Plate Motion Vector Research Articles

Transverse tectonics control on the Late Quaternary development of the Central Main Ethiopian Rift

Extension of continental plates should typically result in the development of normal faults perpendicular or sub-perpendicular to the plate motion vector. However, several factors such as linkage of rift segments and the presence of pre-existing fabrics may result in the development of complex fault systems, with structures oblique or even sub-parallel to the extension vector. This paper is focused on the extension-related deformation of the central sector of the Main Ethiopian Rift (MER) and shows the presence of a transverse structure (ie., sub-parallel to extension) in the central MER that played an important role during the Late Quaternary, influencing the hydrographic evolution in this area. Structural data evidence its tectonic activity and the analysis of distribution of magmatic centres and other volcanic features shows that interaction between transverse structures and the faults delimiting the rift may favour formation of volcanic centres. The orientation of this transverse structure, sub-parallel to extension, is interpreted as being caused by the linkage of major segments of the MER that likely exploited pre-existing structures, as widely documented in the MER.

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

Figure 13. Regional fault orientations and plate boundary parameters used for kinematic analysis and interpretation. Solid orange arrows are maximum and minimum horizontal stress orientations, ! and " , respectively (Hardebeck and Hauksson, 2001), large black and white outlined arrows are plate motion vectors, and the average orientations for Group-1, Group-2, and La Nacion fault zones are solid black, toothed lines. Large black hatched lines are regional faults. G1, Group 1; G2, Group 2. Background fault traces (thin red lines) are from USGS Quaternary fault database (USGS, 2019). Background topography and bathymetry are from the National Centers for Environmental Information Southern California Coastal Relief Model (v2) (Calsbeek et al., 2013).

Looking beyond kinematics: 3D thermo-mechanical modelling reveals the dynamics of transform margins

Abstract. Transform margins represent ∼ 30 % of non-convergent margins worldwide. Their formation and evolution have traditionally been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high-resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15∘ (high obliquity) to 75∘ (low obliquity) every 15∘ for rheologies of strong and weak lower continental crust. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low- (75∘) to intermediate-obliquity (45∘) cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20∘ to 40∘ with the plate motion vector, while for higher obliquities (30∘ to 15∘) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain re-orient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) initiation of strain in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strike-slip structures; (3) formation of highly localized plate boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins.

Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases

SUMMARY We present the results of a shear wave splitting analysis performed on the teleseismic SK(K)S and direct S wave recordings of 68 temporary broad-band stations to investigate the mantle deformation on the northern side of the Arabia–Eurasia collision zone in NW Iran. We used the Reference Station Technique to overcome potential contamination from the source-side anisotropy on the direct S wave signals. This method enabled us to expand our splitting measurement database beyond the usual SK(K)S phases. The average splitting delay time over the entire region was found to be 1.14 ± 0.42 s for the SK(K)S wave and 1.36 ± 0.26 s for the direct S wave. In most parts of the study area, the fast polarization directions for both shear phases are consistent and show a uniform NE–SW direction with an average of 36° and 37° for SK(K)S and S wave-derived results, respectively. This direction is in close agreement with the direction of the absolute plate motion vector in NW Iran (N39°E). The fast directions are associated with neither the surface geological trends, nor the geodetic strain fields. We propose that the observed anisotropy is mainly controlled by the LPO fabric developed due to the shearing of the asthenospheric layer in response to the motion of the lithosphere relative to the deeper mantle. Only in a narrow region near the tectonic boundaries of central Iran with NW Iran and the Alborz, NW–SE oriented SK(K)S fast directions tend to align with the major geological structures. Fast directions obtained from direct S wave indicate significantly smoother variations in the same regions and mostly continue to be aligned in the NE–SW direction. We attribute these differences to the change in the structure of the lithosphere in the tectonic boundary zone. The western margins of central Iran possess a strong deformational fabric as evidenced by the major active strike-slip zones there. Considering that the depth extent of this fabric expands over a relatively narrow zone in the mantle, it can locally influence the SK(K)S phases. The direct S waves, on the other hand, have a larger footprint and therefore average over a larger region, and relative to the SK(K)S phases, are influenced more strongly by the asthenospheric fabric due to their larger angles of incidence, which results in a larger zone of influence for station average anisotropy parameters.

