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The Sierran microplate is a northwest-translating block entrained in distributed motion east of the Pacific plate. To the north, the Oregon coast block (OCB) moves northward within the hanging wall of the Cascadia subduction zone, above the obliquely converging Juan de Fuca plate. Analysis of GPS velocity data indicates that relative motion between the rigid Sierran and OCB microplates is characterized by several millimeters per year of dextral shear directed ∼N70°W, which is distinct from and counterclockwise to macroscopic dextral shear in the Walker Lane east of the Sierran microplate. We present a new analysis of focal mechanisms from small earthquakes in an 80-km-wide zone that spans the geodetically defined Sierran-OCB boundary to evaluate patterns of distributed deformation. We find that the direction of macroscopic dextral shear in this region is parallel to Sierran-OCB motion derived from GPS data. The seismogenic deformation is consistent with postulated dextral shear within an incompletely studied west-northwest–trending zone of faults and lineaments that traverses the northern Sierra Nevada; the faults and lineaments terminate westward against Quaternary folds in the northern Sacramento Valley. Active deformation at the Sierran-OCB boundary accommodates the relative motion of the bounding microplates and probably does not represent discrete transfer of Walker Lane motion to the Cascadia subduction zone in a restraining left step across the northern Sierra and Sacramento Valley.

We report new paleomagnetic data from the sheeted dike complex of the Troodos ophiolite (Cyprus) that indicate that its northern limit is marked by a hitherto unrecognized oceanic transform fault system. The style, magnitude, and scale of upper crustal fault-block rotations in the northwestern Troodos region mirror those observed adjacent to the well-known Southern Troodos transform fault zone along the southern edge of the ophiolite. A pattern of increasing clockwise rotation toward the north, coupled with consistent original dike strikes and inclined net rotation axes across this region, is compatible with distributed deformation adjacent to a dextrally slipping transform system with a principal displacement zone just to the north of the exposed ophiolite. Combined with existing constraints on the spreading fabric, this implies segmentation of the Troodos ridge system on length scales of ∼40 km, and suggests that a coherent strip of Neotethyan lithosphere, representing a complete ridge segment bounded by transforms, has been uplifted to form the currently exposed Troodos ophiolite. Moreover, the inferred length scale of the ridge segment is consistent with formation at a slow-spreading rate during Tethyan seafloor spreading and with a supra-subduction zone environment, as indicated by geochemical constraints.

We show that a belt of clockwise vertical-axis block rotation associated with dextral-oblique rifting in the Basin and Range province in Mexico hosted the localization of plate-boundary strain that led to formation of the Gulf of California ocean basin. Paleomagnetism of Miocene ignimbrites distributed widely across the rift reveals the magnitude, distribution, and timing of rotation. Using new high-precision paleomagnetic vectors (α 95 ≈ 1°) from tectonically stable exposures of these ignimbrites in Baja California, we determine clockwise rotations up to 76° for intrarift sites. Low reference-site error permits isolation of intrarift block rotation during proto-Gulf time, prior to rift localization ca. 6 Ma. We estimate that 48% (locally 0%–75%) of the net rotation occurred between 12.5 Ma and 6.4 Ma. Sites of large (>20°) block rotation defi ne an ~100-km-wide belt, associated with strike-slip faulting, herein named the Gulf of California shear zone, which was embedded within the wide rift Basin and Range province and kinematically linked to the San Andreas fault. After a protracted history of diffuse extension and transtension, rift localization was accomplished by focusing of Pacifi c– North America dextral shear into the Gulf of California, which increased strain rates and connected nascent pull-apart basins along the western margin of the province. Oblique rifting thus helped to localize and increase the rate of continental break up and strongly controlled the three-dimensional architecture of the resultant passive margins.

Displacement of the Miocene Cascade volcanic arc (northwestern North America) from the active arc is in the same sense and at nearly the same rate as the present clockwise block motions calculated from GPS velocities in a North American reference frame. Migration of the ancestral arc over the past 16 m.y. can be explained by clockwise rotation of upper-plate blocks at 1.0°/m.y. over a linear melting source moving westward 1–4.5 km/m.y. due to slab rollback. Block motion and slab rollback are in opposite directions in the northern arc, but both are westerly in the southern extensional arc, where rollback may be enhanced by proximity to the edge of the Juan de Fuca slab. Similarities between post–16 Ma arc migration, paleomagnetic rotation, and modern GPS block motions indicate that the secular block motions from decadal GPS can be used to calculate long-term strain rates and earthquake hazards. Northwest-directed Basin and Range extension of 140 km is predicted behind the southern arc since 16 Ma, and 70 km of shortening is predicted in the northern arc. The GPS rotation poles overlie a high-velocity slab of the Siletzia terrane dangling into the mantle beneath Idaho (United States), which may provide an anchor for the rotations.

Late Cenozoic tectonic reconstructions of southern California (United States) have been hindered for decades by disagreement over the total displacement on the southern San Andreas fault; disparate estimates of dextral offset range from 160 km to 240 km. Prior estimates were based on purely translational models that neglect the effects of transrotational strain on the orientation and distribution of key geologic markers. We present a new reconstruction that integrates published lithologic, structural, and paleocurrent data with the effects of clockwise rotation in the eastern Transverse Ranges (ETR). This model yields 200 ± 14 km of cumulative offset on the southern San Andreas fault system in the Salton Trough. The disparity between our estimate and a lower estimate of 160 km for the Mojave segment can be explained by a combination of transtensional elongation along the southwest boundary of the ETR, transfer of strain to the eastern California shear zone, and diffuse crustal shortening in the Big Bend region. This reconstruction reconciles previously incompatible and long-debated models, and places an important new constraint on the tectonic evolution of the Pacific–North America plate boundary in southern California.

