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At least 379 records · Page 21Linked to original sources
USGS2001

Slip distribution and tectonic implication of the 1999 Chi-Chi, Taiwan, earthquake

We report on the fault complexity of the large (Mw = 7.6) Chi-Chi earthquake obtained by inverting densely and well-distributed static measurements consisting of 119 GPS and 23 doubly integrated strong motion records. We show that the slip of the Chi-Chi earthquake was concentrated on the surface of a "wedge shaped" block. The inferred geometric complexity explains the difference between the strike of the fault plane determined by long period seismic data and surface break observations. When combined with other geophysical and geological observations, the result provides a unique snapshot of tectonic deformation taking place in the form of very large (>10m) displacements of a massive wedge-shaped crustal block which may relate to the changeover from over-thrusting to subducting motion between the Philippine Sea and the Eurasian plates.

Geophysical Research Letters↗
USGS2003

Three-dimensional velocity structure of crust and upper mantle in southwestern China and its tectonic implications

Using P and S arrival times from 4625 local and regional earthquakes recorded at 174 seismic stations and associated geophysical investigations, this paper presents a three‐dimensional crustal and upper mantle velocity structure of southwestern China (21°–34°N, 97°–105°E). Southwestern China lies in the transition zone between the uplifted Tibetan plateau to the west and the Yangtze continental platform to the east. In the upper crust a positive velocity anomaly exists in the Sichuan Basin, whereas a large‐scale negative velocity anomaly exists in the western Sichuan Plateau, consistent with the upper crustal structure under the southern Tibetan plateau. The boundary between these two anomaly zones is the Longmen Shan Fault. The negative velocity anomalies at 50‐km depth in the Tengchong volcanic area and the Panxi tectonic zone appear to be associated with temperature and composition variations in the upper mantle. The Red River Fault is the boundary between the positive and negative velocity anomalies at 50‐km depth. The overall features of the crustal and the upper mantle structures in southwestern China are a low average velocity, large crustal thickness variations, the existence of a high‐conductivity layer in the crust or/and upper mantle, and a high heat flow value. All these features are closely related to the collision between the Indian and the Asian plates.

Journal of Geophysical Research B: Solid Earth↗
USGS2004

Contemporary tectonic deformation of the Basin and Range province, western United States: 10 years of observation with the Global Positioning System

[1] We have estimated patterns and rates of crustal movement across 800 km of the Basin and Range at ∼39° north latitude with Global Positioning System surveys in 1992, 1996, 1998, and 2002. The total rate of motion tangent to the small circle around the Pacific‐North America pole of rotation is 10.4 ± 1.0 mm/yr, and motion normal to this small circle is 3.9 ± 0.9 mm/yr compared to the east end of our network. On the Colorado Plateau the east end of our network moves by ∼1–2 mm/yr westerly with respect to North America. Transitions in strain rates delimit six major tectonic domains within the province. These deformation zones coincide with areas of modern seismicity and are, from east to west, (1) east‐west extension in the Wasatch Fault zone, (2) low rate east‐west extension centered near the Nevada‐Utah border, (3) low rate east‐west contraction between 114.7°W and 117.9°W, (4) extension normal to and strike‐slip motion across the N10°E striking Central Nevada Seismic Zone, (5) right lateral simple shear oriented N13°W inside the Walker Lane Belt, and (6) shear plus extension near the Sierra Nevada frontal faults. Concentration of shear and dilatational deformation across the three westernmost zones suggests that the Walker Lane Belt lithosphere is rheologically weak. However, we show that linear gradients in viscosity and gravitational potential energy can also effectively concentrate deformation. In the Basin and Range, gradients in gravitational potential are spatially anticorrelated with dilatational strain rates, consistent with the presence of horizontal variations in viscosity of the lithosphere.

