ABSTRACT The Franciscan Complex of California, the type example of an exhumed accretionary complex, records a protracted history of voluminous subduction accretion along the western margin of North America. Recent geochronological work has improved our knowledge of the timing of accretion, but the details of the accretionary history are disputed, in part, due to uncertainties in regional-scale correlations of different units. We present new detrital zircon U-Pb ages from two sites on opposite sides of San Francisco Bay in central California that confirm previously proposed correlations. Both sites are characterized by a structurally higher blueschist-facies unit (Angel Island unit) underlain by a prehnite-pumpellyite-facies unit (Alcatraz unit). The Angel Island unit yields maximum depositional ages (MDAs) ranging from 112 ± 1 Ma to 114 ± 1 Ma (±2σ), and the Alcatraz unit yields MDAs between 94 ± 2 Ma and 99 ± 1 Ma. Restoration of post-subduction dextral displacement suggests these sites were originally 44–78 km apart and much closer to other Franciscan units that are now exposed farther south in the Diablo Range. Comparison with detrital zircon dates from the Diablo Range supports correlations of the Bay Area units with certain units in the Diablo Range. In contrast, correlations with Franciscan units in the northern Coast Ranges of California are not robust: some units are clearly older than those in the Bay Area whereas others exhibit distinct differences in provenance. Integration of age data from throughout the Franciscan Complex indicates long-lived and episodic accretion from the Early Cretaceous to Paleogene. Although minor, sporadic accretion began earlier, significant accretion occurred during the interval 123–80 Ma and was followed by minor accretion at ca. 53–49 Ma. Periods of accretion and non-accretion were associated with arc magmatism in the Sierra Nevada–Klamath region, cessation of arc activity, and reorganization of paleodrainage systems, which implicates plate dynamics and sediment availability as major controls on the development of the Franciscan Complex.

The tectonic evolution of the South Island of New Zealand records the consequences of a transition from nearly translational to transpressional plate motions since the Late Miocene. Although it is clear how that transition was accommodated in the upper crust—primarily through the development of the Southern Alps orogen—how the lithospheric system responds to this change in plate kinematics is unclear. Coupling kinematic and deformational modeling with an analysis of the existing seismic data that images the deformational fabric in the lithospheric mantle leads me to propose that a substantial amount of the plate boundary strain is accommodated by a reorientation of the plate boundary structure and maintenance of simple shear deformation as plate motions change. This leads to a developing geographical mismatch between the location of upper crustal strain and that within the lower crust and lithospheric mantle. One possible result of this offset in the locus of plate boundary strain can be the development of a detachment surface within the lower crust that effectively decouples the upper crust from its underlying foundation. Consequences of this include different styles of deformation of the lower crust with position along the orogen and significantly different strain histories for crustal rocks involved in the Southern Alps orogen as a function of whether they are part of the thin-skinned or thick-skinned regime.

The majority of the San Andreas fault zone is convergently oblique to relative plate motion. The commonness of transpression makes it significant for understanding deformation of the continental lithosphere. We have quantified the distribution of transpressional deformation along the San Andreas fault zone with respect to variations in boundary conditions along its length and distance from the fault zone itself. Rock uplift was used as a proxy for transpressional deformation. The pattern of exhumation along the fault was synthesized based on previously determined apatite fission-track and (U-Th)/He ages from 210 locations within 40 km of the fault trace. Patterns of mean elevation and slope in swaths along the fault were used as rough proxies of surface uplift and erosion. Relatively higher exhumation rates and mean elevations occur most commonly along the most oblique sections of the fault, such as in the Transverse Ranges. The highest rates of exhumation (>0.5 mm/yr) and highest and steepest topography also occur almost exclusively in the near field (i.e., within ∼10 km) of the fault trace. These trends are consistent with the strain-partitioning model of transpression, in which distributed deformation is concentrated in the fault zone and the degree of partitioning between simple and pure shear is a function of obliquity. However, the pattern of rock uplift also exhibits considerable variability. Neither the degree of obliquity nor the distance to the fault trace is enough to predict where high exhumation or mean elevation will occur. This suggests that heterogeneity in boundary conditions, including mechanical weaknesses and variations in erodibility, is equally important for controlling the pattern of transpressional deformation.