Seismic anisotropy accrued by seven unusually deep local earthquakes (between 50 and 60 km) in the Albertine Rift: implications of asthenospheric melt upwelling

We investigated the primary mechanisms triggering the S-wave splitting (SWS) of seven unusually deep local earthquakes (between 50 and 60 km) which originated in the lithosphere beneath the Rwenzori region. We attempted to develop an understanding of the relationship between anisotropic structures in the lithosphere and tectonic deformation processes. A total of 12 out of 44 waveforms showed evidence of SWS on their polarization diagrams. The fast-wave direction (φ) and delay-time (δt) were estimated using the covariance matrix and the cross-correlation coefficient methods, respectively. We observed a clockwise rotation of φ-directions (NW - SE and ~ENE - WSW) at stations located in the rift valley. We related this pattern of φ-directions to anisotropic fabric, probably lattice-preferred orientation of preexisting olivine, whose a-axes are aligned with ESE absolute plate motion (APM) vector. At stations located outside the rift valley, however, we observed WNW - ESE and NNW - SSE patterns of φ-directions. We associated these patterns to the shape-preferred orientation of structures frozen in the lithosphere that are aligned with the present-day APM direction. We observed δt values ranging between 0.04 ± 0.01s and 0.43 ± 0.02 s, which decrease with distance away from the rift axis. This further supported our concept that the anisotropy observed at stations located on the moving plate is related to aligned melt inclusions frozen in the surrounding lithosphere. We further observed that the δt values increase linearly with ray-path length, which could indicate a fairly uniform anisotropy between 50-km and 60-km depth. Our study reported no evidence of multi-layer anisotropy beneath the Rwenzori region.

Worldbuilding from tectonic first principles: Integrating and challenging undergraduate knowledge through a course project

In this integrative, team-based capstone project, students in an upper year Tectonics course built a planet of their own design from the basic concepts of plate tectonics. They created a tectonic map complete with plate boundaries, continental-ocean boundaries, motion vectors, and hot spot traces. In subsequent weeks, students produced a geologic map illustrating features that were a consequence of decisions made in creating their tectonic map. Students designed their own legend and determined the specificity or generality in which they conveyed geologic features at the scale of their map. Students also created a physiographic map reflective of the geology of their planet. Evaluation of student artifacts indicates the project was successful in meeting learning outcomes that challenged students to think in three-dimensions and over geologic timescales, and to develop problem-solving skills; a learning outcome of integrating knowledge from other courses was also met, but to a lesser degree and with more variability among teams. With minor adjustments, this project could be implemented in any tectonics course, or in other subjects including oceanography, atmospheric science, or paleontology.

The structural evolution of pull‐apart basins in response to changes in plate motion

AbstractPull‐apart basins are structural features linked to the interactions between strike‐slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, the basin length/width ratio, and changes in extension direction. In this work, we investigate the effect of a change in the plate motion direction on a pull‐apart basin's structure, using analogue modelling experiments with a two‐layer ductile‐brittle configuration to simulate continental crust rheology. We initially impose orthogonal extension on an interconnected rift and strike‐slip system to drive pull‐apart development. Subsequently, we rotate the relative motion vector, imposing transtensional deformation and continuing with this new relative motion vector to the end of the experiment. To compare with natural examples, we analyse the model using seismic interpretation software, generating 3D fault structure and sedimentary thickness interpretations. Results show that the change in the direction of plate motion produces map‐view sigmoidal oblique slip faults that become normal‐slip when deformation adjusts to the new plate motion vector. Furthermore, sediment distribution is strongly influenced by the relative plate rotation, changing the locus of deposition inside the basin at each model stage. Finally, we compare our observations to seismic reflection images, sedimentary package thicknesses and fault interpretations from the Northern Gulf of California and find good agreement between model and nature. Similar fault arrays occur in the Bohai Basin in northern China, which suggests a rotational component in its evolution. More broadly, such similar structures could indicate a role for oblique extension and fault rotation in any pull‐apart basin.

