Strike-slip Partitioning Research Articles

Tectonic setting and Pan-African structural evolution of the western Saldania Belt, South Africa

Abstract The western Saldania Belt records the closure of the Adamastor Ocean and amalgamation of southwest Gondwana in the latest Neoproterozoic and early Phanerozoic, but the geodynamic setting and evolution of the belt remains controversial. Regional mapping and the integration of structural data presented in this paper document the juxtaposition of domains with distinct fabric development and kinematics. Structures record Pan-African strains and a strike-slip dominated sinistral transpression during southeast-directed subduction and convergence at low angles to the continental margin. Two main, structurally overlying domains – an upper and a lower domain – can be distinguished. Rocks of the upper domain record the partitioning of the transpressional strain into (1) broad regions of more or less upright, north to northwesterly-trending folds (F2) that record horizontal, mainly coaxial, east-northeast to west-southwest directed shortening at high angles to the continental margin, and (2) the northwesterly-trending, sinistral strike-slip Colenso Fault Zone that accommodates the margin-parallel, non-coaxial component of deformation between ca. 555 and 520 Ma. The strike-slip partitioning was promoted by strain localisation into synkinematic plutons of the Cape Granite Suite. In contrast, rocks in the pervasively transposed, mélange-like lower domain in the central Swartland region record episodes of vertical, coaxial shortening alternating with non-coaxial top-to-the-west and -northwest thrusting. The coaxial shortening strains relate to the thickening and gravitational loading of the succession in response to thrusting and tectonic underplating. Non-coaxial strains denote episodes of seismic slip during underthrusting. The different fabric domains are interpreted to represent a section through a fore-arc region, from the deeper level accretionary prism (lower domain) to the overlying, folded fore-arc basin succession (upper domain). Regional-scale klippen structures indicate the transfer of lower-domain phyllites into the overlying fore-arc sequence. The proposed fore-arc setting of the Saldania Belt suggests the subduction of the Adamastor Ocean below the leading edge of the Kalahari Craton. This challenges stratigraphic and structural correlations between the Saldania Belt and the Gariep Belt to the immediate north.

Recency of Faulting and Subsurface Architecture of the San Diego Bay Pull-Apart Basin, California, USA

In Southern California, plate boundary motion between the North American and Pacific plates is distributed across several sub-parallel fault systems. The offshore faults of the California Continental Borderland (CCB) are thought to accommodate ∼10–15% of the total plate boundary motion, but the exact distribution of slip and the mechanics of slip partitioning remain uncertain. The Newport-Inglewood-Rose Canyon fault is the easternmost fault within the CCB whose southern segment splays out into a complex network of faults beneath San Diego Bay. A pull-apart basin model between the Rose Canyon and the offshore Descanso fault has been used to explain prominent fault orientations and subsidence beneath San Diego Bay; however, this model does not account for faults in the southern portion of the bay or faulting east of the bay. To investigate the characteristics of faulting and stratigraphic architecture beneath San Diego Bay, we combined a suite of reprocessed legacy airgun multi-channel seismic profiles and high-resolution Chirp data, with age and lithology controls from geotechnical boreholes and shallow sub-surface vibracores. This combined dataset is used to create gridded horizon surfaces, fault maps, and perform a kinematic fault analysis. The structure beneath San Diego Bay is dominated by down-to-the-east motion on normal faults that can be separated into two distinct groups. The strikes of these two fault groups can be explained with a double pull-apart basin model for San Diego Bay. In our conceptual model, the western portion of San Diego Bay is controlled by a right-step between the Rose Canyon and Descanso faults, which matches both observations and predictions from laboratory models. The eastern portion of San Diego Bay appears to be controlled by an inferred step-over between the Rose Canyon and San Miguel-Vallecitos faults and displays distinct fault strike orientations, which kinematic analysis indicates should have a significant component of strike-slip partitioning that is not detectable in the seismic data. The potential of a Rose Canyon-San Miguel-Vallecitos fault connection would effectively cut the stepover distance in half and have important implications for the seismic hazard of the San Diego-Tijuana metropolitan area (population ∼3 million people).