The Niğde Massif, south-central Turkey, experienced two complete cycles of burial and exhumation during orogenesis and is, therefore, an excellent example of yo-yo tectonics. We propose that burial and exhumation of the metamorphic basement and, in the second cycle, the basement and its sedimentary cover rocks, were driven largely by transpression and transtension in an intracontinental strike-slip zone. The eastern margin of the massif, where it is adjacent and subparallel to the sinistral Central Anatolian fault zone, is comprised of Upper Cretaceous basement that was the source of, and is unconformably overlain by, early Tertiary sedimentary rocks. The contact between the Tertiary rocks and basement is an unconformity that is locally sheared and characterized by a low-angle oblique-normal shear system with cataclasite in the basement and brittle-ductile shear zones in the sedimentary rocks. These relationships, documented by geo/thermochronology to encompass 80 million years, define the timing and magnitude of the yo-yo process: burial and heating of Mesozoic sedimentary rocks during Late Cretaceous transpression to form the high-grade metamorphic basement (peak metamorphism at 85–91 Ma); Late Cretaceous (ca. 80–60 Ma) unroofing by transtension and erosion, with early Tertiary deposition of massif-derived clastic material at the edge of a marine basin along the Central Anatolian fault zone; reburial of basement and cover rocks involving folding, shearing, and greenschist facies metamorphism of the sedimentary cover in late Eocene through early Miocene time (ca. 50–20 Ma); and final exhumation in the middle Miocene (17–9 Ma) along strike-slip and normal faults.

The Walker Lane, a zone of northwest-striking dextral faults east of the Sierra Nevada, accommodates 15%–25% of Pacific–North American plate motion. A distinctive feature of the Walker Lane is the coexistence of parallel dextral and normal faults, which either developed sequentially or are related by strain partitioning. In the northern Walker Lane, three en echelon dextral faults strike northwest parallel to relative motion between the Sierra Nevada and Great Basin. Each fault cuts major basins but is parallel to and 1–5 km basinward of range-front normal faults. Basins in this area have anomalous northwest trends, whereas basins and major normal faults in the adjoining Basin and Range province trend north to north-northeast. Major normal fault systems straddling the northern Walker Lane dip toward one another, thereby producing a structural low. Significant exhumation related to strike-slip faulting is restricted to one, more westerly striking, probably transpressional segment of one dextral fault. Geologic data suggest that northwest-striking range-front faults were active during a ca. 3 Ma episode of extension but have been inactive in the Quaternary. Strike-slip faulting probably started immediately after 3 Ma, either cutting or reactivating deeper parts of the northwest-striking range-front normal faults as dextral faults. North-striking normal faults are active and kinematically compatible with the northwest-striking dextral faults. The unusual northwest strike of normal faults in the northern Walker Lane may reflect reactivation of a major northwest-striking basement structure indicated by gravity data. Strain partitioning between parallel dextral and normal faults is unnecessary because the dextral faults parallel Sierra Nevada–Great Basin motion and can take up all required displacement. In contrast, Sierra Nevada–Great Basin motion in Owens Valley in the southern Walker Lane is strongly oblique to faults, so strain partitioning between parallel dextral and normal faults may be necessary.

