are folded into several plunging anticlines, of northwesterly trend, along whose crests the earlier pre-Cambrian rocks below the thrust are exposed. Lying unconformably upon the folded thrust and chaos is the Funeral fanglomerate, probably of late Pliocene age, which consists of fanglomerates and basaltic lava flows. These rocks are deformed by folds and faults so recent that they are still reflected in the topography; Death Valley in the Virgin Spring area is primarily a syncline, modified by faulting, in the Funeral fanglomerate. Features similar to the Amargosa thrust and chaos occur throughout an area of at least 8000 square miles that borders and includes the Death Valley trough. The present paper deals mainly with the structure of the Virgin Spring area but it, also notes related structures elsewhere in the Death Valley region, outlines the stratigraphy of the region, and describes a geologic section across it.