Arizona, California, Nevada, Utah↗
USGS2005

Northwest Basin and Range tectonic deformation observed with the Global Positioning System, 1999-2003

We use geodetic velocities obtained with the Global Positioning System (GPS) to quantify tectonic deformation of the northwest Basin and Range province of the western United States. The results are based on GPS data collected in 1999 and 2003 across five new quasi-linear networks in northern Nevada, northeast California, and southeast Oregon. The velocities show ???3 mm/yr westward movement of northern Nevada with respect to stable North America. West of longitude 119??W the velocities increase and turn northwest, parallel to Sierra Nevada/Great Valley microplate motion, and similar to velocities previously obtained to the south. The observations are explained by a kinematic model with three domains that rotate around Euler poles in eastern Oregon and western Idaho. Northeast California experiences internal dextral shear deformation (11.2 ?? 3.6 nstrain/yr) subparallel to Pacific/North America motion. Relative motions of the domains imply 2-5 mm/yr approximately east-west extension in northwest Nevada and 1-4 mm/yr approximately north-south contraction near the California/Oregon border. The northward decreasing approximately east-west extension in northwest Nevada is consistent with the northern termination of Basin and Range deformation, faulting and characteristic topography. No significant extension is detected in the Oregon Basin and Range. The Oregon Cascade arc moves north at ???3.5 mm/yr and is possibly influenced by the approximately eastward motion of the Juan de Fuca plate. These results disagree with secular northwest trenchward motion of the Oregon forearc inferred from paleomagnetic rotations. South of latitude 43??, however, trenchward motion exists and is consistent with block rotations, approximately east-west Basin and Range extension, and northwest Sierra Nevada translation. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

Volcano-tectonic implications of 3-D velocity structures derived from joint active and passive source tomography of the island of Hawaii

We present a velocity model of the onshore and offshore regions around the southern part of the island of Hawaii, including southern Mauna Kea, southeastern Hualalai, and the active volcanoes of Mauna Loa, and Kilauea, and Loihi seamount. The velocity model was inverted from about 200,000 first-arrival traveltime picks of earthquakes and air gun shots recorded at the Hawaiian Volcano Observatory (HVO). Reconstructed volcanic structures of the island provide us with an improved understanding of the volcano-tectonic evolution of Hawaiian volcanoes and their interactions. The summits and upper rift zones of the active volcanoes are characterized by high-velocity materials, correlated with intrusive magma cumulates. These high-velocity materials often do not extend the full lengths of the rift zones, suggesting that rift zone intrusions may be spatially limited. Seismicity tends to be localized seaward of the most active intrusive bodies. Low-velocity materials beneath parts of the active rift zones of Kilauea and Mauna Loa suggest discontinuous rift zone intrusives, possibly due to the presence of a preexisting volcanic edifice, e.g., along Mauna Loa beneath Kilauea's southwest rift zone, or alternatively, removal of high-velocity materials by large-scale landsliding, e.g., along Mauna Loa's western flank. Both locations also show increased seismicity that may result from edifice interactions or reactivation of buried faults. New high-velocity regions are recognized and suggest the presence of buried, and in some cases, previously unknown rift zones, within the northwest flank of Mauna Loa, and the south flanks of Mauna Loa, Hualalai, and Mauna Kea. Copyright 2009 by the American Geophysical Union.

Hawaii↗
USGS2009

Possible deep fault slip preceding the 2004 Parkfield earthquake, inferred from detailed observations of tectonic tremor

Earthquake predictability depends, in part, on the degree to which sudden slip is preceded by slow aseismic slip. Recently, observations of deep tremor have enabled inferences of deep slow slip even when detection by other means is not possible, but these data are limited to certain areas and mostly the last decade. The region near Parkfield, California, provides a unique convergence of several years of high-quality tremor data bracketing a moderate earthquake, the 2004 magnitude 6.0 event. Here, I present detailed observations of tectonic tremor from mid-2001 through 2008 that indicate deep fault slip both before and after the Parkfield earthquake that cannot be detected with surface geodetic instruments. While there is no obvious short-term precursor, I find unidirectional tremor migration accompanied by elevated tremor rates in the 3 months prior to the earthquake, which suggests accelerated creep on the fault ∼16 km beneath the eventual earthquake hypocenter.