Dextral transpressional shearing and strike-slip partitioning developments in the Central Myanmar Basin during the collision between the India Plate and West Myanmar Block

Central Myanmar Basin (CMB) lies between the two major strike-slip faults: Sagaing Fault to the east and Kabaw Fault to the west. Central Volcanic Line of Myanmar emplaced en-echelon array of P-shears along the fault zone, which was active dextral as surrounding major faults at a low angle about 15° to the shear zone boundary. CMB was developed as strike-slip partitioned transpression model as proposed in this study. The transprsession tectonic is common with oblique convergence subduction as well as collision of India Plate to West Myanmar Block, especially effects on the CMB. Structural evidence are occurred as en-echelon folding and fracturing by satellite photos, field observation and 3D strain ellipsoid construction showing plane strain by pure shear upon ideal strike-slip with simple shear. In this study, two types of transpression distinguished as homogeneous and strike-slip partitioning which are developed within the frame of the strike-slip fault with progressive deformation. Under the condition of the strike-slip partitioned transpression, strain partitioning is distinctly occurred together with progressive deformation and reactivation of pre-existing structures such as cylindrical fold to double plunging fold, normal fault to oblique slip fault and positive flower structure. Deformation process related to the India Plate movement deduced the period of Late Miocene and Pliocene-Pleistocene based on the structural analysis in Rakhine Coastal region, western Myanmar. During the period between the Late Miocene to Pliocene-Pleistocene, pre-existing ophiolite zone in the Indo-Myanmar Ranges (IMR) was split up different zones by the Sagaing Fault displacement, accompanied to the West Myanmar Block movement with India Plate subduction. Accordance with the Sagaing Fault displacement and group of mud volcanoes shifting, the CMB moved northward about 450km with displacement rate 5–7cm per annum by the 45–50°convergent angle of India Plate collision to Eurasia Plate. Transpression deformation of northward compression was developed in the CMB, simultaneously on the other side transtension as pull-apart basin was formed by Andaman Sea opening at the south on the Sagaing Fault. During the Miocene time, collision of India Plate to Eurasia Plate was principal and caused the deformation at northern border the India, in which India Plate moved both in northwards and rotated anti-clockwise, whereas West Myanmar Block together with Sibumasu Block rotated clockwise becoming final closure of the northeasternmost Assam and Myanmar regions. During the Oligocene time, deposition of the shallow marine Barail formation in the Bengal Basin and shallow marine to continental shelf sequences in the CMB started as twin trough. This study provides the transpression deformation in the CMB and related regions during syn- and post-collision of India Plate to West Myanmar Block with position and kinematic structural interpretation.

Structural styles of the Aras fault zone with implications for a transpressive fault system in NW Iran

• NE-striking Aras Fault Zone (AFZ) bounds the Talesh and Lesser Caucasus blocks. • The AFZ is characterized by well-developed geological and geomorphological features. • Paleostress results along the AFZ indicate a similar tectonic regime since Pliocene. • The transpressive tectonic regime controls the structural evolution of the AFZ. • Transpressive tectonic regime leads to strain partitioning along the AFZ. Aras Fault Zone (AFZ) is one of the most important intra-plate fault zones within the Turkish-Iranian Plateau. The AFZ is divided into five fault branches based on geometrical and structural properties called the Aras Main, Horadiz, Khodafarin, Asadkandi, and Aslanduz fault branches. Our geometric and kinematic analysis indicates that these faults exhibit compressional left-lateral strike-slip fault characteristics with general directions of NE-SW and dipping to SE. Structural mapping has revealed that the NE section of the AFZ shows typical horsetail fault geometry. Few meters to approximately 17 km of left-lateral displacements were observed along the AFZ, indicating the occurrence of numerous historical earthquake events. Paleostress analysis of fault slip data reveals that the AFZ has developed in the dominant NNE-SSW and NE-SW oriented compressional stress regime. Evaluating the Ratio of Principal Stress Differences (R), Stress Regime Index (R'), and principal stress directions show that the zone is under the transpressional tectonic regime. Paleostress orientations are consistent with orientations of the GPS velocity vectors, which indicates that the region, including the AFZ, is under the effect of the same tectonic regime, at least since Pliocene. The amounts of strike-slip partitioning on discrete strike-slip faults are determined as 33–41%, which means at least 41% of total wrench components in the region have been accommodated by discrete strike-slip faulting along the AFZ. Results indicate that the simple shear component of total stress applied to the area is consistent with the GPS slip velocities.