We use vertical axis rotations from new paleomagnetic data to constrain off-fault deformation within the San Andreas fault system in central California. Samples were collected from 177 sites in the Miocene Monterey Formation adjacent to the Rinconada fault. Reliable means from 57 sites have 3 prominent patterns: (1) the largest clockwise rotations are close to the Rinconada and Nacimiento faults; (2) no significant rotation is observed near the San Andreas fault; and (3) counterclockwise rotations are observed at several sites northwest of Paso Robles. These paleomagnetic results are compared to two other measures of off-fault deformation where rotation can be calculated. Results from a fold-based kinematic model show increasing clockwise rotations toward the Rinconada fault, consistent with pattern 1. Few folds are observed in rocks on Salinian basement near the San Andreas fault, suggesting that little deformation has occurred, and providing an explanation for the negligible paleomagnetic rotations in pattern 2. Rotations calculated from the global positioning system velocity field predict counterclockwise rotations that coincide with those observed from paleomagnetic data in pattern 3. Broad patterns in the velocity field appear to be controlled by the transition from creeping to locked behavior along the San Andreas fault, and the region of counterclockwise rotation is linked to this transition. Thus, we suggest that the creeping segment has been aseismic over geologic time scales in order to produce the observed paleomagnetic rotations. The integration of all three data sets demonstrates that the San Andreas fault borderlands record an important portion of fault-parallel plate motion over geologic and geodetic time scales.

Detailed petrological study of mid-oceanic ridge basalt−derived high-pressure amphibolite blocks from a fragment of the Caribbean subduction channel (La Corea serpentinite-matrix melange, eastern Cuba) has revealed contrasted zoning patterns of garnet porphyroblasts, including well-defi ned complex oscillatory prograderetrograde concentric zoning in one sample. Calculated pressuretemperature (P-T) conditions for this sample using mineral inclusion assemblages and isochemical P-T projections reveal large P-T recurrences best explained by large-scale convective movement of the tectonic block in a serpentinitic subduction channel. The P-T conditions attending garnet growth followed an overall counterclockwise path as a consequence of continued refrigeration of the subduction channel during ongoing underfl ow after its onset ca. 120 Ma. These fi ndings constitute the fi rst report of large-scale convective circulation of deeply subducted material in the subduction channel, and are consistent with the thermomechanical behavior of the channel predicted by numerical models.

Latest Mesoproterozoic to mid-Neoproterozoic (1030–710 Ma) sedimentation and orogenic activity that developed on the northeast Laurentian substrate around the North Atlantic borderlands and is currently exposed in Scotland, Shetland, East Greenland, Svalbard, and Norway, is herein defined as the Valhalla orogen. The site for the orogen was initiated by ∼95° of clockwise rotation of Baltica with respect to Laurentia at the end of the Mesoproterozoic. This created a triangular ocean basin, the Asgard Sea, which received orogenic detritus from the Grenville-Sveconorwegian-Sunsas orogen. Sedimentary successions within the orogen accumulated during two cycles at 1030–980 Ma and 910–870 Ma, with each cycle terminated and the successions stabilized during tectonothermal episodes involving crustal thickening and igneous activity, some of calc-alkaline affinity, associated with the Renlandian (980–910 Ma) and Knoydartian (830–710 Ma) orogenic events. The Valhalla orogen represents an exterior accretionary orogen that developed along the margin of Laurentia and the Asgard Sea. The early stages of the Valhalla orogen are coeval with the final stages of the Grenville-Sveconorwegian-Sunsas orogen to the south, but are tectonically discrete; they constitute part of an exterior orogen that is entirely distinct from the interior orogen formed between collision of Laurentia, Baltica, and Amazonia.

Global positioning system (GPS) site velocities and earthquake focal mechanisms reveal an active left-lateral shear zone cutting across Kyushu in southwest Japan. Surprisingly, no active faults have been identified in association with this zone of rapid contemporary deformation. To explain the existence of this shear zone, we propose a model comprising subduction of an aseismic ridge (Kyushu-Palau Ridge) at the southwest end of the Nankai Trough. Because of rapid (~40 mm/yr) along-strike migration of the ridge, we suggest that the ridge subduction point (and resulting left-lateral shear zone) is never in one place long enough to enable the development of a through-going fault zone that can be identified at the ground surface, reconciling the mismatch between the GPS, seismological, and geological data in this region. Our conceptual model is supported by numerical modeling results. We also suggest that the along-strike change in subducting plate buoyancy explains the recent counterclockwise rotation of the Kyushu forearc documented in paleomagnetic studies, as is found in many other western Pacific subduction margins.

Stress measurements in deep boreholes have universally shown that stresses in the Earth’s crust are in equilibrium with favorably oriented faults with friction coefficients in the range 0.6‐0.7 and with nearly hydrostatic pore-pressure gradients. Because of the lack of any fault-adjacent heat-flow anomaly as predicted by a conductive model of frictional heating, the San Andreas fault has long been thought to be an exception, i.e., far weaker than this standard case. Borehole stress measurements near the San Andreas fault have failed to confirm this weak-fault hypothesis, being either inconclusive or in conflict with it. Directions of maximum horizontal stresses reported to be nearly fault normal in central California are now known not to be regional stresses but a result of active folding within folds that have been rotated 20°‐30° clockwise from their original orientations. Everywhere in southern California it is observed that the maximum stress directions rotate to smaller angles (30°‐60°) with the San Andreas, within 20 km of it. The sense of this rotation is opposite to that expected from the weak-fault hypothesis and indicates that the shear stress on the San Andreas is comparable in magnitude to all other horizontal stresses in the system. In the “big bend” section of the fault, this rotation is predicted from a transpressional plate-boundary model in which the San Andreas is loaded by a deep shear zone with a locking depth of 10 km. If the adjacent minor thrust faults are assumed to obey Byerlee friction, the crustal-average shear stress on the San Andreas in that region must be in the range 100‐160 MPa, regardless of the pore pressure in the fault. These stresses are many times greater than permitted by the weak-fault hypothesis. In the more transcurrent regions farther south, the San Andreas shear stress will be smaller than this estimate, but similar stress rotations observed there indicate that the San Andreas cannot be weak relative to minor faults in that region. These stress rotations can only be consistent with the weakfault hypothesis if it were assumed that all faults in California were equally weak, which is known to be untrue. The conclusion is that the heat-flow model is flawed, probably in its assumption that all heat transfer is governed by conduction.