Geophysical Research Letters↗
USGS2019

The 12 November 2017 Mw 7.3 Ezgeleh–Sarpolzahab (Iran) earthquake and active tectonics of the Lurestan arc

The 12 November 2017 M w 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic.

Journal of Geophysical Research: Solid Earth↗
USGS2019

The August 2018 Kaktovik earthquakes: Active tectonics in northeastern Alaska revealed With InSAR and seismology

The largest earthquakes recorded in northern Alaska (M w 6.4 and M w 6.0) occurred ~6 hours apart on August 12, 2018 in the northeastern Brooks Range. The earthquakes were captured by Sentinel-1 InSAR satellites and Earthscope Transportable Array seismic data, giving insight into the little-known active tectonic processes of Arctic Alaska, obscured until recently by sparse data availability. In this study, InSAR modelling, teleseismic back projections, calibrated hypocentral relocations and regional moment tensor solutions resolve two previously unknown, SSW-dipping right-lateral fault segments. These are the first active faults identified as conjugate to the NE-trending sinistral Canning Displacement Zone directly to the west, which is therefore a more complex zone of diffuse faulting than previously thought. The northeastern Brooks Range has been characterized as an area of low to moderate seismic hazard, but these earthquakes illustrate the potential for larger, possibly destructive events in a region earmarked for rapid resource development.

Alaska↗
USGS2021

Cataloging tectonic tremor energy radiation in the Cascadia subduction zone

For the past ∼12 years the Pacific Northwest Seismic Network has been automatically detecting and locating tectonic tremor across the Cascadia subduction zone, resulting in a catalog of more than 500,000 tremor epicenters to date, which has served as a valuable resource for tremor and slip research. This manuscript presents an updated methodology for routine tremor detection in Cascadia and a new catalog of over 180,000 tremor epicenters including amplitudes detected along the subduction zone margin from 2017 to 2021. The events are detected via cross-correlation of continuous vertical envelope data of 128 stations from northern California to northern Vancouver Island. The modified approach results in less scatter and a 55% increase in detected epicenters than previously observed, as well as a newly identified tremor source offset updip from the main tremor and slip region at the southern edge of the subduction zone. Radiated seismic energy in the 1.5–5 Hz band is used to assign epicenters an energy magnitude ( M eL ), which is calibrated to the M L of local earthquakes. Southern Cascadia is most active, but the highest tremor energy rates occur in northern Cascadia. Tremor in central Cascadia is systematically weaker and less frequent. Individual epicenter magnitudes range from ∼0.5–2 and spatiotemporally cluster into 1,060 swarms with cumulative M eL ranging from ∼0.8 to 3.7. The swarms reflect underlying slow slip events and occur with an earthquake-like energy distribution with a b value ∼1. Tremor epicenters, however, follow a tapered Gutenberg-Richter distribution with high b values, suggesting individual tremor bursts and their constituent low-frequency earthquakes are fault-dimension limited.

California, Oregon, Washington↗
USGS2026

Interplay between tectonics and submarine mass transport deposits in Cortes Basin: New high-resolution geophysics in the Outer California Borderland