Plate boundary localization, slip-rates and rupture segmentation of the Queen Charlotte Fault based on submarine tectonic geomorphology

Linking fault behavior over many earthquake cycles to individual earthquake behavior is a primary goal in tectonic geomorphology, particularly across an entire plate boundary. Here, we examine the 1150-km-long, right-lateral Queen Charlotte-Fairweather fault system using comprehensive multibeam bathymetry data acquired along the Queen Charlotte Fault (QCF) offshore southeastern Alaska and western British Columbia. Fine-scale analysis of tectonic geomorphology allowed us to identify and reconstruct 184 strike-slip piercing points over a 630 km stretch of the QCF. Age constraints from glacial recession and offshore sedimentation patterns yield a consistent slip-rate of ∼50–57 mm/yr since ∼17–12 ka, the fastest rate for a continent-ocean strike-slip fault on Earth. These slip-rates equal or exceed estimates of Pacific-North America (PA-NA) relative motion from global plate reconstructions, indicating that PA-NA motion is highly localized. The QCF cuts the seafloor along a narrow and unusually straight trace for its entire length and multiple fault traces are observed only at local step-overs. The geometry and behavior of the QCF over many earthquake cycles is simple and typical of mature faults with relatively homogeneous stress fields. Since the QCF is the primary PA-NA plate boundary, we used the trace of the QCF to define the small circle path for relative plate motion and computed the associated Euler pole. Predicted along-strike obliquity variations based on the new pole agree with observed tectonic geomorphology and suggest that previous global plate reconstructions overestimated the degree of oblique convergence along the QCF. We also find that subtle, long-wavelength (75–150 km) bends and discrete step-overs appear to define the endpoints of M>7 earthquakes, suggesting that obliquity and resultant fault geometry may control rupture segmentation and asperity development. Lastly, the agreement between predicted obliquity and tectonic geomorphology along the entire length of QCF compelled a reevaluation of regional tectonic models. In the north, the eastern Yakatat Terrane appears to be translating northwest with the Pacific plate, and slip transferred from the QCF to the Fairweather Fault results in ∼20 mm/yr of convergence along the southern St. Elias mountains. In the south, we predict a reduced rate of convergence along the QCF west of Haida Gwaii (∼5–6 mm/yr of shortening, on average) relative to previous studies. Our results support a model for transpression and strike-slip partitioning along the edge of a hot and weak Pacific Plate, leading to crustal thickening and growth of the Queen Charlotte Terrace to the west of Haida Gwaii.

Neogene oblique convergence and strain partitioning along the North Tabriz Fault, NW Iran

The North Tabriz Fault is one of the main active geological structures in NW Iran. It is reactivated under a pure-shear dominated transpressional tectonic regime induced by high angle (∼50°) oblique convergence between the Arabia and Eurasia plates. The first stage structures in the study area are folds and thrust faults that are mostly observed in Miocene sedimentary rocks and are cut by right-lateral strike-slip faults that are part of the North Tabriz Fault. Structural analysis of the North Tabriz Fault revealed flower structure geometry with thrust faults and related folds on both sides of the fault and strain partitioning. From GPS data, 7.3mm/year of right lateral plate motion has been calculated for NW Iran with 57–72% strike-slip partitioning, resulting in a maximum right-lateral slip rate for the North Tabriz Fault of 4.2–5.3mm/year. Since the total strike-slip displacement along the NTF has been estimated at 17±3km, with the slip-rate of 4.2–5.3mm/year, the maximum age of inception of the NTF strike-slip movement was at 3.2–4Ma (Upper Pliocene).

Quaternary slip-rates of the Kazerun and the Main Recent Faults: active strike-slip partitioning in the Zagros fold-and-thrust belt

The aim of this work is to constrain the Late Quaternary activity of two major dextral strike-slip faults of the Zagros fold-and-thrust belt of Southern Iran, within the framework of right-oblique convergence between Arabia and Eurasia. The NW-trending Main Recent fault marks the rear of the belt along two thirds of its length. Its southeastern tip connects to the northern termination of the N-trending Kazerun Fault, which affects the entire width of the belt. Horizontal slip rates have been estimated on these two faults over the last 140 ka from lateral offsets of streams and fans and in situ cosmogenic 36Cl exposure dating of cobbles sampled on the surface of these geomorphic features. Compared to GPS data, the obtained minimum slip rate of 3.5–12.5 mm yr−1 on the Main Recent Fault implies strike-slip partitioning of the convergence along this fault. Minimum slip rate of the Kazerun Fault is 2.5–4 mm yr−1 for its northern strand, 1.5–3.5 mm yr−1 for its central segment and is negligible for its southern segment. These results are consistent with southward distribution of the slip from along the Main Recent Fault to the longitudinal thrusts and folds of the fold-and-thrust belt through the Kazerun Fault, with a decrease of slip from the southeastern tip of the Main Recent Fault towards the southern termination of the Kazerun Fault. The Kazerun and associated faults form the horsetail termination of the Main Recent fault and may be seen as the propagating southeastern front of the fault system that accommodates indentation of Eurasia by Arabia.