4 pages, 3 figures, 1 table, The Cocos-Nazca spreading center is one of the few examples of the formation of a spreading center by splitting of oceanic lithosphere. It was created when the Farallon plate, broke up in the early Miocene following the collision of the Pacific-Farallon spreading center with the North American continent. Much of the ancient Farallon plate corresponding to the area of opening is lost to subduction beneath Central America and South America, but new data from the conjugate area on the Pacific plate allow the first detailed reconstruction of the break-up process. The opening began after chron 7 (25 Ma) at a location of focused crustal extension caused by overlapping spreading centers that had evolved in response to a slight reorientation of a Pacific-Farallon ridge segment. Beginning at chron 6B (22.7 Ma), eastward progressing seafloor spreading started along an axis that most likely migrated toward the region of weak lithosphere created by the Galapagos hotspot. By chron 6 (19.5 Ma), plate splitting from the spreading center to the trench was complete, allowing the fully detached Cocos and Nazca plates to move independently. This kinematic change resulted in a significant ridge jump of the newly established Pacific-Nazca spreading center, a change in plate motion direction of the Nazca plate by 20°clockwise, and a large increase in Pacific-Cocos plate velocity in the middle Miocene. © 2008 The Geological Society of America, This work was supported by the German Ministry of Education and Research (BMBF), project 03G0180A, and by National Science Foundation grant OCE-0242205

Ongoing plate convergence between India and Eurasia provides a natural laboratory for studying the dynamics of continental collision, a fi rst-order process in the evolution of continents, regional climate, and natural hazards. In southeastern Tibet, the fast directions of seismic anisotropy determined using shear-wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld. These observations are consistent with a clockwise rotation of material around the eastern Himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern Tibet. At the southeastern edge of the Tibetan Plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent E-W direction and a clockwise rotation of the surface velocity, surface strain fi eld, and fault network toward Burma. Around the eastern Himalayan syntaxis, the coincidence between structural crustal features, surface strain, and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust-mantle interface and that the lower crust is suffi ciently strong to transmit stress. At the southeastern margin of the plateau in Yunnan province, a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation. Lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the Himalayan orogen and Tibetan Plateau and require the development of three-dimensional models that incorporate lateral heterogeneity.

A small quantity of helically coiled spheroidal fossils has been recovered from acid digestion of phosphorite samples from the Ediacaran Doushantuo Formation, South China. These fossils consist of an internal body enclosed in a sculptured envelope that is very similar to that of Doushantuo animal eggs and blastula embryos such as Megasphaera ornata . A hallmark of these fossils is a three-dimensional spiral structure, which always consists of three clockwise coils, and occurs on both the envelope and the internal body. The spiral structure consists of a spiral tunnel or canal flanked by two raised levees, and it is punctured by a series of holes. Some specimens show evidence of uncoiling, invagination along the spiral structure, or bipectinate furrowing on the band between canals. A possible ontogenetic link between these helical spheroidal fossils and Megasphaera ornata is suggested by similar size, similar envelope sculptures, and co-occurrence. We tentatively interpret these fossils as postblastula embryos related to Megasphaera ornata . Thus, they may represent the most advanced embryonic fossils so far known from the Ediacaran, although their adult morphologies and phylogenetic affinity remain unknown.

Recent estimates of the rotation between Nubia and Somalia have resulted in disparate poles of rotation for the motion since 3.16 Ma (southwest of South Africa) compared with that since 11.03 Ma (near the east tip of Brazil). Here we use magnetic anomaly profiles unavailable in prior Nubia-Antarctica motion studies to significantly revise the estimate of the rotation between Nubia and Antarctica since 11.03 Ma. We use this newly estimated rotation to construct revised estimates of Nubia-Somalia, Pacific–North America, and India-Eurasia motion. The new Nubia-Somalia rotation indicates substantial displacement of Somalia relative to Nubia over the past 11.03 m.y.: 129 ± 62 km extensional, 90 ± 42 km right-lateral transtensional, and 52 ± 21 km right-lateral transtensional near the northern extremity of the East African Rift, the northern Mozambique Basin, and the Andrew Bain Fracture Zone complex, respectively. The substantial rotation between Nubia and Somalia implies that prior plate motion estimates based on a circuit through Africa are biased by 60–85 km at 11.03 Ma and perhaps by much more for earlier reconstructions. Our results imply that India-Eurasia motion since 11.03 Ma has been ∼12% (∼5 mm yr−1) slower than, and ∼20° clockwise of, estimates that neglect Nubia-Somalia motion. Our results further imply that Pacific–North America displacement since 11.03 Ma has been 5°–10° clockwise of prior estimates and require 58–75 km less extensional displacement across the Basin and Range since 11.03 Ma than inferred before.

We have reanalyzed the pattern of paleomagnetically detected rotations in the central Andes (the central Andean rotation pattern) in order to investigate the temporal and spatial distributions of rotations. The dominant pattern of rotation is well known with counterclockwise rotations in the northern Andes (clockwise in the southern Andes) linked to Neogene orogenesis and shortening. However, much of the rotation in the forearc of northern Chile (23–30°S) is distinctly anomalous because of markedly high rotations that appear to predate rotations observed elsewhere in the central Andes. We argue that the data define a domain located in the forearc of northern Chile marked by major (>25°) clockwise crustal rotations related to late Paleocene–early Eocene highly oblique convergence. This rather diffuse and cryptic deformation style accommodated strong shortening perpendicular to the Andean margin by vertical axis rotations rather than by conventional fold-thrust belts or transpressional fault systems.

Global positioning system (GPS) data from the central Andes record vertical axis rotations that are consistently counterclockwise in Peru and Bolivia north of the bend in the mountain belt, and clockwise to the south in southern Bolivia, Argentina, and Chile. These geologically instantaneous rotations have the same sense as rotations that have accrued over millions of years and are recorded by paleomagnetic and geologic indicators. The change in sign of the rotation at both decadal and million-year time scales occurs across the axis of topographic symmetry that defines the Bolivian orocline. When extrapolated to a common time interval, the magnitudes of rotation from geologic features and from GPS are surprisingly similar, given that a significant part of the instantaneous deformation field is probably elastic and due to interseismic locking of the plate boundary. Some of the interseismic deformation field must reflect permanent deformation, and/or some of the current elastic deformation will be converted to upper-plate permanent deformation over time rather than be recovered by elastic rebound during interplate earthquakes. We suggest that the spatial patterns of the elastic and the permanent modes of bending are similar because they are driven by the same stress field.