The impact of oblique inheritance and changes in relative plate motion on the development of rift-transform systems

In transform margins, oblique structural inheritance and plate motion vector changes have a direct impact on the margin's morphology and duration of transform activity. We investigate the effect of these two factors using numerical modelling. To simulate oblique inheritance in continental lithosphere, we model an initial rift-transform-rift configuration oriented at a range of angles (−45o to +45o) with respect to the extension direction. In a second suite of calculations, we first extend a rift-transform-rift system orthogonally and then vary the extension direction to simulate rotation of the far-field stress directions, and hence the relative plate motion. We found that transpressional deformation on the transform results in increased duration of fault activity and diffuse, longer transform zones at higher angles, while the opposite is true for the transtensional case. These observations are in good agreement with natural examples such as the Ungava Transform Zone, the Gulf of California and the Gulf of Aden, indicating that relative plate rotation plays an important role in the structural evolution of transform margins. Finally, we present a metric that links current transform margin morphology to past evolution.

The influence of inherited continental margin structures on the stress and strain fields of the south-central Taiwan fold-and-thrust belt

SUMMARYIn this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2 and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (SH) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counter-clockwise. The direction of the horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, εH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east–northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.

Linked thick- to thin-skinned inversion in the central Kirthar Fold Belt of Pakistan

Abstract. The Kirthar Fold Belt is part of the transpressive transfer zone in Pakistan linking the Makran accretionary wedge with the Himalaya orogeny. The region is deforming very obliquely, nearly parallel to the regional S–N plate motion vector, indicating strong strain partitioning. In the central Kirthar Fold Belt, folds trend roughly N–S and their structural control is poorly understood. In this study, we use newly acquired 2-D seismic data with pre-stack depth migration, published focal mechanisms, surface and subsurface geological data, and structural modelling with restoration and balancing to constrain the structural architecture and kinematics of the Kirthar Fold Belt. The central Kirthar Fold Belt is controlled by Pliocene to recent linked thick-skinned to thin-skinned deformation. The thick-skinned faults are most likely partially inverting rift-related normal faults. Focal mechanisms indicate dip-slip faulting on roughly N–S-trending faults with some dip angles exceeding 40∘, which are considered too steep for newly initiated thrust faults. The hinterland of the study area is primarily dominated by strike-slip faulting. The inverting faults do not break straight through the thick sedimentary column of the post-rift and flexural foreland; rather, the inversion movements link with a series of detachment horizons in the sedimentary cover. Large-scale folding and layer-parallel shortening has been observed in the northern study area. In the southern study area progressive imbrication of the former footwall of the normal fault is inferred. Due to the presence of a thick incompetent upper unit (Eocene Ghazij shales) these imbricates develop as passive roof duplexes. In both sectors the youngest footwall shortcut links with a major detachment and the deformation propagates to the deformation front, forming a large fault-propagation fold. Shortening within the studied sections is calculated to be 18 %–20 %. The central Kirthar Fold Belt is a genuine example of a hybrid thick- and thin-skinned system in which the paleogeography controls the deformation. The locations and sizes of the former rift faults control the location and orientation of the major folds. The complex tectonostratigraphy (rift, post-rift, flexural foreland) and strong E–W gradients define the mechanical stratigraphy, which in turn controls the complex thin-skinned deformation.