Mechanical relation between crustal rheology, effective fault friction, and strike-slip partitioning among the Xiaojiang fault system, southeastern Tibet

The north–south trending Xiaojiang fault system accommodates ∼10–12 mm/yr sinistral motions between southeastern Tibet and south China. In the south segment, the fault system composes mainly of four parallel strike-slip faults, namely from west to east, the Luzhijiang fault, the Yimen fault, the Puduhe fault, and the Xiaojiang fault. Geological and Seismological observations have shown that these strike-slip faults are all of active, while the slip rates of the Luzhijiang, the Yimen, and the Puduhe faults are much less than that of the Xiaojiang fault. We use finite element modeling to explore the mechanical relation between crustal rheology, effective fault friction and long-term slip rate partitioning among the four parallel faults. The individual faults are simplified as vertical discontinuities embedded in the crust as geophysical explorations have predicted. A large number of models are tested, associating with variations of the crustal rheolohy and the effective fault friction of individual faults. Result shows that if crust bounding the faults trends to behave like rigid blocks and decoupled mechanically from underlying layer, the modeled result is hard to approximate slip rates of the individual faults. To better fit slip rates of the individual faults, viscous deformation of the lower crust should be included. With a heterogeneously viscous lower-crust model that is built upon thermal structure of the heat flow data, associating with relatively low effective friction of the Xiaojiang fault, the modeled results fit the geological slip rates well, with ∼1–1.5 mm/yr for the Luzhijiang, the Yimen and the Puduhe faults, and ∼6–6.5 mm/yr for the Xiaojiang fault. Thus, in the southward movement of the Tibetan plateau around the eastern Himalayan syntaxis, slip partitioning among the Xiaojiang fault system should be related to viscous deformation of the lower crust associated with different strength of the individual faults, highlighting that deformation of this fault system is coupled mechanically between the frictional upper crust and the viscous lower crust.

Slip partitioning and inclined dextral transpression along the Zagros Thrust System, Iran

Flexural duplex structures and shear zones are well developed in the Heneshk area, in southwestern Iran. The Zagros Thrust System in this area consists of eight sheets of NW-striking, NE-dipping dextral strike-slip duplexes that are linked with imbricate fans and oblique-slip thrusts. The Zagros Thrust System is a portion of the internal zone of the Zagros orogenic belt. This internal zone is characterized by penetrative plastic deformation and metamorphism. Based on the kinematic vorticity number ( W k = 0.73 ± 0.02) calculated from quartz porphyroclasts of the Goshti shear zones along the Zagros Thrust System, the estimated θ angle between the maximum instantaneous strain axis (ISA) and the transpressional zone boundary is 33°. The estimated α angle of the plate convergence or the flow apophysis of the displacement field is 25°.The presence of dominantly dextral shear sense indicators in the area is consistent with dextral inclined transpressional convergence. The mean estimated finite deformation ( W m ) value indicates relative contributions of 47% pure shear and 53% simple shear for the deformation which involved both strike-slip and oblique slip displacements. In this inclined transpression, about 40% strike-slip partitioning was required to accommodate the finite strain and re-orientation of instantaneous strain.

Transpression and strain partitioning in the Caribbean Island-arc: Fabric development, kinematics and Ar–Ar ages of syntectonic emplacement of the Loma de Cabrera batholith, Dominican Republic

