Relative Plate Motion Research Articles

Coherence between geodetic and seismic deformation in a context of slow tectonic activity (SW Alps, France)

A dense, local network of 30 geodetic markers covering a 50×60km2 area in the southwestern European Alps (Briançon region) has been temporarily surveyed in 1996, 2006 and 2011 by GPS. The aim is to measure the current deformation in this seismically active area. The study zone is characterized by a majority of extensional and dextral focal mechanisms, along north–south to N160 oriented faults. The combined analysis of the three measurement campaigns over 15 years and up to 16 years of permanent GPS data from the French RENAG network now enables to assess horizontal velocities below 1mm/year within the local network. The long observation interval and the redundancy of the dense campaign network measurement help to constrain a significant local deformation pattern in the Briançon region, yielding an average E–W extension of 16±11nanostrain/year. We compare the geodetic deformation field to the seismic deformation rate cumulated over 37 years, and obtain good coherencies both in amplitude and direction. Moreover, the horizontal deformation localized in the Briançon region represents a major part of the Adriatic-European relative plate motion. However, the average uplift of the network in an extensional setting needs the presence of buoyancy forces in addition to plate tectonics.

Maximum‐Likelihood Recurrence Parameters and Conditional Probability of a Ground‐Rupturing Earthquake on the Southern Alpine Fault, South Island, New Zealand

Abstract The Alpine fault in south Westland, New Zealand, releases strains of Pacific–Australian relative plate motion in large earthquakes with an average interevent spacing of ∼330 years. A new record of earthquake recurrence has been developed at Hokuri Creek, with evidence for 22 events. The youngest Hokuri Creek earthquake overlaps in time and is believed to be the same as the oldest of another site about 100 km to the northwest near Haast. The combined record spans the last 7900 years and includes 24 events. We study the recurrence rate and conditional probability of ground ruptures from this record using a new likelihood‐based approach for estimation of recurrence model parameters. Paleoseismic parameter estimation includes both dating and natural recurrence uncertainties. Lognormal and Brownian passage time (BPT) models are considered. The likelihood surface has distribution location and width parameters as axes, the mean and standard deviation of the log recurrence for the lognormal, and the mean and coefficient of variation for the BPT. The maximum‐likelihood (ML) point gives the parameters most likely to have given rise to the data. The ML point, 50‐year conditional probabilities of a ground‐rupturing earthquake are 26.8% and 26.1% for the lognormal and BPT models, respectively. Contours of equal likelihood track the parameter pairs that are equally probable to have given rise to the observed data. Conditional probabilities on the lognormal 95% boundary around the ML point range from 18.2% to 35.8%. An empirical distribution model completely based on past recurrence times gives a similar conditional probability of 27.1% (9.6%–50.2%). In contrast, the time‐independent conditional probability estimate of 13.6% (8.8%–19.1%) is about half that of the time‐dependent models. A nonparametric test of earthquake recurrence at Hokuri Creek indicates that time‐dependent recurrence models best represent the southern Alpine fault of the South Island, New Zealand.

Ophiolitic rocks of the Indo-Myanmar Ranges, NE India: relicts of an inverted and tectonically imbricated hyper-extended continental margin basin?