In the western Great Basin of North America, a system of dextral faults accommodates 15%–25% of the Pacific–North American plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this system. This young evolving part of the plate boundary offers insight into how strike-slip fault systems develop and may reflect the birth of a transform fault. A belt of overlapping, left-stepping dextral faults dominates the northern Walker Lane. Offset segments of a W-trending Oligocene paleovalley suggest ∼20–30 km of cumulative dextral slip beginning ca. 9–3 Ma. The inferred long-term slip rate of ∼2–10 mm/yr is compatible with global positioning system observations of the current strain field. We interpret the left-stepping faults as macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. The strike-slip faults end in arrays of ∼N-striking normal faults, suggesting that dextral shear diffuses into extension in the Great Basin. Coeval extension and dextral shear have induced slight counterclockwise fault-block rotations, which may ultimately rotate Riedel shears toward the main shear zone at depth, thus facilitating development of a throughgoing strike-slip fault.

Paleomagnetic analysis from lower Pleistocene (Sicilian) mudstones in the Sant’Arcangelo basin (Southern Apennines, Italy) shows no vertical-axis rotations. These results define the upper time constraint on the 238 counterclockwise rotations previously measured in the underlying lower Pleistocene (Santernian–Emilian) units of the Sant’Arcangelo basin. These rotations occurred before the Jaramillo subchron in a time span of ,0.5 m.y. Paleomagnetic rotations are coeval with the major phases of thrusting along the outer front of the Apennines, do not extend to the Adriatic foreland, and were limited to the hanging wall of the active thrust sheets along the outer front of the Apennines. Rapid counterclockwise rotations, coeval with thrusting and offset on left-lateral faults, represent the surface manifestation of the differential slab retreat and southwestward rollback of the trench in the Calabrian arc region. The unusually fast vertical-axis rotations indicate that the left-lateral component of deformation was particularly intense during the early Pleistocene in the Sant’Arcangelo region, located along the edge of the retreating subduction zone. These data suggest that the lateral breakoff of the African- Ionian-Adriatic subducting lithosphere, imaged by seismic tomography and deep seismicity, occurred in the Sant’Arcangelo region during the early Pleistocene.

The Walker Lane and northern part of the eastern California shear zone form a boundary zone accommodating differential motion between the Sierra Nevada and western Great Basin. Within the boundary zone, Global Positioning System velocities show a westward increase from 2–3 mm/yr in the central Great Basin to ∼14 mm/yr in the Sierra Nevada and a clockwise rotation from west-northwest to northwest. In the same region, incremental extensional strain axes recorded by earthquake focal mechanisms and fault-slip inversion show an east to west counterclockwise rotation of 50°, from parallel with the velocity field in the central Great Basin to nearly orthogonal to the velocity field along the eastern flank of the Sierra Nevada. Unlike plane-strain deformation within the central Great Basin, the progressive deviation between the orientation of extension axes and the velocity field in the boundary zone is a product of nonplane strain. The boundary zone records active constriction formed in conditions varying from wrench-dominated to extension-dominated transtension.

This reply touches on several issues that reflect Brown's and my different emphasis on the tectonic setting of granulites. Essentially, Brown suggests that the tectonic switching model should produce counterclockwise pressure-temperature-time ( P - T - t ) paths, then argues that most documented

Active strike-slip and normal faults along the eastern margin of the Sierra Nevada primarily accommodate northwestward translation of the Sierra Nevada–Central Valley (i.e., Sierran) microplate with respect to stable North America. Strike-slip faults bordering the eastern Sierran microplate are subparallel to small circles about the Sierra Nevada–North American Euler pole. Normal faults of the Sierra Nevada frontal fault system strike ∼45° clockwise of the small circle trajectories and exhibit well-defined, left-stepping en echelon patterns, consistent with formation in dextral transcurrent regime. Major graben bordering the northeastern Sierran microplate are located in regions where the locus of range-front deformation steps abruptly eastward in a releasing geometry relative to Sierra Nevada–North American motion. Crustal shortening occurs at the northern end of the Sierran microplate, where a component of northwest dextral shear steps westward in a left-restraining geometry across the Sierran crest to the Sacramento Valley. Kinematic inversions of earthquake focal mechanisms from the Walker Lane belt bordering the eastern Sierra Nevada indicate that seismogenic deformation primarily is characterized by horizontal shearing and oblique crustal thinning. Directions of macroscopic dextral shear inferred from the inversions are subparallel to the trajectories of small circles about the Sierra Nevada–North American Euler pole. Normal faulting along most of the eastern Sierran range front thus appears to primarily accommodate microplate translation rather than Sierran uplift or regional Basin and Range extension.

The tectonic convergence between the South China block and Qinling belt that occurred in Mesozoic time was not homogeneous along strike; it was mainly concentrated in the Shengnongjia and Hannan domes, which formed a pair of indentors, along which the South China block penetrated the Qinling belt. As a consequence, the Tongbai‐Dabie Shan belt, the eastern part of the Qinling orogenic belt, may have undergone large-scale eastward extrusion, clockwise rotation, and uplift, as indicated by its boundary deformation features. It was originally located beneath the narrowest part of the Qinling belt, where it underwent ultrahigh-pressure metamorphism in the Early Triassic.

The Charleston-Nebo salient is highly convex toward the foreland of the Sevier fold-thrust belt in Utah. To determine the importance of vertical-axis rotations to the kinematic history of the salient, paleomagnetic samples were collected from red Triassic strata of the Woodside Shale and Ankareh Formation from 68 sites distributed around the curvature of the salient. The paleomagnetic data indicate counterclockwise rotation of 33.3° ± 4.8° in the northern part of the salient and clockwise rotation of 66.7° ± 5.1° in the southern part of the salient. These data support divergent-flow models for thrust-salient development, augmented by relatively minor contributions from original basin geometry and local strike-slip along salient margins.

Vertical-axis, clockwise block rotations in the Northeast Mojave block are well documented by numerous authors. However, the effects of these rotations on the crust to the north of the Northeast Mojave block have remained unexplored. In this paper we present a model that results from mapping and geochronology conducted in the north and central Owlshead Mountains. The model suggests that some or all of the transtension and rotation observed in the Owlshead Mountains results from tectonic response to a combination of clockwise block rotation in the Northeast Mojave block and Basin and Range extension. The Owlshead Mountains are effectively an accommodation zone that buffers differential extension between the Northeast Mojave block and the Basin and Range. In addition, our model explores the complex interactions that occur between faults and fault blocks at the junction of the Garlock, Brown Mountain, and Owl Lake faults. We hypothesize that the bending of the Garlock fault by rotation of the Northeast Mojave block resulted in a misorientation of the Garlock that forced the Owl Lake fault to break in order to accommodate slip on the western Garlock fault. Subsequent sinistral slip on the Owl Lake fault offset the Garlock, creating the now possibly inactive Mule Springs strand of the Garlock fault. Dextral slip on the Brown Mountain fault then locked the Owl Lake fault, forcing the active Leach Lake strand of the Garlock fault to break.

A new three-dimensional tomographic velocity model and depth-converted reflection images of the melt sills beneath the 9°03′N overlapping spreading center on the East Pacific Rise show that the upper crustal construction at this ridge discontinuity is highly asymmetric with reference to the bathymetric ridge crests of the overlapping limbs. Despite the similarly curved ridge crests, the asymmetries are markedly different under the two limbs and appear to be related to the contrasting evolutionary history of the limbs. The overlap basin is closely related to the propagating eastern limb in terms of its seismic structure. By contrast, the western limb forms a distinct morphologic region that displays little structural relationship to the adjacent overlap basin and other relict basins. As the overlapping spreading center is migrating southward, the differential development of melt sills and ridge crests may be inferred from the results of this study. Ridge propagation appears to involve two major processes: the advancement of the melt sill at the ridge tip and the development of ridge-crest morphology and the neovolcanic axis to the north of the overlap basin region near the existing propagating limb. The latter process may result in the abandonment of the current neovolcanic axis, leading to the self-decapitation of the propagating limb. By contrast, the self-decapitation of the western limb is related to the receding melt sill, which lags behind the anticlockwise rotational motion of the ridge crest.

Paleomagnetic data were obtained from Tertiary red sedimentary rocks at two locations separated by several hundred kilometers within the Qaidam Basin. In the east-central part of the basin, 30 sites from the lower Pliocene Youshashan Formation yielded characteristic remanent magnetization (ChRM) directions with intermediate unblocking temperatures (100‐600 8C); ChRM with high unblocking temperatures (to 680 8C) was isolated from 14 sites. In the same area, ChRM directions were obtained from six sites within the Oligocene Lower Gancaigou Formation. Characteristic magnetization was also determined from 16 sites within the Lower Gancaigou Formation exposed in the E Bo Liang range of the north-central Qaidam Basin. When compared with equivalentage expected directions for Eurasia, the mean paleomagnetic directions indicate no Neogene vertical-axis rotation of the Qaidam Basin or the Altyn Tagh fault. The Qaidam Basin may act as an indentor translating without rotation toward the North China block and driving clockwise vertical-axis rotations by differential shortening within the Nan Shan fold-and-thrust belt.

Paleomagnetic results from 31 Neogene sites in the Peruvian Andes yield primary magnetizations, as demonstrated by positive fold and reversal tests. Strata dated as 18–9 Ma record a significant counterclockwise rotation (−11° ± 5°), whereas unconformably overlying younger strata (7–6 Ma) are not rotated. The age of rotation thus is between 9 and 7 Ma, a period that coincides with the widespread Quechua 2 deformation phase. Moreover, eight independent studies on 107–9 Ma rocks from Peru between 9°S and 15°S reveal similar and significant rotations (−15° ± 6°). This suggests that the region rotated during a 2 m.y. period of deformation ca. 8 Ma, when the Andes underwent rapid uplift and important deformation commenced in the Subandean zone.

Four to six years of continuous measurements at 10 permanent Global Positioning System sites in the Western Alps show horizontal residual velocities of ,2 mm/yr with respect to stable Europe; uncertainties range from 0.3 to 1.4 mm/yr. These velocities and the associated strain-rate field indicate that the central part of the range is currently dominated by east-west extension, whereas the southern part shows north-south to northwest-southeast compression. The geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the Adriatic microplate with respect to Eurasia. This rotation, together with the arcuate shape of the contact between the Adriatic microplate and the Alps, induces dextral shear kinematic boundary conditions across the Western Alps, with an additional divergence component in their central part and in Switzerland, and a convergence component in their southern part.

Quartz diorite dikes in the footwall of the 1.85 Ga Sudbury impact structure were emplaced within fractures formed during hypervelocity impact. The anisotropy of magnetic susceptibility of the radial Copper Cliff dike reveals a steeply dipping foliation that is counterclockwise oblique along both the eastern and western sides of the dike, and a steeply northwest plunging lineation. The data suggest that the Copper Cliff dike acted as a melt-filled dextral transfer fault during collapse of the inner rim of the central peak ring of the impact crater to accommodate ultrahigh-strain-rate displacements. Peak-ring rim collapse may have caused sulfide-rich melts to be injected into the Copper Cliff dike, leading to the formation of large Cu–Ni–platinum group element deposits.

Volcanic and sedimentary strata of the Late Cretaceous MacColl Ridge Formation were sampled and demagnetized to reevaluate the paleomagnetically derived paleolatitude of the allochthonous Wrangellia terrane. Characteristic directions from 15 sites representing ∼750 m of the MacColl Ridge Formation (80 Ma) reveal a reversed-polarity primary magnetization yielding a paleomagnetic pole at 126°E, 68°N, A 95 = 9°. Comparison of this pole with the Late Cretaceous reference pole for North America indicates 15° ± 8° of latitudinal displacement (northward) and 33° ± 25° of counterclockwise rotation. In contrast to previously reported low paleolatitudes (32° ± 9°N) for the MacColl Ridge Formation, these new results place the Wrangellia terrane at a moderate paleolatitude (53° ± 8°N) in the Late Cretaceous.

Relict high-pressure granulite-facies assemblages have been discovered in mafic sheets within Neoproterozoic Moine Supergroup psammites of northern Scotland. Minerals and fabrics associated with the main 440–430 Ma Caledonian deformation and metamorphism (D2) overprint these assemblages, which preserve peak pressure-temperature ( P-T ) conditions of 11–12 kbar at 650–700 °C, indicating burial to ∼35 km. The maximum age of the sheets is constrained by the post-Grenvillian ca. 1000 Ma age of the youngest detrital zircons in the host psammites. The high- P assemblage is constrained by the age of anatexis in the psammites at 467 ± 10 Ma. Assembling these data with P-T and chronological data for the D2, D3, and later cooling events, two partial clockwise P-T-t-D (time-deformation) paths emerge, demonstrating that the high- P event is not part of the evolution of the P granulite assemblages are interpreted to be a relict of an earlier, Taconic 470–460 Ma history. This new evidence suggests that the tectonic evolution of the Caledonian nappes east of the Moine thrust is more complex than previously thought.

The Hosgri fault is located immediately offshore of south-central California and is part of the transform boundary between the Pacific and North American plates. This fault terminates to the southeast into east-trending folds and reverse-separation faults of the western Transverse Ranges. Our new structure-contour maps of deformed horizons show a spatial relationship between faulting and folding consistent with right-lateral slip. Restoration of these digital maps quantifies post-Miocene right-lateral slip across the southern Hosgri fault to be 3.5 km. This slip is absorbed by folding, thrust overlap, and clockwise vertical-axis rotation of elongate blocks between strands of the fault. The restored part of a block located to the east has rotated 8° clockwise. Extrapolating this restored rotation to the 50 km block length produces an estimate of 7 km of dextral shear for a total, including the 3.5 km fault slip, of 10.5 km of post-Miocene displacement. Our three-dimensional approach precludes interpretations for reverse slip on the Hosgri fault, and is not consistent with models for more than 80 km of late Cenozoic right-lateral fault slip.

The Early Cretaceous northeast Brazilian Sergipe microplate, formed at the northern end of the South Atlantic rift between South America and Africa, closely resembles the modern Sinai microplate at the northern end of the Red Sea in size, shape, and relative motion. Both formed where east-northeast–trending transverse shear zones arrested northward rift propagation, causing the Tucano-Reconcavo and Gulf of Suez rifts to fail and be replaced by northeast-trending leaky transforms (Sergipe-Alagoas and Dead Sea transforms) as the new paths of continental breakup. Bordered by the failed rift, the leaky transform, and the transverse shear zone, both microplates were rotated counterclockwise by drag along their eastern transform margins. Rotation thrust the edge of the Sergipe microplate over part of its northern border, creating the Arcoverde thrust wedge. The northwest-trending Vaza-Barris fault sheared the microplate, transferring the rift and evaporite sequence from the Sergipe-Alagoas to the Gabon continental margin. In Albian time, heating of the lithosphere in the Cabo igneous province near Recife permitted the South Atlantic rift to propagate across the Arcoverde thrust wedge, completing continental breakup.

Well-preserved eclogitic rocks near Faro (Yukon-Tanana terrane, Canada) underwent a prograde evolution from ~510 °C and ♢1.1 GPa to 690 °C and ♢1.5 Gpa followed by nearisobaric cooling to ~540 °C. A remarkable feature of the garnet porphyroblast cores is the presence of minute garnet “inclusions” of distinctly different composition. Preservation of a sharp compositional gradient at the interface between the host and the inclusion garnets and results of diffusion modeling indicate that the counterclockwise pressure-time evolution took place on a very short time scale of about 0.2 m.y. Minimum rates of heating (950 °C/m.y.) and burial (7 cm/yr) calculated along the prograde part of the path are in good agreement with values extracted from thermal models of newly initiated subduction zones.

The Andean deformational pattern of the northern Chilean Precordillera between 26°30′ and 27°00′S suggests significant influence of an orogen-oblique (northwest-trending) pre-Andean basement structure. The left-lateral reactivation of this structure and the associated deformation can be explained by clockwise vertical-axis rotations of forearc-wide crustal blocks during the middle-late Eocene (ca. 39 ± 3 Ma) induced by the coupling stress due to oblique northeastward subduction of the oceanic Farallon plate beneath the South American continent. The compressive stress at the leading edges of the blocks may have been accommodated 200 km away from the Peru-Chile trench in what is now the Precordillera, where the magmatic arc was then located. The postulated large-scale domino mechanism explains the paleomagnetically observed in situ block rotations, as well as the orientation, segmentation, and shear sense of major structural elements within large parts of the present forearc in northern Chile.

Neogene deformation, paleomagnetic rotations, and sparse geodetic data suggest the Cascadia fore arc is migrating northward along the coast and breaking up into large rotating blocks. Deformation occurs mostly around the margins of a large, relatively aseismic Oregon coastal block composed of thick, accreted seamount crust. This 400-km-long block is moving slowly clockwise with respect to North America about a Euler pole in eastern Washington, thus increasing convergence rates along its leading edge near Cape Blanco, and creating an extensional volcanic arc on its trailing edge. Northward movement of the block breaks western Washington into smaller, seismically active blocks and compresses them against the Canadian Coast Mountains restraining bend. Arc-parallel transport of fore-arc blocks is calculated to be up to 9 mm/yr, sufficient to produce damaging earthquakes in a broad deformation zone along block margins.

Inversions of earthquake focal mechanisms for brittle strain show that the dextral Hayward fault forms the western boundary of a distinct seismotectonic domain in the eastern San Francisco Bay region. East of the Hayward fault, the seismogenic strain field is characterized by subhorizontal extension oriented northwest-southeast and subhorizontal shortening oriented northeast-southwest. Major folds and thrust faults east of the Hayward fault are oriented normal to the direction of maximum shortening, but have formed in a predominantly strike-slip tectonic setting and exhibit a right-stepping, en echelon geometry typical of dextral wrench folds. The direction of maximum right-lateral shear strain in this domain is ∼N20°W, which generally is parallel to the strike of the dextral Calaveras, Greenville, and Concord faults, but oriented ∼15°–25° more northerly than the strike of the San Andreas and Hayward faults to the west. The clockwise rotation of maximum right-lateral shear strain from west to east across the Hayward fault is confirmed by geodetic measurements. The kinematic consequences of this variation in regional strain for deformation east of the Hayward fault include the following. (1) Dextral faults must form left-restraining contractional stepovers in order to maintain continuity and conserve slip along strike, thus creating localized fold-and-thrust belts. (2) Dextral faulting accommodates a component of shortening normal to the boundary between the Pacific plate and the Sierra Nevada–Central Valley microplate, thus obviating the need for a laterally continuous, boundary-parallel zone of thrust faulting along the eastern margin of the Coast Range (Coast Ranges–Sierran block boundary zone) to accommodate plate-normal motion at the latitude of the East Bay domain. The transpressional kinematics of the East Bay domain may be present in other obliquely convergent orogenic belts, and they have implications for seismic hazard assessment in the highly urbanized San Francisco Bay area.

Global Positioning System measurements performed over a 2–4 yr period confirm the Xianshuihe-Xiaojiang fault system as one of the primary active structures in southwest China. Stations southwest of these faults show southerly motions of 5–15 mm/yr relative to the western Sichuan basin, and stations in a 200 km geodetic network located northwest of the basin move at only 0–5 mm/yr. These results are consistent with clockwise rotation of southwestern Sichuan and western Yunnan about the eastern Tibetan syntaxis, accommodated by left-lateral slip of 12 ± 4 mm/yr on the Xianshuihe-Xiaojiang fault system. The results imply that if eastward extrusion of crustal material from the plateau occurs at present, it is not accommodated by east-west shortening along the margin of the plateau and must involve wholesale eastward motion of low-lying regions to the east.

Newly compiled Russian and U.S. seismological data support an independent Bering block in motion relative to the North American plate. This motion is likely to be driven by the westward extrusion of southwestern Alaska, resulting from compression in southern Alaska due to subduction of the Pacific plate and terrane accretion. Seismicity extends from central Alaska, through the Bering Strait, and into Chukotka. In eastern Chukotka several southwest trends are evident, some of which continue through the Koryak Highlands to Kamchatka. The seismicity outlines the Bering block, which includes most of the Bering Sea, Chukchi Peninsula, Seward Peninsula, and parts of western Alaska. Focal mechanisms, young basaltic volcanism, and normal faults in western Alaska and Chukotka indicate that the Bering Strait is under northeast-southwest extension. This, in conjunction with thrust faulting in the Koryak Highlands, indicates that the Bering block is rotating clockwise relative to the North American plate.

The 500-km-long Late Jurassic Independence dike swarm in California extends from the central Sierra Nevada through the Mojave Desert to the Eastern Transverse Range. The dikes are mostly vertical, and their azimuth in the Sierra Nevada is ∼ 315°. However, in the Mojave region, their azimuth varies significantly: 300°–320° in the Argus Range, the Spangler Hills, and the Stoddard Well and Soda Mountain areas, and 340°–350° in the Granite Mountain, Alvord Mountain, and Eagle Mountain areas. Apparently the dikes in the Granite, Alvord, and Eagle mountains areas have rotated ∼ 50° clockwise about vertical axis relative to the Sierra Nevada and the Stoddard Well area. These rotations are remarkably consistent with known post–early Miocene paleomagnetically derived rotations. The clockwise rotations are associated with the east-west–trending left-lateral strike-slip faults in the northeastern Mojave and the Eastern Transverse range regions. In contrast, the data show that little or no rotation occurred in the central Mojave domain, which is characterized by northwest-trending faults.

Structural, petrological, and geochronological data from the middle Korean peninsula indicate that the Qinling-Dabie-Sulu collisional belt of east-central China crosses the Yellow Sea and extends into the Imjingang belt. The Yeoncheon complex, first identified as the western Imjingang belt, comprises primarily north-dipping metamorphic sequences: (1) the northern Jingok unit, consisting of Barrovian-type metapelites, and (2) the southern Samgot unit, consisting of calc-silicate and amphibolitic rocks. South-vergent structures with reverse-sense shearing are dominant in the Jingok unit, whereas late normal-sense shearing is pervasive in the Samgot unit and the deformed granitoid to the south. These structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt. Pressure-temperature ( P-T ) estimates from the amphibolites suggest a high-P amphibolite-facies metamorphism (8–13 kbar and 630–790 °C), possibly evolving from eclogite facies conditions along a clockwise P-T path. Sm-Nd and Rb-Sr geochronological data suggest that the amphibolites emplaced in Late Proterozoic time were metamorphosed during Permian-Triassic time.

A plate reconstruction of the Bay of Biscay to a predrift stage is proposed on the basis of (1) an Aptian-Albian tectonic pattern of the Basque-Cantabrian region in northern Spain, which suggests major left-lateral strike-slip movements, and (2) a continental margin that matches predrift stages. Plate reconstruction results from the juxtaposition of the two continental margins of the Bay of Biscay, so that the eastern North Spanish margin lies laterally adjacent to the southwest French margin (Landes Plateau) and the western North Spanish margin borders the eastern part of the Meriadzek Terrace in the Armorican margin. The restoration obtained is supported by both the implied amount of clockwise rotation of Iberia (37°, very close to the 35° suggested by paleomagnetism) and the positive matching of north-south-oriented transform structures from the South Biscay margin with northeast-southwest-oriented transform structures from the North Biscay margin.

Proterozoic rocks of the southwestern United States exhibit a variety of tectonometamorphic histories that can all be explained by a characteristic looping style of pressure-temperature ( P-T ) path. The path is fundamentally clockwise. However, because the rocks were cooled isobarically at 3 kbar after tectonism, the cooling path crosses the prograde path, leading to a geometry with a counterclockwise character. The absolute size of the loop in the P-T path varies across the region because of different amounts of overthickening (and resulting exhumation) during the contractional event, combined with variable amounts of heating from syntectonic plutons. This style of looping P-T path may be characteristic of high- T , low- P metamorphic terranes and middle crustal tectonism, particularly where syntectonic plutons contribute to elevated geotherms and polymetamorphism and where rocks become stabilized and reside for long periods at middle-crustal depths before being exhumed during a later tectonic event.