An integrative structural and geochronologic study of the Loma de Cabrera batholith (LCB, Cordillera Central, Dominican Republic) and its country rocks reveals the interplay of deformation, metamorphism and plutonism produced in the Caribbean island-arc during Late Cretaceous oblique convergence. The results emphasize the interference between three contemporaneous strain fields: (1) a northern and southern domains produced by (<95 Ma) arc-perpendicular NE- and SW-vergent folding and thrusting, respectively; (2) arc-parallel sinistral strike-slip shearing along the La Meseta shear zone (LMSZ), active during the 88–74 Ma interval; and (3) the adjacent syn-kinematic emplacement of the LCB (90–74 Ma; 40Ar/ 39Ar in hornblende) during sinistral transpressional shearing. Comparison of the structural data with strain models of oblique plate convergence suggest that the LMSZ is a preserved ductile signature of strike-slip partitioning within a sinistral transpressional intra-oceanic subduction zone. In the LCB, microstructural data indicate that the magmatic to high-temperature solid-state deformation initially occurred over a wider band of heterogeneously distributed shear deformation, and was partitioned in narrow bands of mid- to low-temperature deformation connected with the LMSZ during the cooling of the batholith. Field and geochronologic studies also suggest that shortening across the southern domain took place concurrently with sinistral strike-slip movement along the crustal-scale La Guácara and Macutico fault zones, also consistent with a transpressional setting for the Late Cretaceous Caribbean magmatic arc. Shear and fault zones were variably reactivated during Upper Eocene-Oligocene thrusting and Miocene to Recent uplift of the Cordillera Central.

Application of stereoscopic satellite images for studying Quaternary tectonics in arid regions

We introduce a flexible method for creating stereoscopic pairs of images from any interesting sub-area of the same scene of Landsat Thematic Mapper (TM)/Enhanced Thematic Mapper (ETM) and Indian Remote Sensing (IRS)-1C Pan remote sensing data by setting the Z scale. As a test of this method, stereoscopic images were used to study Quaternary deformation along the Tian Shan Orogenic Belt, north-west China. The new stereoscopic images can then provide detailed information of Quaternary deformation structures, including spatial distribution and arrangement pattern of fold structures, fault scarps and displacement of alluvial fans, terraces and drainage systems along active faults, in three dimensions. The strike–slip partitioning has been revealed by interpretation of stereoscopic images within Chinese Tian Shan. Structural interpretations derived from stereoscopic analysis were confirmed to a high degree of accuracy during a subsequent field study. The satellite remote sensing stereoscopic technique is an effective method of analysing Quaternary tectonic deformation in remote arid to semi-arid regions such as the Tian Shan.

Structure and kinematics of oblique continental convergence in northern Fiordland, New Zealand

Southeast of the Alpine Fault in Fiordland, New Zealand, Late Tertiary faults envelope exposures of Early Cretaceous high- P (12–16 kbar) granulite facies orthogneisses. These exposures allowed us to examine how pre-Cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence. Fault-slip data and kinematic modelling show that oblique-slip and reverse displacements preferentially were partitioned east of the Alpine Fault onto curved NE-striking surfaces and moderately dipping (30–52°), N- and NW-striking surfaces, respectively. Approximately 3.0–5.5 km of exhumation and 6.5–7.0 km of oblique-dextral displacement occurred on faults that display palm tree-style geometries and subhorizontal detachments in profile. This heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the Alpine Fault north of Milford Sound. The principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes. Our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7–9 Ma. The effects of these pre-existing structures and a focusing of contractional deformation along the northeast margin of Fiordland resulted in an unusually high degree of strike-slip partitioning across the southernmost onshore segment of the Alpine Fault.

Pull-apart formation and strike-slip partitioning in an obliquely divergent setting, Leka Ophiolite, Norway

The Leka Ophiolite Complex (LOC) is located on the island of Leka, Norway, and belongs to the Uppermost Allochthon of the Scandinavian Caledonides. The rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex. Paleostress analysis, gravity inversion, and regional geology support a fault-bounded rhombochasm geometry for the LOC. The paleostress inversions revealed two types of tensors, interpreted as small strains: (1) horizontal extension, generally E–W to NE–SW, and (2) horizontal extension in the same direction with an added component of perpendicular horizontal contraction. A strong positive gravity anomaly (25 mGal) is centered on Leka, and gravity inversion indicates that the LOC lies directly below its surface exposures with steep-sided walls and a flat bottom located at ∼7 km depth. The faults bounding the LOC probably initiated during postorogenic extension in the Scandinavian Caledonides. The faults are regional in scale and are parallel to other NE–SW trending en echelon faults along the Norwegian coastline and on the adjacent mainland. A pull-apart structure explains the down-dropping and subsequent preservation of the LOC, as it is surrounded by rocks from lower structural positions within the nappe stack. The paleostress directions from Leka support a sinistral component of shear along these faults. The gravity inversion is consistent with a fault-bounded geometry. This pull-apart structure, as uniquely recorded by the dense ophiolitic rocks, suggests that strike-slip partitioning was active in an obliquely divergent setting.