Abstract The Indo-Myanmar Ranges (IMR) of NE India are host to various ophiolitic rocks, including metamorphosed Alpine-type harzburgite and lherzolite. Compared to abyssal peridotites of normal oceanic lithosphere, these ultramafic rocks are enriched in trace and rare earth elements. Spilitic pillow lavas along with mafic dykes and sills locally intruded into the serpentinized ultramafic rocks and associated pelagic sediments exhibit alkaline compositional affinities. Ophiolite formation and emplacement were by a process analogous to that described for mantle exhumation in hyper-extended continental margin settings and ophiolites in parts of the European Alps, involving very slow passive continental margin rifting accompanied by slow upwelling or extensional unroofing of the subcontinental upper mantle up to the seafloor. Preliminary palaeomagnetic measurements conducted on ultramafic rocks within the IMR ophiolite belt give a virtual geomagnetic pole (VGP) at 47° N, 045° E for thermal demagnetization (TDM) measurements and 33° N, 013° E for the alternating field demagnetization (AfD) measurements, requiring an anticlockwise rotation of the ultramafic bodies by 14° during the subduction process. The original trend of the spreading axis of the ophiolites was probably NE–SW, with spreading directed NW–SE. Computation of palaeolatitude of the ultramafic rocks gives an average value of 24.67°. Comparison between the palaeolatitude and the present latitude of the sample sites provides a mere latitudinal shift of less than 1°. Field studies, combined with an analysis of structural and tectonic features in the IMR, suggest a generalized WNW–ESE (east–west) compression and NNE–SSW (north–south) extension contradictory to the NNE–SSW contraction indicated by seismic data. Area balancing techniques employed along sections orientated perpendicular to regional tectonic strike in the IMR reveal systematic variations in the amount of crustal shortening, with a maximum of approximately 60% recorded in the Nagaland–Manipur segment along 25.644° N, 93.826° E–25.076° N, 95.897° E. The amount of shortening gradually decreases away from the axis of maximum shortening and on both sides. Calculations of relative plate motion based on rotation vectors given by different workers for various plate pairs represented in the region reveal that the interaction between the Indian and Myanmar plates can ideally produce the structural and tectonic features of this range. Dextral shear coupled to oblique subduction of the Indian Plate below the Myanmar Plate can best explain all of the structural and tectonic features present in the IMR.

Kinematic Barrier Constraints on the Magnitudes of Additional Great Earthquakes Off the East Coast of Japan

Online Material: Figures showing potential areas and magnitudes of earthquakes rupturing partially coupled regions of the subduction zones. In the wake of the 2011 great Tohoku‐Oki, Japan, earthquake, questions were asked about the extent to which this earthquake ruptured a part of the subduction zone that had been strongly coupled in the decades prior to the event (Avouac, 2011; Lay and Kanamori, 2011; Loveless and Meade, 2011). The primary means for investigating this question is the analysis of interseismic geodetic data. These data record both tectonic motions and the accumulation of elastic strain the upper crust. Combined with elastic dislocation and block models, Global Navigation Satellite System (GNSS) data from the GNSS Earth Observation Network System (GEONET) array had been used to estimate the spatial variation in interseismic coupling (defined as the fraction of relative motion between tectonic plates that contributes to elastic strain accumulation) along the Japan trench subduction interface prior to the occurrence of the 2011 event (Nishimura et al. , 2004; Suwa et al. , 2006; Hashimoto et al. , 2009; Loveless and Meade, 2010). Each of these estimated interseismic coupling distributions identified a partially to strongly coupled region, extending for ∼400 km between 36° and 40° north latitude along the Japan trench, approximately bounding the lateral and down‐dip extents of the coseismic rupture (Loveless and Meade, 2011). However, none of these estimates show a perfect spatial correspondence between the coseismic rupture extent and complete preseismic coupling. Rather, the coseismic rupture was laterally bounded by regions of the Japan subduction zone with a coupling fraction c of ∼0.3, in which 30% of the relative plate motion contributes to elastic strain accumulation during the interseismic regime (Loveless and Meade, 2011). This spatial relationship could result from insufficient elastic strain accumulation around portions of the interface characterized by …

The role of thrust faulting in the formation of the eastern Alaska Range: Thermochronological constraints from the Susitna Glacier Thrust Fault region of the intracontinental strike-slip Denali Fault system

Horizontal-slip along restraining bends of strike-slip faults is often partitioned into a vertical component via splay faults. The active Susitna Glacier Thrust Fault (SGTF), as shown by its initiation of the 2002 M7.9 Denali Fault earthquake, lies south of, and intersects the dextral strike-slip Denali Fault. Geochronology and thermochronology data from samples across the SGTF constrain the region's tectonic history and the role of thrusting in the formation of the eastern Alaska Range south of the Denali fault. U-Pb zircon ages indicate intrusion of plutons in the footwall (~57 Ma) and hanging wall (~98 Ma). These U-Pb zircon ages correlate to those from the Ruby Batholith/Kluane Terrane ~400 km east along the Denali Fault, supporting geologic correlations and hence constraints on long-term slip rates. 40Ar/39Ar mica and K-feldspar data from footwall and hanging wall samples (~54 to ~46 Ma) reflect cooling following magmatism and/or regional Eocene metamorphism related to ridge subduction. Combined with apatite fission track data (ages 43–28 Ma) and thermal models, both sides of the SGTF acted as a coherent block during the Eocene and early Oligocene. Contrasting apatite (U-Th)/He ages across the Susitna Glacier (~25 Ma footwall, ~15 Ma hanging wall) suggest initiation of faulting during the middle Miocene. Episodic cooling and exhumation is related to thrusting on known or hypothesized faults that progressively activate due to varying partition of strain along the Denali Fault associated with changing kinematics and plate interaction (Yakutat microplate collision, flat-slab subduction and relative plate motion change) at the southern Alaskan plate margin.