Paleomagnetic and geochronologic data from mafic intrusive rocks, inferred to contain magnetizations of early Late Cretaceous age, and upper Tertiary volcanic rocks, all part of the upper plate of the Silver Peak extensional complex in the southern Silver Peak Range, add to the growing body of results suggesting that Neogene displacement transfer within the central Walker Lane involved components of modest magnitude crustal tilting and, at least locally, rotation of structural blocks. Mesozoic intrusions and upper Tertiary volcanic rocks yield paleomagnetic data that are discordant to expected field directions. The data from 49 accepted sites in mafic dikes that cut granitic rocks, 4 sites in a single Oligocene(?) ash flow tuff, 20 sites in mid-Miocene andesite flows, and 28 sites in upper Miocene to lower Pliocene pyroclastic rocks may imply a systematic progression in the magnitude of vertical axis rotation and tilting with age. At a minimum, the data are consistent with at least some 20° of clockwise rotation of upper-plate rocks in this part of the Silver Peak Range and demonstrate a greater regional extent to the area affected by clockwise rotation during Neogene displacement transfer. Eight new 40 Ar/ 39 Ar age determinations from the mafic dikes and adjacent host rocks, all somewhat disturbed age spectra, imply that these rocks cooled below ∼300 °C during the Late Cretaceous between about 90 and 80 Ma. Four mafic dike groundmass concentrates yield integrated apparent ages between 86.31 Ma ± 0.12 Ma and 80.80 Ma ± 0.11 Ma, and four age spectra from biotite from the host granite yield integrated values between 93.6 ± 0.9 Ma and 78.6 Ma ± 0.2 Ma. The mafic dikes yield in situ exclusively normal polarity results consistent with an early Late Cretaceous age of magnetization acquisition, with an overall group mean (D = 25.1°, I = 55.4°, α 95 = 3.4°) that is discordant to an early Late Cretaceous expected field (D = 337°, I = 66°). Ten of 20 sites from steeply dipping mid-Miocene andesite flows and 21 of 28 sites in gently tilted upper Miocene ash flow tuffs yield overall stratigraphically corrected group means (D = 24.4°, I = 36.7°, α 95 = 7.1°) and (D = 16.5°, I = 53.5°, α 95 = 7.6°, respectively) that are discordant in a clockwise sense to the Miocene expected direction (D = 358°, I = 55°). The paleomagnetic data support a history of tilting and vertical axis rotation of the southern Silver Peak Range, most of which occurred coincidently with latest Miocene and Pliocene exhumation of the lower-plate rocks in the extensional complex. In addition, it is possible that the paleomagnetic data from Mesozoic intrusions record an additional, modest phase of deformation that predated development of the extensional complex. The observations are consistent with a tectonic model where deformation of upper-plate rocks in this area involved a small component of west- to southwest-side-down tilting, likely related to range-scale folding during the late Miocene and Pliocene, accompanied by modest clockwise vertical axis rotation.

We have examined the deformation associated with a right-releasing stepover along the dextral Walker Lane belt where it traverses Wild Horse Mesa in eastern California. We use a micropolar inversion of both seismic focal mechanism and fault-slickenline data and compare the results to the micropolar deformation parameters inferred from paleomagnetically determined block rotations and GPS velocities. The focal mechanisms, fault-slickenlines, and GPS velocities all show horizontal shear with a consistent ENE–WSW to E–W maximum extension-rate axis (d 1). A subset of data shows crustal thinning with a similarly oriented d 1 . We interpret these results as a reflection of divergent strike-slip (i.e., transtensional) boundary conditions in a negative flower structure developed in the right-releasing stepover. The fault-slickenline data also show a crustal thickening solution that we attribute to the local accommodation of block rotations. Paleomagnetic data demonstrate clockwise-looking-down rotations of 12.0° ± 2.6° (68% confidence limits) in ca. 3 Ma volcanic rocks, relative to the same rocks outside the stepover. Assuming rotations took 2–3 m.y. gives average microspins (block rotation rates) of 4.0° ± 0.9°/m.y. to 6.0° ± 1.3°/m.y. GPS velocities define a current macrospin (half the continuum rotation rate) of 3.9° ± 0.6°/m.y. to 6.1° ± 1.5°/m.y. These spin components are consistent with expectations for transtension. Our calculations of relative vorticity W from the GPS and paleomagnetic data are generally consistent with values obtained from the inversion of the fault-slickenline data, but the uncertainties in the data do not permit a definitive test of these results.

A well-defined axis of maximum dilation within the ca. 148 Ma Independence dike swarm is significantly offset across Owens Valley. Dilation by diking within the axis of maximum dilation is greater than 5%, commonly exceeds 10%, and locally ranges over 40%. Elsewhere in the swarm, dilation rarely ranges above 2%. The axis of maximum dilation steps ∼75–130 km rightward across Owens Valley, although the offset is difficult to measure precisely because the dike swarm is diffuse and intersects the valley at a relatively low angle. Comparison with other recently investigated geologic markers favors 65 ± 5 km of dextral offset since 83.5 Ma and perhaps an additional 10–65 km of offset prior to 83.5 Ma. Although Owens Valley is a locus of modern dextral slip, regional relations suggest that most of the 65 km of dextral displacement accumulated in Latest Cretaceous–early Paleogene (Laramide) time, when Cordilleran subduction was strongly right-oblique. Thermobarometric, structural, stratigraphic, and geochronologic evidence from the southern Sierra Nevada have previously been interpreted to reflect south-directed tectonic unroofing of deep-crustal rocks to form a metamorphic core complex during Laramide time. Large-magnitude Laramide right slip across Owens Valley thus may have been transferred southward into extension in the southern Sierra Nevada. Linked systems of late-Laramide to post-Laramide strike-slip faults and metamorphic core complexes have long been recognized in the plutonic-metamorphic core of the northern Cordillera. Recognition of this tectonic style in California suggests that it may have characterized most of the western Cordilleran orogen at this time.

The Paria Península in eastern Venezuela exposes an E-W–oriented mountain belt composed of deformed and metamorphosed sediments that were deposited on the northern South American passive margin in early Mesozoic time. The metamorphic grade, mostly greenschist facies, decreases from north to south in a direction perpendicular to the trend of the metamorphic belt. Foliation (S 1) dips steeply to the south along the southern coast and progressively gentler to the north to 25–30°. S 1 strikes ∼ 060–075° subparallel to oblique to the general trend of the metamorphic belts. Stretching lineation (L 1) plunges variably to the SW. The pattern of crystallographic preferred orientation (CPO) of quartz c -axes indicates a transition from symmetric coaxial strain to weak top-to-SW or oblique-normal sense of shear. The fabric patterns suggest activation of the basal <a>, prism <a>, and rhomb <a> slip systems under relative low-temperature (300–400 °C). Apatite fission-track ages range from 29 Ma in the south to 5 Ma in the north. Similarly, samples in the northern and central zone yielded the youngest zircon fission track (FT) ages, ranging from 5 Ma to 9 Ma, and the southern zone yielded slightly older ages ca. 13 Ma. From the FT ages we estimate a diachronous cooling from south to north and a cooling rate in the range of 16–56 °C/m.y. (∼1–2 mm/yr of exhumation). That many of the cooling ages postdate pre–10 Ma transpression suggests that tectonically driven vertical extrusion alone cannot account for the observed exhumation. The topography of the Paria Península and its current precipitation pattern are both asymmetric. Exhumation, deformation, topography, erosion, and precipitation patterns from the transpressional orogen of the Paria Península are comparable to those described in the Southern Alps of New Zealand. A general model for these two-sided transpressional wedges is proposed based on geologic observations. Obliquity of the compression and erosion seems to play an important role in the evolution, exhumation, and deformation of these two naturally deformed orogens.

The Zhangbaling metamorphic belt and the Fucha Shan metamorphic zone are metamorphic terranes that occur along the Tan-Lu fault, the major strike-slip fault that separates the ultra-high-pressure metamorphic belts of Dabie Shan and Sulu in SE China. The greenschist-facies Zhangbaling metamorphic belt is characterized by subhorizontal foliation, belt-parallel lineation, and a top-to-south sense of shear; 40 Ar/ 39 Ar analysis of synkinematic white mica dates deformation at ca. 235–240 Ma, coeval with reported peak metamorphic ages of ultra-high-pressure metamorphism in Dabie Shan. The Fucha Shan metamorphic zone is composed of felsic to mafic mylonite and is characterized by subvertical foliation and subhorizontal lineation. Ductile fabrics record a sinistral sense of shear and provide evidence of a zone-perpendicular contraction. Fabric-forming biotites yield 40 Ar/ 39 Ar cooling ages of ca. 120 Ma to ca. 135 Ma. The lower-amphibolite-facies Fucha Shan zone may represent an Early Cretaceous transpressional shear zone that developed independent of the Zhangbaling belt. Alternatively, the Zhangbaling belt might have operated as an attachment zone coupling transpressional shear in the underlying Fucha Shan zone with brittle upper crustal deformation during Triassic movement on the proto–Tan-Lu shear zone. Deformation may have been associated with oblique convergence between South and North China. In this model, Early Cretaceous argon ages from the deeper Fucha Shan zone are thought to record a later exhumation event.

The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40 Ar/ 39 Ar, and fission-track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. New U-Pb zircon and 40 Ar/ 39 Ar data from the Anaconda complex and published geochronology from the Priest River complex indicate a similar timing for the onset of major extension and exhumation. 40 Ar/ 39 Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. The Lewis and Clark fault zone separates ENE-directed extension in the Priest River complex from ESE-directed extension in the Bitterroot and Anaconda complexes. Large-scale extension was transferred eastward on the south side of this fault zone, where stretching lineations in core complex mylonites are oriented ∼104°–110° and coincide with the general trend of the transcurrent faults. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone was concentrated in areas that also experienced voluminous Eocene midcrustal magmatism. Extension was probably initiated by a change in plate boundary conditions combined with the rapid influx of heat from the asthenosphere as a slab window opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.