A data set comprising 110 spreading rates, 78 transform fault azimuths, and 142 earthquake slip vectors has been inverted to yield a new instantaneous plate motion model, designated Relative Motion 2 (RM2). The model represents a considerable improvement over our previous estimate, RM1 [Minster et al., 1974]. The mean averaging interval for the spreading rate data has been reduced to less than 3 m.y. A detailed comparison of RM2 with angular velocity vectors which best fit the data along individual plate boundaries indicates that RM2 performs close to optimally in most regions, with several notable exceptions. The model systematically misfits data along the India-Antarctica and Pacific-India plate boundaries. We hypothesize that these discrepancies are manifestations of internal deformation within the Indian plate; the data are compatible with northwest-southeast compression across the Ninetyeast Ridge at a rate of about 1 cm/yr. RM2 also fails to satisfy the east-west trending transform fault azimuths observed in the French-American Mid-Ocean Undersea Study area, which is shown to be a consequence of closure constraints about the Azores triple junction. Slow movement between North and South America is required by the data set, although the angular velocity vector describing this motion remains poorly constrained. The existence of a Bering plate, postulated in our previous study, is not necessary if we accept the proposal of Engdahl and others that the Aleutian slip vector data are biased by slab effects. Absolute motion models are derived from several kinematical hypotheses and compared with the data from hot spot traces younger than 10 m.y. Although some of the models are inconsistent with the Wilson-Morgan hypothesis, the overall resolving power of the hot spot data is poor, and the directions of absolute motion for the several slower-moving plates are not usefully constrained.

Present-day plate motions

A data set comprising 110 spreading rates, 78 transform fault azimuths and 142 earthquake slip vectors was inverted to yield a new instantaneous plate motion model, designated RM2. The mean averaging interval for the relative motion data was reduced to less than 3 My. A detailed comparison of RM2 with angular velocity vectors which best fit the data along individual plate boundaries indicates that RM2 performs close to optimally in most regions, with several notable exceptions. On the other hand, a previous estimate (RM1) failed to satisfy an extensive set of new data collected in the South Atlantic Ocean. It is shown that RM1 incorrectly predicts the plate kinematics in the South Atlantic because the presently available data are inconsistent with the plate geometry assumed in deriving RM1. It is demonstrated that this inconsistency can be remedied by postulating the existence of internal deformation with the Indian plate, although alternate explanations are possible.

/pdf/present-day-plate-motions-1cfamgs3qp.pdf

Present‐day plate motions

Orogenic gold and geologic time: a global synthesis

Geochronological, structural, and sedimentological data provide the basis for a regional synthesis of the evolution of the Cordilleran retroarc thrust belt and foreland basin system in the western USA In this region, the Cordilleran orogenic belt became tectonically consolidated during Late Jurassic time ( 155 Ma) with the closure of marginal oceanic basins and accretion of fringing arcs along the western edge of the North American plate Over the ensuing 100 Myr, contractile deformation propagated approximately 1000 kilometers eastward, culminating in the formation of the Laramide Rocky Mountain ranges At the peak of its development, the retroarc side of the Cordillera was divided into five tectonomorphic zones, including from west to east the Luning-Fencemaker thrust belt; the central Nevada (or Eureka) thrust belt; a high-elevation plateau (the "Nevadaplano"); the topographically rugged Sevier fold-thrust belt; and the Laramide zone of intraforeland basement uplifts and basins Mid-crustal rocks beneath the Nevadaplano experienced high-grade metamorphism and shortening during Late Jurassic and mid- to Late Cretaceous time, and the locus of major, upper crustal thrust faulting migrated sporadically eastward By Late Cretaceous time, the middle crust beneath the Nevadaplano was experiencing decompression and cooling, perhaps in response to large-magnitude ductile extension and isostatic exhumation, concurrent with ongoing thrusting in the frontal Sevier belt The tectonic history of the Sevier belt was remarkably consistent along strike of the orogenic belt, with emplacement of regional-scale Proterozoic and Paleozoic mega- thrust sheets during Early Cretaceous time and multiple, more closely spaced, Paleo- zoic and Mesozoic thrust sheets during Late Cretaceous-Paleocene time Coeval with emplacement of the frontal thrust sheets, large structural culminations in Archean- Proterozoic crystalline basement developed along the basement step formed by Neoproterozoic rifting A complex foreland basin system evolved in concert with the orogenic wedge During its early and late history ( 155 - 110 Ma and 70 - 55 Ma) the basin was dominated by nonmarine deposition, whereas marine waters inundated the basin during its midlife ( 110 - 70 Ma) Late Jurassic basin development was controlled by both flexural and dynamic subsidence From Early Cretaceous through early Late Creta- ceous time the basin was dominated by flexural subsidence From Late Cretaceous to mid-Cenozoic time the basin was increasingly partitioned by basement-involved Laramide structures Linkages between Late Jurassic and Late Cretaceous Cordilleran arc- magmatism and westward underthrusting of North American continental lithosphere beneath the arc are not plainly demonstrable from the geological record in the Cordilleran thrust belt A significant lag-time ( 20 Myr) between shortening and coeval underthrust- ing, on the one hand, and generation of arc melts, on the other, is required for any linkage to exist However, inferred Late Jurassic lithospheric delamination may have provided a necessary precondition to allow relatively rapid Early Cretaceous continen- tal underthrusting, which in turn could have catalyzed the Late Cretaceous arc flare-up

/pdf/late-jurassic-to-eocene-evolution-of-the-cordilleran-thrust-jx3x655k2a.pdf

Late Jurassic to Eocene evolution of the Cordilleran thrust belt and foreland basin system, western U.S.A.

stress provinces indicate that these transitions can be abrupt, occurring over <75 km in places. In the western United States, a region of active tectonism characterized by high levels of seismicity and generally high heat flow, the stress pattern is complex, but numerous stress provinces can be well delineated. Despite relative tectonic quiescence in the eastern and central United States, a major variation in principal stress orientation is apparent between the Atlantic Coast and midcontinent areas. Most of the eastern United States is marked by predominantly compressional tectonism (combined thrust and strike slip faulting), whereas much of the region west of the southern Great Plains is characterized by predominantly extensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreas fault and in parts of the Sierra Nevada is nearly pure strike slip. Exceptions to this general pattern include areas of compressional tectonics in the western United States (the Pacific Northwest, the Colorado Plateau interior, and the Big Bend segment of the San Andreas fault) and the normal growth faulting along the Gulf Coastal Plain. Sources of stress are constrained not only by the orientation and relative magnitude of the stresses within a given province but also by the manner of transition of the stress field from one province to another. Much of the modem pattern of stress in the western United States can be attributed to present transform motion and residual thermal and dynamic effects of Tertiary subduction along the western edge of the North American plate. Abrupt stress transitions around actively extending regions in the western United States probably reflect shallow sources of stress and anomalously thin lithosphere. Large areas characterized by a uniform stress field in the central and eastern United States suggest broad scale plate tectonic forces. In the midcontinent region, both ridge push and asthenospheric viscous drag resistance to lithospheric motion can explain the NE-SW compression in the cold, thick lithosphere of the craton, although drag-induced stress directions (resistance to absolute plate motion) correlate better with the data than do the ridge push directions. Asthenospheric counterflow models do not apply in this region, as predicted stress orientations are about 90 o off. A region of compression oriented approximately perpendicular to the continental margin and Appalachian fold belt is defined by the stress data along the Atlantic Coast. This NW-SE compression is in direct contrast with previous models predicting extension perpendicular to passive continental margins due to lateral density contrasts at the continental-oceanic crust interface. Ridge push forces, while capable of producing a component of compression across the coastal area, do not explain the observed orientation of the stress. Two speculative mechanisms are suggested to explain the observed orientations: (1) rotation of stress (or strain) due to anisotropic Appalachian basement structure and (2) flexural effects associated with erosion and isostatic rebound of the Appalachians.

/pdf/state-of-stress-in-the-conterminous-united-states-12cs1w0fvw.pdf

State of stress in the conterminous United States

Tectonic overview of the Cordilleran orogen in the western United States

Structural framework and evolution of the southern part of the Cordilleran orogen, western United States

The northeast- to east-striking Garlock fault of southern California is a major strike-slip fault with a left-lateral displacement of at least 48 to 64 km. It is also an important physiographic boundary since it separates along its length the Tehachapi–Sierra Nevada and Basin and Range provinces of pronounced topography to the north from the Mojave Desert block of more subdued topography to the south. Previous authors have considered the 260-km-long fault to be terminated at its western and eastern ends by the northwest-striking San Andreas and Death Valley fault zones, respectively. We interpret the Garlock fault as an intracontinental transform structure which separates a northern crustal block distended by late Cenozoic basin and range faulting from a southern, Mojave block much less affected by dilational tectonics. Earlier ideas that the Garlock fault terminates eastward at the Death Valley fault zone appear to us to be in error, although right-lateral offsetting of the Garlock along that zone by about 8 km is necessary. Displacement along the Garlock fault must increase westward from its eastern terminus, a point of zero offset now buried beneath alluvial deposits in Kingston Wash to the east of the Death Valley fault zone. Much of the displacement on the Garlock fault due to east-west components of basin and range faulting appears to have been derived from block faulting in the area between Death Valley and the Nopah Range. Westward displacement of the crustal block north of the Garlock by extensional tectonics within it totals 48 to 60 km in the Spangler Hills–Slate Range area and probably continues to increase westward at least as far as the eastern frontal fault of the Sierra Nevada. Westward shifting of the northern block of the Garlock has probably contributed to the westward bending or deflection of the San Andreas fault where the two faults meet. Many earlier workers have considered that the left-lateral Garlock fault is conjugate to the right-lateral San Andreas fault in a regional strain pattern of north-south shortening and east-west extension, the latter expressed in part as an eastward displacement of the Mojave block away from the junction of the San Andreas and Garlock faults. In contrast, we regard the origin of the Garlock fault as being directly related to the extensional origin of the Basin and Range province in areas north of the Garlock. Recent models for development of that province related to intracontinental spreading east of an east-dipping subduction zone along the Cenozoic margin of western North America may best account for the differential east-west extension which has occurred in the crustal blocks to the north and south of the Garlock fault. Other possible examples of intracontinental transform faults in the southwestern Cordillera with geometries similar to that of the Garlock fault include the left-lateral Santa Cruz–Sierra Madre fault zone along the southern margin of the western Transverse Ranges, and the right-lateral Las Vegas shear zone and Agua Blanca fault of Baja California.

B. C. Burchfiel

Papers

Tectonic overview of the Cordilleran orogen in the western United States

Structural framework and evolution of the southern part of the Cordilleran orogen, western United States

Garlock Fault: An Intracontinental Transform Structure, Southern California

Mesozoic Construction of the Cordilleran “Collage”, Central British Columbia to Central California

Mojave desert and environs