Pn anisotropy beneath the South Island of New Zealand and implications for distributed deformation in continental lithosphere

AbstractPn travel times from regional earthquakes recorded both by stations on New Zealand and by ocean bottom seismographs deployed offshore indicate anisotropy in the uppermost mantle beneath the region. The largest anisotropy of ~8% (±2%, 1σ) lies beneath the deforming part of the South Island to just off its West Coast, a zone roughly 100–200 km wide. The fastest propagation is aligned N60°E (±3°), essentially parallel to the largely strike‐slip relative plate motion since 20 Ma, also ~ N60°E. The magnitude of anisotropy decreases abruptly northwest and southeast of this zone, and on the southeast side of the island, the orientation of fastest propagation is between N32°W and N‐S. The ~ N60°E orientation of fast propagation is consistent with finite strain within the uppermost part of the mantle lithosphere if the measured 850 km of displacement of the Pacific plate past the Australia plate is spread over a region with a width of 100–200 km. The agreement of this orientation of fast propagation with the orientation or relative plate motion suggests the possibility of but does not require some dynamic recrystallization in rock as cold as 500–800°C, where Peierls creep seems to be the likely deformation mechanism. Such a strain distribution matches deformation of a thin viscous sheet that obeys a constitutive relationship of the form , where is the average strain rate and τ is the operative deviatoric stress, with an average value of n ≈ 3–10. Presumably, the NW‐SE fast propagation in the region southeast of the island results from strain that precedes the Cenozoic deformation that has shaped the island.

The 23 June 2014Mw7.9 Rat Islands archipelago, Alaska, intermediate depth earthquake

On 23 June 2014, the largest intermediate depth earthquake (M_w 7.9) of the last 100 years ruptured within the subducting Pacific plate about 100 km below the Rat Islands archipelago of the Western Aleutian Islands, Alaska. The unusual faulting orientation, strike = 206°, dip = 24°, and rake = −14°, is possibly related to curvature of the underthrust slab and high obliquity of the relative plate motions. The first ~15 s of the rupture generated relatively weak seismic waves, followed by strong energy release for the next 25 s. The seismic moment is 1.0 × 10^(21) Nm, and slip of up to ~10 m is concentrated within a 50 km × 50 km region. The radiated energy is 1.1 to 2.7 × 10^(16) J, assuming attenuation t* of 0.4 to 0.7 s. This type of intraplate faulting can be very damaging for populated regions above subduction zones such as Japan, Taiwan, Chile, and Indonesia.

Assembly of a large earthquake from a complex fault system: Surface rupture kinematics of the 4 April 2010 El Mayor–Cucapah (Mexico) Mw 7.2 earthquake

The 4 April 2010 moment magnitude (M_w) 7.2 El Mayor–Cucapah earthquake revealed the existence of a previously unidentified fault system in Mexico that extends ∼120 km from the northern tip of the Gulf of California to the U.S.–Mexico border. The system strikes northwest and is composed of at least seven major faults linked by numerous smaller faults, making this one of the most complex surface ruptures ever documented along the Pacific–North America plate boundary. Rupture propagated bilaterally through three distinct kinematic and geomorphic domains. Southeast of the epicenter, a broad region of distributed fracturing, liquefaction, and discontinuous fault rupture was controlled by a buried, southwest-dipping, dextral-normal fault system that extends ∼53 km across the southern Colorado River delta. Northwest of the epicenter, the sense of vertical slip reverses as rupture propagated through multiple strands of an imbricate stack of east-dipping dextral-normal faults that extend ∼55 km through the Sierra Cucapah. However, some coseismic slip (10–30 cm) was partitioned onto the west-dipping Laguna Salada fault, which extends parallel to the main rupture and defines the western margin of the Sierra Cucapah. In the northernmost domain, rupture terminates on a series of several north-northeast–striking cross-faults with minor offset (<8 cm) that cut uplifted and folded sediments of the northern Colorado River delta in the Yuha Desert. In the Sierra Cucapah, primary rupture occurred on four major faults separated by one fault branch and two accommodation zones. The accommodation zones are distributed in a left-stepping en echelon geometry, such that rupture passed systematically to structurally lower faults. The structurally lowest fault that ruptured in this event is inclined as shallowly as ∼20°. Net surface offsets in the Sierra Cucapah average ∼200 cm, with some reaching 300–400 cm, and rupture kinematics vary greatly along strike. Nonetheless, instantaneous extension directions are consistently oriented ∼085° and the dominant slip direction is ∼310°, which is slightly (∼10°) more westerly than the expected azimuth of relative plate motion, but considerably more oblique to other nearby historical ruptures such as the 1992 Landers earthquake. Complex multifault ruptures are common in the central portion of the Pacific North American plate margin, which is affected by restraining bend tectonics, gravitational potential energy gradients, and the inherently three-dimensional strain of the transtensional and transpressional shear regimes that operate in this region.