Analogue Modeling of Plate Rotation Effects in Transform Margins and Rift‐Transform Intersections

AbstractTransform margins are first‐order tectonic features that accommodate oceanic spreading. Uncertainties remain about their evolution, genetic relationship to oceanic spreading, and general structural character. When the relative motion of the plates changes during the margin evolution, further structural complexity is added. This work investigates the evolution of transform margins and associated rift‐transform intersections, using an analogue modeling approach that simulates changing plate motions. We investigate the effects of different crustal rheologies by using either (a) a two‐layer brittle‐ductile configuration to simulate upper and lower continental crust, or (b) a single layer brittle configuration to simulate oceanic crust. The modeled rifting is initially orthogonal, followed by an imposed plate vector change of 7° that results in oblique rifting and plate overlap (transpression) or underlap (transtension) along each transform margin. This oblique deformation reactivates and overprints earlier orthogonal structures and is representative of natural examples. We find that (a) a transtensional shift in the plate direction produces a large strike‐slip principal displacement zone, accompanied by en‐echelon oblique‐normal faults that accommodate the horizontal displacement until the new plate motion vector is stabilized, while (b) a transpressional shift produces compressional structures such as thrust fronts in a triangular zone in the area of overlap. These observations are in good agreement with natural examples from the Gulf of California (transtensional) and Tanzania Coastal Basin (transpressional) shear margins and illustrate that when these deformation patterns are present, a component of plate vector change should be considered in the evolution of transform margins.

Upper mantle anisotropy and deformation beneath the major thrust-and-fold belts of Zagros and Alborz and the Iranian Plateau

SUMMARY We present new SKS splitting measurements obtained from a temporary seismic broad-band network in western Iran across the Arabia–Eurasia collision zone. The average delay time over the entire network was found to be 1.27 ± 0.27 s. In the Zagros where the lithosphere attains its greatest thickness, the fast-axes are predominantly subparallel to the trend of the mountain ranges, suggesting a lithospheric origin of the anisotropy caused by transpressional deformation. In contrast in the Alborz, the fast-axes become subparallel to the absolute plate motion vectors and we propose that anisotropy is mainly controlled by the direction of the asthenospheric flow field. The central Iran region shows a more complex pattern of anisotropy that could be the result of variable and small-scale deformation fields in the crust and the shallow sublithospheric mantle.

Strain histories from the eastern Central Range of Taiwan: A record of advection through a collisional orogen

In the eastern Central Range of Taiwan there is a regional variation in the orientation of maximum finite stretch across the slate belt, with down-dip maximum stretch found in the western Central Range and along-strike maximum stretch in the eastern Central Range. Incremental strain histories from syntectonic fibers in pyrite pressure shadows indicate a progressive change in extension direction from down dip to along strike during deformation, there is a corresponding temporal variation in stretching direction shown in samples from the eastern edge of the Central Range, a pattern that mimics the regional west-to-east spatial variation. These observed temporal and spatial strain distributions are used to evaluate the kinematics associated with slaty cleavage development during advection through the Taiwan orogenic system. The subduction zone beneath the island of Taiwan is influenced by two types of obliquity that have the potential to generate the observed along-strike stretching. First, the plate motion vector of the Philippine Sea plate relative to the Eurasian plate is slightly oblique to the regional strike of the mountain range, which could result in partitioning of strike slip shearing into the interior of the collision. Second, the north-south Luzon volcanic arc on the Philippine Sea Plate is obliquely oriented relative to the northeast-southwest edge of the Eurasian continental margin, which could result in lateral extrusion of the ductile core of the range. Incremental strain histories in cleavage-parallel samples represent a time-for-space equivalence where the stretching direction is fixed relative to the position within the mountain belt architecture (e.g., the topographic divide), and temporal variations in the eastern central Range reflect lateral advection through the strain field in response to accretionary and erosional fluxes. Incremental strain histories in cleavage perpendicular samples show both clockwise and counter-clockwise rotation of strain, indicating the possibility of bulk coaxial strain and lateral extrusion.

Simulation of tectonic evolution of the Kanto Basin of Japan since 1 Ma due to subduction of the Pacific and Philippine Sea plates and the collision of the Izu-Bonin arc

The Kanto Basin, the largest lowland in Japan, developed by flexure as a result of (1) the subduction of the Philippine Sea (PHS) and the Pacific (PAC) plates and (2) the repeated collision of the Izu-Bonin arc fragments with the Japanese island arc. Geomorphological, geological, and thermochronological data on vertical movements over the last 1My suggest that subsidence initially affected the entire basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modeled the tectonic evolution of the Kanto Basin following the method of Matsu'ura and Sato (1989) for a kinematic subduction model with dislocations, in order to quantitatively assess the effects of PHS and PAC subduction. We include the steady slip-rate deficit (permanent locking rate at the plate interface) in our model to account for collision process. We explore how the latest collision of the Izu Peninsula block has been affected by a westerly shift in the PHS plate motion vector with respect to the Eurasian plate, thought to have occurred between 1.0–0.5Ma, using long-term vertical deformation data to constrain extent of the locked zone on the plate interface. We evaluated the change in vertical deformation rate for two scenarios: (1) a synchronous shift in the orientation of the locked zone as PHS plate motion shifts and (2) a delayed shift in the orientation of the locked zone following the shift in plate motion. Observed changes in the uplift/subsidence pattern are better explained by scenario (2), suggesting that recent (<1My) deformation in the Kanto Basin shows a lag in crustal response to the plate motion shift. We also calculated stress accumulation rates and found a good match with observed earthquake mechanisms, which shows that intraplate earthquakes serve to release stress accumulated through long-term plate interactions.

The role of farfield tectonic stress in oceanic intraplate deformation, Gulf of Alaska

An integration of geophysical data from the Pacific Plate reveals plate bending anomalies, massive intraplate shearing and deformation, and a lack of oceanic crust magnetic lineaments in different regions across the Gulf of Alaska. We argue that farfield stress from the Yakutat Terrane collision with North America is the major driver for these unusual features. Similar plate motion vectors indicate that the Pacific plate and Yakutat Terrane are largely coupled along their boundary, the Transition Fault, with minimal translation. Our study shows that the Pacific Plate subduction angle shallows toward the Yakutat Terrane and supports the theory that the Pacific Plate and Yakutat Terrane maintain coupling along the subducted region of the Transition Fault. We argue that the outboard transfer of collisional stress to the Pacific Plate could have resulted in significant strain in the NE corner of the Pacific Plate, which created pathways for igneous sill formation just above the Pacific Plate crust in the Surveyor Fan. A shift in Pacific Plate motion during the late Miocene altered the Yakutat collision with North America, changing the stress transfer regime and potentially terminating associated strain in the NE corner of the Pacific Plate. The collision further intensified as the thickest portion of the Yakutat Terrane began to subduct during the Pleistocene, possibly providing the impetus for the creation of the Gulf of Alaska Shear Zone, a &gt; 200 km zone of intraplate strike‐slip faults that extend from the Transition Fault out into the Pacific Plate. This study highlights the importance of farfield stress from complex tectonic regimes in consideration of large‐scale oceanic intraplate deformation.

Relationships between kinematic indicators and strain during syn-deformational exhumation of an oblique slip, transpressive, plate boundary shear zone: The Alpine Fault, New Zealand

In order to assess how different elements of plate boundary-scale shear zone tectonite fabrics reflect the boundary kinematics and strain history, we have examined a range of such structures within the active Alpine Fault mylonite zone, New Zealand. In this oblique slip zone we find that quartz CPO fabric symmetry and individual mica grain lineations reflect incremental stretches but develop stable orientations parallel to the finite stretching direction (S1). This S1 trends oblique to the plate motion vector because the shear zone experiences some extrusion toward the free surface. Conversely, asymmetric structures (such as shear bands and mica fish) reflect simple shear-dominated deformation late in the shear zone history, parallel to the imposed relative plate motion vector. Finally, linear fabrics are a poor record of the mylonitic deformation in this zone, because (1) they are dominantly inherited and have been only partially re-oriented during mylonitisation, and (2) macroscopically they become less visible at higher strains due to fabric homogenisation. Generally, these results indicate that, particularly in heterogeneous, polyphase shear zones, a full understanding of the movement history can only be obtained by considering kinematic information from as many different sources as possible.

Crustal stress and strain patterns in the Indian plate interior: implications for the deformation behaviour of a stable continent and its seismicity

Terra Nova, 23, 407–415, 2011AbstractThe present analyses indicate that the maximum principal stress axis of the Indian plate interior does not coincide with the plate motion vector. The strain rate curves for the Indian plate indicate interference of two different orders of deformation. The magnitudes of the first‐order maximum strain rates are lower than local extreme strain rates of the second‐order deformation. It has been shown herein that the oblique motion of the Indian plate with respect to a state of dominant strike‐slip deformation in the Indian plate interior is partitioned into a principal deformation zone, synthetic shear and antithetic shear. The strain rates along these regional shear zones show complex association of second‐order high shortening rate coinciding with high extension rate. The earthquakes of the Indian plate interior are interpreted to be results of periodic accumulation of stress/strain due to shortening and release of strain along the same directions during the extension.

Lithospheric control on the spatial pattern of Azores hotspot seafloor anomalies: Constraints from a model of plume-triple junction interaction

[1] The Azores hotspot is located near a plate boundary triple junction (TJ) consisting of the Terceira Rift (TER) and two branches of the Mid-Atlantic Ridge (MAR). The seafloor expression of the Azores hotspot has a complex spatial pattern. Latitudinal anomalies in seafloor depth and other data along the MAR extend farther to the south of the inferred location of the mantle heterogeneity than to the north. Longitudinal anomalies span a greater distance to the east of the MAR (along the TER) than to the west. A finite element model is used to investigate how the divergence of three plates away from a TJ may affect the spatial dispersion of thermally buoyant material simulating a mantle plume. Prescribed plate motion vectors approximate the kinematics of the Azores TJ during a main phase of plateau formation ∼7 Ma. The plume is located off axis to the southeast of the simulated triple junction, following several studies that suggest that the present-day conduit is located near the islands of Faial and Pico. Asymmetry in the divergence of the three plates with respect to the triple junction tends to drive plume material preferentially southward and eastward, consistent with observed anomalies.

Correction to “Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake”

[1] In the paper “Coseismic slip distribution of the February 27, 2010 Mw 8.8 Maule, Chile earthquake” by Fred F. Pollitz et al. (Geophysical Research Letters, 38, L09309, doi:10.1029/2011GL047065, 2011) the captions for Figure 1–3 are ordered incorrectly. The correct captions for Figures 1–3 appear here. [2] Figure 1. Rupture areas associated with historic earthquakes along the Andean megathrust and aftershocks of the February 27, 2010 event. Historical epicenters are provided by Centro Regional de Sismologia para America del Sur and aftershock locations are provided by the National Earthquake Information Center. White lines indicate the surface projection of the fault plane used to model the 2010 event. Nazca–South America relative plate motion vector is from Ruegg et al. [2009]. [3] Figure 2. Observed coseismic GPS offsets (black vectors) with 95% uncertainties compared with model horizontal offsets using the coseismic slip model obtained by the joint InSAR/GPS inversion, which is contoured in gray (values in meters). White lines indicate the surface projection of the fault plane. [4] Figure 3. Coseismic slip models derived from (a) InSAR data only, (b) GPS data only, and (c) combined GPS and InSAR data, with seismic moment indicated for each model. Contour interval is 3 m. Star symbol indicates the Global CMT epicenter.

Where is the real transform boundary in California?

The zone of highest geodetically defined strain rate in California does not everywhere coincide with the surface trace of the San Andreas Fault (SAF). To determine whether this reflects the pattern of long‐term, permanent deformation, we analyzed the velocity field on swaths across the transform, located so as to avoid intersections among the major fault strands. Slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model, representing the interseismic strain accumulation. Our slip rates compare well with current geologic estimates, which suggests that the present‐day velocity field is representative of long‐term motions. We find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the SAF and has an overall trend closer to the relative plate motion vector than the SAF. Most sections of the SAF take up less than half of the total slip rate, and slip is transferred from one part of the system to another in a way that suggests that the SAF should not be considered as a unique locator of the plate boundary. Up to half of the total displacement takes place on faults outside the high strain rate zone, distributed over several hundred kilometers on either side. Our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere, around which stress and strain rate decrease on a length scale controlled by the length of the transform.