Geologic and seismic data reveal a set of parallel, active, strike-slip faults in east-central Alaska between the Denali and Tintina fault systems. The faults strike northeast to north-northeast, at a high angle to the bounding dextral fault systems, and exhibit sinistral slip. We hypothesize that this set of faults divides the crust into long blocks that are rotating clockwise in response to northerly compression resulting from Pacific–North American plate convergence. We suggest that these faults have produced most of the large historical earthquakes in east-central Alaska between the Alaska Range and the Yukon River.

Plate reconstructions indicate that early Miocene Pacific–North American relative motion was not parallel to the transform, but was instead oblique and transtensional. We propose that the western edge of the North American plate east of the transform gravitationally collapsed to the southwest in response to this divergence. The northern and eastern edges of the collapsed region underwent extension between 24 and 20 Ma. Whereas extension in southeastern California and southern Arizona during this interval was coaxial with the overall S50°–60°W collapse direction, the extension direction along the northern oundary (Mojave extensional belt) was north-south and the result of the combined effects of the southwestward collapse and the independent northwestward translation of the region to the north. Collapse continued between 20 and 16 Ma, and extension persisted in southeastern California and southern Arizona; extension along the northern boundary waned owing to the apparent cessation of motion of the region to the north. Thus, the kinematics of the northern boundary changed to dextral shear and extension along what is here named the “Trans–Mojave-Sierran shear zone.” This east-west zone of mainly right shear oroclinally folded the southern tail of the Sierra Nevada and the central Mojave Desert; rocks within this 83–110-km-wide zone were rotated (Approx.)40°–60° (clockwise) about vertical axes. Finally, as collapse moved the North American plate progressively west across the locus of transform shear, its western edge became increasingly subject to the motion of the Pacific plate. Over time, the edge of North America was truncated by faults of the San Andreas system and transferred to the Pacific plate.

Staurolite with a complex internal metamorphic and structural relationship occurs in metapelite from the Silvretta thrust sheet (eastern Alps). Stepwise leaching of a core and rim fraction of a single staurolite crystal resulted in a total Pb-Pb isochron of 308 ± 28 Ma. The Variscan metamorphism has been confirmed by a further single bulk staurolite Pb-Pb age of 298 ± 42 Ma, which is in accordance with U-Pb ages of 306 ± 8 Ma and 304 ± 6 Ma, respectively, obtained from combined U-Pb leaching of the same crystals. Direct dating of the staurolite-in reaction isograd was therefore successful. Our results date the prograde part of a Variscan counterclockwise pressure-temperature path in the Silvretta thrust sheet. Proximity with white-mica Rb-Sr and K-Ar cooling ages suggests that the whole Variscan episode was fast and took place within a few million years.

Paleomagnetic data from Pliocene sedimentary strata in the south Wanganui basin, New Zealand, document clockwise tectonic rotation west of and increasing toward the main zone of deformation (axial tectonic belt) associated with the Australia-Pacific plate boundary zone in the North Island of New Zealand. Measured rotation with respect to the Australian plate increases from 1.4° ± 3.9° 80 km west of the axial tectonic belt to 6.5° ± 2.0° and 13.2° ± 3.1° at 45 km and 20 km west of the axial tectonic belt, respectively. These measurements indicate that (1) deformation associated with plate convergence occurs across a wide region up to 270 km west of the Hikurangi Trench, and (2) in the past 2.5 m.y. at least 30 km of transcurrent motion and 10 km of shortening at the plate boundary have been accommodated within the south Wanganui basin by distributed shear west of the plate boundary zone proper. The data support a model suggesting that the Wanganui basin is a lithospheric flexure basin driven by high friction at the plate interface. This strain in the south Wanganui basin and inferred high friction across the plate interface at the basin9s eastern margin have occurred since 2.5 Ma.

Paleomagnetic data from latest Miocene to Pliocene sedimentary rocks in southeast Kyushu, as well as those previously reported from middle to late Miocene formations, show that a large part of south Kyushu has been rotated ∼30° counterclockwise with respect to north Kyushu and Eurasia during the past 2 m.y. This rotation started synchronously with rifting of the continental crust beneath south Kyushu and the northern Okinawa trough, which commenced around the Pliocene-Pleistocene boundary and continues to the present. The coincidence of rotation and extension implies that arc rotation can take place during rifting of continental crust.

Comparison of a new ∼1700–1600 Ma segment of the Proterozoic apparent polar wander path (APWP) for Australia with the time-equivalent segment of the North American APWP that has been rotated clockwise by 117° about the Euler pole, long 100°E, lat 38°N, superposes the North American APWP onto the Australian APWP and shows the segments to be similar. The same rotation makes the Pacific margins of the North American and Australian cratons adjacent, as predicted by the Southwest U.S.–East Antarctic (SWEAT) hypothesis, but North America is located farther north relative to Australia than originally suggested. However, the reconstruction is consistent with the identification of western sediment sources for the Belt-Purcell basin in western North America, with matching of basement provinces, and with correlation of major lineaments of the two continents. The rotation gives only very broad agreement between the younger Proterozoic pole sets. This is probably partly due to the sparseness of poles on the Australian APWP whereby prominent features that are found on the North American APWP, such as the Grenville loop, are eliminated on the Australian path by smoothing. Nevertheless, some discrepancies between the younger Proterozoic poles cannot be accounted for without revision of the APWPs.

Structural development of the Altiplano during the past 10 m.y. involved crustal thickening and enhanced curvature of the central Andes. Oroclinal bending of 30° explains reasonably the pattern of vertical-axis rotations observed from units with ages 10 to 30 Ma. New paleomagnetic data from Late Cretaceous through Paleocene strata of southeastern Peru and the Eastern Cordillera of Bolivia document rotations that are also counterclockwise on the north limb of the orocline and clockwise on the south limb. However, the rotations of units with ages 30 to 70 Ma are more variable; nearby areas have undergone differential vertical-axis rotations of up to 60° . These larger rotations that have affected 30–70 Ma units in the central Andes may represent (1) local rotations during late Oligocene to present orogeny, (2) local rotations during Eocene orogeny, and/or (3) oroclinal bending during Eocene orogency.