Far-Field Deformation Resulting from Rheologic Differences Interacting with Tectonic Stresses: An Example from the Pacific/Australian Plate Boundary in Southern New Zealand

The Miocene in Southern New Zealand was dominated by strike-slip tectonics. Stratigraphic evidence from this time attests to two zones of subsidence in the south: (a) a middle Cenozoic pull-apart basin and (b) a regionally extensive subsiding lake complex, which developed east and distal to the developing plate boundary structure. The lake overlay a block of crust with a significantly weak mid-crustal section and we pose the question: can rheological transitions at an angle to a plate boundary produce distal subsidence and/or uplift? We use stratigraphic, structural and geophysical observations from Southern New Zealand to constrain three-dimensional numerical models for a variety of boundary conditions and rheological scenarios. We show that coincident subsidence and uplift can result from purely strike-slip boundary conditions interacting with a transition from strong to weak to strong mid-crustal rheology. The resulting pattern of vertical displacement is a function of the symmetry or asymmetry of the boundary conditions and the extent and orientation of the rheological transitions. For the Southern New Zealand case study, subsidence rates of ~0.1 mm/yr are predicted for a relative plate motion of 25 mm/yr, leading to ~500 m of subsidence over a 5 Ma time period, comparable to the thickness of preserved lacustrine sediments.

Derivation of paleolongitude from the geometric parametrization of apparent polar wander path: Implication for absolute plate motion reconstruction

AbstractObtaining ancient longitude position of continents in the past has always been a challenge for plate tectonic reconstructions. Paleomagnetism has been commonly used to reconstruct paleolatitudes and relative rotations but not paleolongitudes. In this work, we present a synthesized method to derive paleolongitude by geometrically parametrizing apparent polar wander path (APWP). Great and small circle modeling are implemented concurrently to the identified APWP tracks to calculate the paleomagnetic Euler parameters (stage rotation pole and rotation angle). From the Euler parameters of the optimal fitting option, the absolute motion history can be restored for the reference geometries. Using our method as well as the results from relative plate motion studies, we reevaluate the dispersion history of East Gondwana since 140 Ma. To further test the validity of our method, we compare the predictions from four other absolute motion models mainly in paleolatitude movement, longitudinal variation, and great circle distance, which suggest the most similarity with the global hybrid reference frame.

Gravimetric structure for the abyssal mantle massif of Saint Peter and Saint Paul peridotite ridge, Equatorial Atlantic Ocean, and its relation to active uplift

This paper presents gravimetric and morphologic analyses based on the satellite-derived data set of EGM2008 and TOPEX for the area of the oceanic mantle massif of the Saint Peter and Saint Paul peridotite ridge, Equatorial Atlantic Ocean. The free-air anomaly indicates that the present plate boundary is not situated along the longitudinal graben which cuts peridotite ridge, but about 20 km to the north of it. The high Bouguer anomaly of the peridotite ridge suggests that it is constituted mainly by unserpentinised ultramafic rocks. The absence of isostatic compensation and low-degree serpentinisation of the ultramafic rocks indicate that the peridotite ridge is sustained mainly by active tectonic uplift. The unparallel relation between the transform fault and the relative plate motion generates near north-south compression and the consequent tectonic uplift. In this sense, the peridotite massif is a pressure ridge due to the strike-slip displacement of the Saint Paul Transform Fault.

Steady contemporary deformation of the central Basin and Range Province, western United States

AbstractWe use data from western U.S. GPS networks to estimate the rate, pattern, and style of tectonic deformation of the central Basin and Range Province (BRP). Previous geodetic investigations have found the crust of eastern Nevada and western Utah to act as a rigid microplate, with zero deformation rates to within measurement uncertainty. Observed transients in GPS time series have led others to propose a megadetachment model, predicting that the central BRP behaves as a microplate, but with time‐varying translation. Here we reassess these hypotheses, benefiting from a significant increase in GPS stations and time span, and innovations in analysis techniques. Our results show that the BRP crust deforms everywhere and all the time. In a group of 24 stations between longitude −113.5° and −116.8°, we find strain rates of 1.9 ± 0.2 × 10−9 yr−1 extension directed N55°W and 2.2 ± 0.2 × 10−9 yr−1 contraction directed N35°E, inconsistent with microplate behavior. The linearity of time series of strain from GPS station triplets is inconsistent with episodic translation of quasi‐rigid domains. One exception is station EGAN that exhibits nonlinear motion not found in adjacent stations. The dominant signal in Nevada is distributed shear consistent with Pacific/North America relative plate motion, suggesting that stresses are transmitted through the lithosphere at least 800 km from the plate boundary. The observed active extension is consistent with earthquake focal mechanisms and is in agreement with integrated rates estimated from earthquake geology. Our results do not support the proposed megadetachment in the BRP.

Localized slip and distributed deformation in oblique settings: the example of the Denali fault system, Alaska

In most fault systems the direction of the relative plate motion is oblique to the azimuth of the existing faults. Hence, during earthquakes the displacement may be partitioned between several faults that accommodate different components of the total motion. Here, we quantify the effect of the obliquity of the fault system relatively to the plate-motion direction on the distribution of the deformation in the fault system, during distinct periods of the seismic cycle. The 2002 November, M w 7.9, Denali strike-slip earthquake ruptured 341 km of the Denali fault. The azimuth of the fault varies by more than 50 • over the total rupture length, making the Denali fault an ideal system to test the effect of obliquity. From west to east, thrust dominates the first part of the rupture while strike-slip dominates the central and eastern sections. Using a kinematic model that considers the obliquity of the plate-motion direction relative to the local fault azimuth, we explored how much of the far-field tectonic loading is accommodated on the main strike-slip fault during the earthquake, and how much is accommodated by distributed deformation off the main fault, on secondary structures. Using a dataset of 735 focal mechanisms, we represent the deformation using strain rosettes and we compare seismological data with model results using the areal strain. Then we developed the parameter Ca, the coefficient of accommodation, which allows a direct quantification of the efficiency of a fault to accommodate oblique motion. Using these indicators, we show that in oblique setting, such as in the Denali case, the aftershocks and the background seismicity are organized to accommodate a significant part of the deformation that is not taken on the Denali strike-slip fault during the main earthquakes. The westward increase of the obliquity actually increases the amount of such deformation accommodated through distributed thrust faults, leading to the westward widening of the Alaska Range, located north of the Denali fault. Therefore, the strain partitioning between localized slip on pre-established major faults and distributed deformation accommodated through aftershocks and background seismicity on smaller faults (highlighted here by the longer-term topography) seems to be needed during the seismic cycle to accommodate the boundary conditions in such oblique settings.

Accreted Kula plate fragment at 94Ma in the Yokonami-melange, Shimanto-belt, Shikoku, Japan

Abstract The Cretaceous Shimanto belt that is distributed along the Pacific side of southwest Japan contains one of the best records of accretionary processes and history of oceanic plate subduction in the circum-Pacific region. Yokonami melange, Shimanto belt, Shikoku, SW Japan is composed of pillowed basalt that erupted in the equatorial region, about 200 m-thick bedded radiolarian chert sequence that contains radiolarians with age determinations spanning ca. 135 Ma (Upper Valanginian) to 93.9 Ma (Cenomanian), younging southward with repetition, and alternated sandstone and mudstone. Although the chronology of the bedded chert sequence is well studied using the radiolarian, the chronology of the clastic units was not reported sufficiently. We performed a detailed geologic mapping investigation of rock outcrops along the Goshiki-no-Hama Coast, the Yokonami melange. We have determined U–Pb ages of zircon grains extracted from tuffs and sandstones to determine the depositional age of the turbidite and hemipelagic units. The youngest depositional age of these clastic sediments is interpreted to be the arrival time at the trench. We reconstructed the ocean plate stratigraphy with our new chronological data, which includes mid-oceanic basalt, bedded chert, hemipelagic sediments, and trench-turbidite deposits. We recognize that the turbidite sequence has been repeated by layer-parallel thrust in the study area. We have delineated the travel trajectory of the oceanic lithosphere based on (1) the relative plate motion model of the Paleo-Pacific Ocean, (2) the arrival time of the subducted lithosphere at the trench (94.32 Ma) and (3) paleomagnetic measurement of the basalt and turbidite. The trajectory suggests that the MORB in the Goshiki-no-Hama Coast, Yokonami melange, erupted at the Kula–Pacific Ridge in the Paleo-Pacific Ocean, traveled northward, and eventually arrived at the continental margin of East Asia at 94.32 Ma.

Localized shear in the deep lithosphere beneath the San Andreas fault system

Research Article| April 01, 2014 Localized shear in the deep lithosphere beneath the San Andreas fault system Heather A. Ford; Heather A. Ford * 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA *Current address: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA; E-mail: heather.ford@yale.edu. Search for other works by this author on: GSW Google Scholar Karen M. Fischer; Karen M. Fischer 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Search for other works by this author on: GSW Google Scholar Vedran Lekic Vedran Lekic 2Department of Geology, University of Maryland, College Park, Maryland 20742, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Heather A. Ford * 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Karen M. Fischer 1Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02912, USA Vedran Lekic 2Department of Geology, University of Maryland, College Park, Maryland 20742, USA *Current address: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA; E-mail: heather.ford@yale.edu. Publisher: Geological Society of America Received: 19 Sep 2013 Revision Received: 31 Dec 2013 Accepted: 10 Jan 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (4): 295–298. https://doi.org/10.1130/G35128.1 Article history Received: 19 Sep 2013 Revision Received: 31 Dec 2013 Accepted: 10 Jan 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Heather A. Ford, Karen M. Fischer, Vedran Lekic; Localized shear in the deep lithosphere beneath the San Andreas fault system. Geology 2014;; 42 (4): 295–298. doi: https://doi.org/10.1130/G35128.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Seismic images of the base of the lithosphere across the San Andreas fault system (California, USA) yield new constraints on the distribution of deformation in the deep lithosphere beneath this strike-slip plate boundary. We show that conversions of shear to compressional waves (Sp) across the base of the lithosphere are systematically weaker on the western side of the plate boundary, indicating that the drop in seismic shear-wave velocity from lithosphere to asthenosphere is either smaller or occurs over a larger depth range. In central and northern California, the lithosphere-asthenosphere boundary changes character across a distance of <50 km, and does so directly beneath the San Andreas fault along its simple central segment, and beneath the Calaveras–Green Valley–Bartlett Springs faults to the north. Given the absolute velocities of the North America and Pacific plates, and low viscosities inferred for the asthenosphere, these results indicate the juxtaposition of mantle lithospheres with different properties across these faults. The spatial correlation between the central San Andreas fault and the laterally abrupt change in the velocity structure of the deepest mantle lithosphere points to the accommodation of relative plate motion on a narrow shear zone (<50 km in width), and a rheology that enables strain localization throughout the thickness of the lithosphere. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The geometry of the North Anatolian transform fault in the Sea of Marmara and its temporal evolution: implications for the development of intracontinental transform faults

The North Anatolian Fault is a 1200 km long strike-slip fault system connecting the East Anatolian convergent area with the Hellenic subduction zone and, as such, represents an intracontinental transform fault. It began forming some 13–11 Ma ago within a keirogen, called the North Anatolian Shear Zone, which becomes wider from east to west. Its width is maximum at the latitude of the Sea of Marmara, where it is 100 km. The Marmara Basin is unique in containing part of an active strike-slip fault system in a submarine environment in which there has been active sedimentation in a Paratethyan context where stratigraphic resolution is higher than elsewhere in the Mediterranean. It is also surrounded by a long-civilised rim where historical records reach well into the second half of the first millennium BCE (before common era). In this study, we have used 210 multichannel seismic reflexion profiles, adding up to 6210 km profile length and high-resolution bathymetry and chirp profiles reported in the literature to map all the faults that are younger than the Oligocene. Within these faults, we have distinguished those that cut the surface and those that do not. Among the ones that do not cut the surface, we have further created a timetable of fault generation based on seismic sequence recognition. The results are surprising in that faults of all orientations contain subsets that are active and others that are inactive. This suggests that as the shear zone evolves, faults of all orientations become activated and deactivated in a manner that now seems almost haphazard, but a tendency is noticed to confine the overall movement to a zone that becomes narrower with time since the inception of the shear zone, i.e., the whole keirogen, at its full width. In basins, basin margins move outward with time, whereas highs maintain their faults free of sediment cover, making their dating difficult, but small perched basins on top of them in places make relative dating possible. In addition, these basins permit comparison of geological history of the highs with those of the neighbouring basins. The two westerly deeps within the Sea of Marmara seem inherited structures from the earlier Rhodope–Pontide fragment/Sakarya continent collision, but were much accentuated by the rise of the intervening highs during the shear evolution. When it is assumed that below 10 km depth the faults that now constitute the Marmara fault family might have widths approaching 4 km, the resulting picture resembles a large version of an amphibolite-grade shear zone fabric, an inference in agreement with the scale-independent structure of shear zones. We think that the North Anatolian Fault at depth has such a fabric not only on a meso, but also on a macro scale. Detection of such broad, vertical shear zones in Precambrian terrains may be one way to get a handle on relative plate motion directions during those remote times.

Global Positioning System Constraints on Active Crustal Deformation in Central Panama

Online Material: Figures of three‐component GPS displacements and residuals; table of derived velocities. The tectonic setting of central Panama has been controlled primarily by the collision of the Central American Arc with the South American continent, beginning in Miocene time (e.g., Coates et al. , 2004). Ongoing collision at a rate of about 25 mm/yr has been inferred using campaign‐style Global Positioning System (GPS) surveys from a sparse, long‐baseline network covering the collision zone (e.g., Trenkamp et al. , 2002; Fig. 1). The central portion of the Panama block (Figs. 1 and 2) is commonly described as a nondeforming microplate (e.g., Trenkamp et al. , 2002; Bird, 2003; Argus et al. , 2011). However, appreciable Pleistocene crustal deformation has been suggested in central Panama based on an analysis of the landscape and paleoseismological trenches (Rockwell et al. , 2010a), and is likely a consequence of ongoing collision diffusely absorbing a fraction of Cocos–Nazca–Caribbean–South America relative plate motion (Rockwell, Bennett et al. , 2010). Figure 1. The regional tectonic setting of Central America. Plate boundaries from Bird (2003) are shown with thick lines, superimposed on shaded topography and bathymetry. The political border of Panama is highlighted with a thin black line. Vectors show the approximate sense of motion of the various plates, including the Panama block, relative to a South America fixed‐velocity reference frame. Ongoing collision of the Panama block with the Northern Andes block continent results in appreciable deformation of central Panama and northwest South America. NPDB shows the location of the North Panama deformed belt, which separates the Caribbean plate from the Panama block. The small square in the Panama block shows the area of Figure 2, which encompasses the Rio Gatun, Limon, and Pedro Miguel faults, …

Total and incremental left-lateral displacement across the East Gobi Fault Zone, southern Mongolia: Implications for timing and modes of polyphase intracontinental deformation

The East Gobi Fault Zone (EGFZ) is a dominant structural feature in southeastern Mongolia. The EGFZ's protracted history includes at least five distinct deformation events occurring over the last 250 million years. Two of these phases are marked by left-lateral motion, but the total and incremental displacement history is poorly known. A better understanding of the displacement history across the EGFZ is required to place its Mesozoic and Cenozoic evolution within the larger context of central Asian tectonics, including evaluating the EGFZ's possible role as an intracontinental extrusion feature. This study presents analysis of sedimentary and igneous rocks as proposed offset markers on either side of the EGFZ. New geochronologic data, integrated with existing structural and stratigraphic data, are used to characterize and quantify two periods of left-lateral displacement along the EGFZ. The slip history of the EGFZ includes a total of ∼250–300 km of left-lateral displacement. Approximately 150–200 km of slip occurred along a major shear zone during the Late Triassic (∼225–210 Ma), based on restoration of Carboniferous intrusive suites and Permian remnant ocean basin deposits. During this first period of left-lateral displacement, the EGFZ acted as a continental extrusion feature responding to oblique collision between the South and North China Blocks and northern Mongolia and Siberia, an event analogous to the later India–Asia collision during the Cenozoic. A second period of ∼90–100 km of left-lateral slip occurred in the Late Oligocene, based on offset of Lower Cretaceous strata and cross-cutting relationships. This brittle deformation phase was contemporaneous with dynamic boundary conditions along the Pacific margin, and the Indo–Asia collision. Cenozoic movement along the EGFZ may have coincided kinematically with the Altyn Tagh Fault in China via dispersed fault zones in the Alxa region. Major displacement along the EGFZ likely ceased in the early Miocene, coincident with changing relative plate motions at the Pacific margin. Magnitudes of Cenozoic offset across the EGFZ are significantly smaller than estimated for the Altyn Tagh Fault, which likely reflects distributed deformation in heterogeneous crust of the southern Central Asian Orogenic Belt.

Upper mantle seismic anisotropy at a strike-slip boundary: South Island, New Zealand

New shear wave splitting measurements made from stations onshore and offshore the South Island of New Zealand show a zone of anisotropy 100–200 km wide. Measurements in central South Island and up to approximately 100 km offshore from the west coast yield orientations of the fast quasi-shear wave nearly parallel to relative plate motion, with increased obliquity to this orientation observed farther from shore. On the eastern side of the island, fast orientations rotate counterclockwise to become nearly perpendicular to the orientation of relative plate motion approximately 200 km off the east coast. Uniform delay times between the fast and slow quasi-shear waves of nearly 2.0 s onshore continue to stations approximately 100 km off the west coast, after which they decrease to ~1 s at 200 km. Stations more than ~300 km from the west coast show little to no splitting. East coast stations have delay times around 1 s. Simple strain fields calculated from a thin viscous sheet model (representing distributed lithospheric deformation) with strain rates decreasing exponentially to both the northwest and southeast with e-folding dimensions of 25–35 km (approximately 75% of the deformation within a zone 100–140 km wide) match orientations and amounts of observed splitting. A model of deformation localized in the lithosphere and then spreading out in the asthenosphere also yields predictions consistent with observed splitting if, at depths of 100–130 km below the lithosphere, typical grain sizes are ~ 6–7 mm.

The kinematics of central-southern Turkey and northwest Syria revisited

Central-southern Turkey, NW Syria, and adjacent offshore areas in the NE Mediterranean region form the boundary zone between the Turkish, African and Arabian plates. A great deal of new information has emerged in recent years regarding senses and rates of active crustal deformation in this region, but this material has not hitherto been well integrated, so the interpretations of key localities by different teams remain contradictory. We have reviewed and synthesised this evidence, combining it with new investigations targeted at key areas of uncertainty. This work has led to the inference of previously unrecognised active faults and has clarified the roles of other structures within the framework of plate motions provided by GPS studies. Roughly one third of the relative motion between the Turkish and Arabian plates is accommodated on the Misis–Kyrenia Fault Zone, which links to the study region from the Kyrenia mountain range of northern Cyprus. Much of this motion passes NNE then eastward around the northern limit of the Amanos Mountains, as previously thought, but some of it splays northeastward to link into newly-recognised normal faulting within the Amanos Mountains. The remaining two thirds of the relative motion is accommodated along the Karasu Valley; some of this component steps leftward across the Amik Basin before passing southward onto the northern Dead Sea Fault Zone (DSFZ) but much of it continues southwestward, past the city of Antakya, then into offshore structures, ultimately linking to the subduction zone bounding the Turkish and African plates to the southwest of Cyprus. However, some of this offshore motion continues southward, west of the Syrian coast, before linking onshore into the southern DSFZ; this component of the relative motion is indeed the main reason why the slip rate on the northern DSFZ, measured geodetically, is so much lower than that on its southern counterpart. In some parts of this region, notably in the Karasu Valley, it is now clear how the expected relative plate motion has been accommodated on active faults during much of the Quaternary: rather than constant slip rates on individual faults, quite complex changes in the partitioning of this motion on timescales of hundreds of thousands of years are indicated. However, in other parts of the region it remains unclear whether additional major active faults remain unrecognised or whether significant relative motions are accommodated by distributed deformation or on the many smaller-scale structures present.