The Outer California Borderland (OCB) is an active transform plate boundary offshore Southern California, where the relationship between faulting and submarine mass transport deposits (MTDs) remains poorly understood. Onshore paleoseismic data provide high-resolution earthquake records, whereas marine geophysical data capture longer-term histories. Offshore fault systems pose hazards to infrastructure and dense coastal populations, particularly when linked to submarine landslides. We present new high-resolution geophysical data set (cruise SR2303), including bathymetric and CHIRP sub-bottom data integrated with legacy seismic reflection data and chronostratigraphic constraints from ODP Site 1012 to examine Quaternary MTD recurrence and tectonic controls in the Cortes Basin, OCB. Bathymetry shows deformational features, including slide scarps and previously unmapped fault segments with evidence of Holocene activity. CHIRP profiles reveal 10 stacked MTDs in the East Cortes Basin and 8 in the West Cortes Basin, spanning ∼752 ka with an average recurrence of ∼83.6 ± 1 ka. Acoustic imaging shows 7 MTD intervals coinciding with fault offset increments and fault growth suggesting earthquake-triggered mass wasting. A strong association between MTD occurrences and sea-level extremes also supports glacio-eustatic contribution to slope failure. Stratigraphic correlations suggest quasi-synchronous MTDs across the eastern and western areas, likely triggered by larger eathquakes in the Quaternary. Although the identified MTDs occur relatively far from the Southern California coast, they still pose a potential tsunamigenic hazard requiring further assessment. Moreover, if linked to earthquakes along major strike-slip faults, for example, the Ferrelo fault, the MTDs may provide valuable proxies to constrain rupture scenarios and fault connectivity within the understudied OCB.

California↗
USGS2025

A glimpse into the future of tectonic tremor monitoring

Tectonic tremor is a weak, long-duration seismic signal often observed in subduction zones and on some other plate-bounding faults. Because of tremor's characteristically low amplitude (and low signal-to-noise) and lack of clear phase arrivals, detecting and locating tremor usually requires techniques distinct from those applied to typical earthquakes. Major advances in detection and understanding of tremor have derived in the past from a powerful combination of new data and new analysis techniques. In a recent study, Sagae et al. (2025, https://doi.org/10.1029/2025jb031348 ) exploit that combination again, developing a new machine-learning based workflow and applying it to the S-net cabled seismic network in the Japan trench offshore northern Honshu. Their approach, although complex, succeeds in detecting several times more tremor activity than earlier studies, resulting in new insights and providing a blueprint for similar approaches that could be applied elsewhere. As real-time earthquake monitoring adopts similar tools, it may present an opportunity to bring tremor monitoring into operational workflows. In turn, this could solidify tremor monitoring as a component of future operational earthquake forecasting.

JGR Solid Earth↗
USGS2026

Role of volcano-tectonic interactions during early-phase magma-assisted continental rifting

We study volcanic-tectonic interactions during the early phase of rifting in the Natron Rift, a magmatic segment of the East African Rift System. We use the open-source crustal deformation modeling code, PyLith, to model deflation during the 2007–2008 explosive eruptions and inflation during 2016–2023. We compare predictions of surface displacements resulting from magma reservoir inflation from 3D models with and without topography and homogeneous and heterogeneous material properties against Global Navigation Satellite System displacements from 2016 to 2023. We prefer the model with heterogeneous material properties and topography for fitting the observed displacements. Including topography improves the fit to GNSS data and modulates stress changes from shallow magma sources. Our numerical modeling of Coulomb stress changes suggests that deflation of a magma reservoir under the active Ol Doinyo Lengai volcano could trigger oblique normal slip on the Natron Fault with Coulomb stress increases exceeding ∼0.1 MPa on the volcano-adjacent fault segment. Fault slip on the Natron Fault is less likely during inflation of the magma reservoir because normal compressive stress increases on the fault. We also demonstrate that Coulomb stress changes are sensitive to the inclusion of topography in our model, particularly because the depth of the magma reservoir is similar to the amplitude of the topographic relief.

Ol Doinyo Lenga↗
USGS2012

Three-dimensional modeling of pull-apart basins: implications for the tectonics of the Dead Sea Basin

We model the three-dimensional (3-D) crustal deformation in a deep pull-apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the Dead Sea Basin. The brittle upper crust is modeled by a boundary element technique as an elastic block, broken by two en echelon semi-infinite vertical faults. The deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress-free “shear zone” in the vicinity of the fault zone. The bottom displacement represents the regional relative plate motion. Results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin-bounding faults. As the width of the shear zone increases, the depth of the basin decreases, the rotation around a vertical axis near the fault tips decreases, and the basin shape (the distribution of subsidence normalized by the maximum subsidence) becomes broader. In contrast, two-dimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone. Our models also predict full-graben profiles within the overlapped region between bounding faults and half-graben shapes elsewhere. Increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin. We suggest that the observed structure of the Dead Sea Basin can be described by a 3-D model having a large overlap (more than 30 km) that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km.

Dead Sea Basin↗
USGS1997

Permian deposition in the north central Brooks Range, Alaska: Constraints for tectonic reconstructions

Two opposing tectonic models have been offered to explain the regional structural relations in the north central Brooks Range fold-thrust belt of northern Alaska. The first suggests that rocks of the northern Endicott Mountains were thrust from south to north over the area of the present Mount Doonerak high and are therefore highly allochthonous. The second implies that the rocks of the northern Endicott Mountains were deposited in a basin that lay north of the Mount Doonerak high and later were thrust a short distance southward onto the northern flank of the high and are thus parautochthonous. To provide stratigraphic constraints for these models, this study examines Permian facies of the north central Brooks Range. Permian rocks in the north central Brooks Range comprise a thin (40 to 160 m thick), fining-upward succession of clastic, storm-influenced shelf deposits. When the rocks of the northern Endicott Mountains are restored south of the Mount Doonerak area, a minimum distance of 80 km, the Permian deposits grade systematically from distal facies (Siksikpuk Formation) in the southwest to proximal facies (Echooka Formation) in the northeast. Facies trends in the reconstructed Permian basin include, from southwest to northeast, (1) an increase in carbonate content and corresponding decrease in silica content, (2) a general darkening and thickening of shaley intervals, (3) an increase in proximal features of storm beds, including coarser, thicker, more abundant, and more closely spaced beds, and (4) an increase in abundance and diversity of the faunal assemblage with a corresponding decrease in age. These stratigraphic relations imply that rocks of the northern Endicott Mountains are allochthonous and structurally overlie a proximal stratigraphie succession similar to that exposed in the Mount Doonerak area and northeastern Brooks Range.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Stratigraphic contrasts and tectonic relationships between Carboniferous successions in the Trans-Alaska Crustal Transect corridor and adjacent areas, northern Alaska

The Carboniferous succession along the Trans-Alaska Crustal Transect (TACT) corridor in the Atigun Gorge area of the central Brooks Range consists of the Kayak Shale (Kinderhookian) and the Lisburne Group (Kinderhookian through Chesterian). The Kayak Shale is at least 210 m thick; it is chiefly black, noncalcareous shale with several limestone beds of pelmatozoan-bryozoan packstone and formed in an open-marine setting. The Lisburne Group is a carbonate rock succession about 650 m thick and consists mainly of skeletal packstone, wackestone, and milestone which contain locally abundant calcispheres, ostracodes, algae, and sponge spicules; it accumulated largely in a shallow water platform environment with restricted circulation. This restriction was probably produced by a coeval belt of skeletal sand shoals recognized 70 km to the west in the Shainin Lake area. Significant and apparently abrupt shifts in the age and lithofacies of Carboniferous strata occur across the central and eastern Brooks Range. These shifts are most marked in a zone roughly coincident with what is interpreted by many workers to be the leading edge of the Endicott Mountains allochthon. Notable lithologie contrasts are also observed, however, between sections in the northern and southern parts of the Endicott Mountains allochthon. This suggests that considerable tectonic shortening has taken place within the allochthon, as well as between it and parautochthonous rocks to the northeast. The Carboniferous section near Mount Doonerak is more similar in age and lithofacies to coeval sections in the central Brooks Range that are considered allochthonous than to parautochthonous sections to the northeast.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Preliminary correlations of MAGSAT anomalies with tectonic features of Africa

An overview of the MAGSAT scalar anomaly map for Africa has suggested a correlation of MAGSAT anomalies with major crustal blocks of uplift or depression and different degrees of regional metamorphism. The strongest MAGSAT anomalies in Africa are closely correlated spatially with major tectonic features. Although a magnetic anomaly caused by a rectangular crustal block would be offset from the block's center by the effects of magnetic inclination, an anomaly caused by real crustal blocks of varying uplift, depression, and degree of regional metamorphism would be located nearer to the locus of greatest vertical movement and highest grade of metamorphism. Thus, the Bangui anomaly may be caused by a central old Precambrian shield, flanked to the north and south by two relatively young sedimentary basins.

Geophysical Research Letters↗
USGS2012

Contemporary block tectonics: California and Nevada

Well-determined fault plane solution and the gross pattern of late-Cenozoic faulting in California and Nevada show a systematic relation between the orientation of fault planes and slip directions. In general, normal faults have northerly strikes, reverse faults have easterly strikes, and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes, respectively. Kinematically, this relation is consistent with the response of clusters of fault-bounded crustal blocks to a regional stress field generated by the relative motion between the Pacific and North American plates. In this stress field, the greatest and least principal (compressive) stresses are restricted to northerly and easterly striking vertical planes, respectively. Clusters of crustal blocks bound by faults having the above attitudes form three basic regimes: (1) a spreading regime with a combination of normal and conjugate, strike slip faults, (2) a converging regime with a combination of reverse and conjugate, strike slip faults, and (3) a wrench regime with sets of subparallel, throughgoing, northwest striking (dextral) faults or northeast striking (sinstral) faults. These three regimes are typified by faulting patterns in the Basin and Range province (spreading), the Transverse Ranges (converging), and the San Andreas-Garlock fault systems (wrench), respectively. The gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north-south shortening and east-west extension with the ratio of extensional to shortening strains (and the areal dilitation) decreasing systematically from spreading to wrench to compressional regimes. The wrench regime involves a component of net rotational deformation (clockwise for dextral slip and counter-clockwise for sinstral slip), while deformation of the spreading and converging regimes is irrotational. Local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes. Maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength (Byerlee's law) along preexisting fractures and that pore pressure in the brittle crust is laterally uniform. A minimum strength difference between active, block-bounding faults and block interiors is 15–30%. Simple arrangements of such block clusters mimic the gross kinematic pattern of Quaternary faulting in California and Nevada. Some implications for contemporary tectonics emphasized by this model involve the westward displacement of the Sierra Nevada block.with respect to the stable interior of the North American plates, oblique thrusting of the Salinian block over the Pacific plate, and a progressive increase in the offset of the San Andreas fault represented by the ‘big bend’ through the Transverse Ranges.

Journal of Geophysical Research Solid Earth↗
USGS2012

Paleomagnetism and geology of Eocene volcanic rocks of southwest Washington, implications for mechanisms of tectonic rotation

Paleomagnetic and geologic investigations in Eocene volcanic rocks of the southwest Washington Coast Range demonstrate a close relationship between tectonic rotations and the local structural geology. The allochthonous middle Eocene submarine basalt basement of the Crescent Formation consists of several fault‐bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation (20° to perhaps as much as 65°) when compared to the Eocene reference pole for North America. Structural analysis shows the differential rotations postdate middle Eocene folding of the Crescent Formation against the continental margin and predate the unconformably overlying upper Eocene Goble Volcanics, which are rotated about 23° and do not show the same domains of rotation as the underlying Crescent Formation. Post‐Goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults. Major north‐northwest trending faults with several kilometers of dextral displacement form the boundaries of cross‐faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west‐northwest trending sinistral R ′ Riedel shears. Thirty‐five to 100% of the observed post‐late Eocene rotations could have occurred by this shear rotation mechanism. Other paleomagnetic study areas in the Coast Range and western Cascades have a similar fault geometry and may also have undergone significant shear rotations. Long‐term northward oblique subduction of the Farallon plate beneath the Coast Range throughout most of the Tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin. Shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large, rigid plates.

Washington↗