Strain partitioning and batholith emplacement at the root of a transpressive magmatic arc

An integrative study of the high-pressure Ecstall batholith and its country rocks (Prince Rupert area, British Columbia) reveals the interplay of deformation and plutonism produced in the largest Cordilleran magmatic arc (i.e. the Coast Plutonic Complex) during mid-Cretaceous convergence between the Farallon oceanic plate and North America. The results emphasize the interference between three strain fields: (1) an early (>92 Ma) crustal wedge produced by orogen-perpendicular, SW-vergent thrusting, (2) an orogen-parallel sinistral strike-slip shear zone active until 87 Ma, and (3) the lateral expansion of the batholith (93–91 Ma) against the shear zone. The batholith expanded in a direction oblique with respect to the shear zone trend (+20°) by extruding country rocks against its head in a direction normal to its expansion direction during sinistral, strike-slip partitioned transpression. The batholith's emplacement combined far-field batholith boundary-normal translation, extrusion-related rotation and radial and concentric elongation in its structural aureole. This case study may typify the process by which a modern magmatic arc grows longitudinally at depth during transpression, the direction and sense of growth being determined by the direction of plate motion relative to the magmatic arc and the degree of strike-slip partitioning of transpression.

Strike-slip partitioned transpression of the San Andreas fault system: a lithospheric-scale approach

Abstract Physical and numerical experiments on transpression indicate that strike-slip partitioning is facilitated by low angle of convergence, such as occurs on the modern San Andreas fault system. Cross-sections across the San Andreas fault overestimate the amount of margin-normal contraction, because they do not include the effect of horizontal stretching caused by the wrench component of deformation. When this correction is made assuming a strike-slip partitioned transpression model, contraction estimated from cross-sections is consistent with both the normal and tangential movements imposed by plate motion: there is no San Andreas discrepancy. Seismic anisotropy (shear-wave splitting) and teleseismic analyses suggest that the upper mantle in the San Andreas region is thick and largely undeformed to the SW of the San Andreas fault, but thin and intensely sheared in a zone 50–100 km wide beneath the NE region of the San Andreas fault system. Seismic anisotropy is best explained by a transpression zone of vertical foliation (and possibly horizontal lineation) which penetrates the asthenospheric mantle. Using a lithospheric-scale approach, the transfer of the displacement field from the penetratively deformed mantle to the strike-slip partitioned upper crust necessitates zones of accommodation in the mid- to lower crust, corresponding to the flatlying detachments that underlie the San Andreas fault system. The mechanics of the San Andreas fault are best considered in connection with the strength of the various lithospheric layers involved in this system.

Role of plate kinematics and plate-slip-vector partitioning in continental magmatic arcs: Evidence from the Cordillera Blanca, Peru

New structural and geochronological data from the Cordillera Blanca batholith in the Peruvian Andes, coupled with Nazca–South American plate-slip-vector data, indicate that oblique convergence and associated strike-slip partitioning strongly influenced continental magmatic arc evolution. Both the strain field and mode of magmatism (plutonism vs. volcanism) in the late Miocene Peruvian Andes were controlled by the degree to which the arc-parallel component of the plate slip vector was partitioned into the arc. Strong strike-slip partitioning at ca. 8 Ma produced arc-parallel sinistral shear, strike-slip intercordilleran basins and east-west–oriented tension fractures that facilitated emplacement of the Cordillera Blanca batholith (ca. 8.2 ± 0.2 Ma). Periods during which the strike-slip component was not partitioned into the arc (ca. 10 and ca. 7 Ma) were associated with roughly arc-normal contraction and ignimbrite volcanism. The data thus support the contention that contraction within continental magmatic arcs favors volcanism, whereas transcurrent shear favors plutonism. The tie between oblique convergence and batholith emplacement in late Miocene Peruvian Andes provides a modern analogue for batholiths emplaced as the result of transcurrent shear in ancient arcs.

Oblique plate motion and continental tectonics

Three-dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins. Kinematic models of strike-slip partitioning for transpression and transtension allow the exact relation among three critical parameters—plate motion, instantaneous strain axes, and degree of strike-slip partitioning—to be calculated. Application to two end-member tectonic environments characterized by a low (South Island, New Zealand) and high (central California) degree of strike-slip partitioning demonstrates a remarkable consistency among the three parameters, suggesting that strike-slip partitioned transpression is a valid model for deformation in these regions. The extreme degree of strike-slip partitioning in wrench-dominated systems, such as central California, is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes.