Relative Plate Motion Research Articles

The Malpelo Plate Hypothesis and implications for nonclosure of the Cocos‐Nazca‐Pacific plate motion circuit

AbstractUsing global multiresolution topography, we estimate new transform‐fault azimuths along the Cocos‐Nazca plate boundary and show that the direction of relative plate motion is 3.3° ± 1.8° (95% confidence limits) clockwise of prior estimates. The new direction of Cocos‐Nazca plate motion is, moreover, 4.9° ± 2.7° (95% confidence limits) clockwise of the azimuth of the Panama transform fault. We infer that the plate east of the Panama transform fault is not the Nazca plate but instead is a microplate that we term the Malpelo plate. With the improved transform‐fault data, the nonclosure of the Nazca‐Cocos‐Pacific plate motion circuit is reduced from 15.0 mm a−1 ± 3.8 mm a−1 to 11.6 mm a−1 ± 3.8 mm a−1 (95% confidence limits). The nonclosure seems too large to be due entirely to horizontal thermal contraction of oceanic lithosphere and suggests that one or more additional plate boundaries remain to be discovered.

How fault evolution changes strain partitioning and fault slip rates in Southern California: Results from geodynamic modeling

AbstractIn Southern California, the Pacific‐North America relative plate motion is accommodated by the complex southern San Andreas Fault system that includes many young faults (<2 Ma). The initiation of these young faults and their impact on strain partitioning and fault slip rates are important for understanding the evolution of this plate boundary zone and assessing earthquake hazard in Southern California. Using a three‐dimensional viscoelastoplastic finite element model, we have investigated how this plate boundary fault system has evolved to accommodate the relative plate motion in Southern California. Our results show that when the plate boundary faults are not optimally configured to accommodate the relative plate motion, strain is localized in places where new faults would initiate to improve the mechanical efficiency of the fault system. In particular, the Eastern California Shear Zone, the San Jacinto Fault, the Elsinore Fault, and the offshore dextral faults all developed in places of highly localized strain. These younger faults compensate for the reduced fault slip on the San Andreas Fault proper because of the Big Bend, a major restraining bend. The evolution of the fault system changes the apportionment of fault slip rates over time, which may explain some of the slip rate discrepancy between geological and geodetic measurements in Southern California. For the present fault configuration, our model predicts localized strain in western Transverse Ranges and along the dextral faults across the Mojave Desert, where numerous damaging earthquakes occurred in recent years.

Constraining shifts in North Atlantic plate motions during the Palaeocene by U-Pb dating of Svalbard tephra layers

Radioisotopic dating of volcanic minerals is a powerful method for establishing absolute time constraints in sedimentary basins, which improves our understanding of the chronostratigraphy and evolution of basin processes. The relative plate motions of Greenland, North America, and Eurasia changed several times during the Palaeogene. However, the timing of a key part of this sequence, namely the initiation of compression between Greenland and Svalbard, is currently poorly constrained. The formation of the Central Basin in Spitsbergen is inherently linked to changes in regional plate motions, so an improved chronostratigraphy of the sedimentary sequence is warranted. Here we present U-Pb zircon dates from tephra layers close to the basal unconformity, which yield a weighted-mean 206Pb/238U age of 61.596 ± 0.028 Ma (2σ). We calculate that sustained sedimentation began at ~61.8 Ma in the eastern Central Basin based on a sediment accumulation rate of 71.6 ± 7.6 m/Myr. The timing of basin formation is broadly coeval with depositional changes at the Danian-Selandian boundary around the other margins of Greenland, including the North Sea, implying a common tectonic driving force. Furthermore, these stratigraphic tie points place age constraints on regional plate reorganization events, such as the onset of seafloor spreading in the Labrador Sea.

Transient Quaternary erosion and tectonic inversion of the Northern Range, Trinidad

Cosmogenic 10Be measured in sediments from Northern Range catchments on the island of Trinidad reveals low millennial-scale rates of erosion (average ~40mm/ka) that increase roughly eightfold, from 11 to 92mm/ka, eastward across the mountain range. These results, in conjunction with an analysis of mountain morphometrics, are consistent with Quaternary east-side-up tilting of the Northern Range, which has occurred recently (~100ka). The highest millennial-scale erosion rates coincide spatially with Quaternary east-side surface uplift (albeit not in magnitude), high modern rainfall rates, low topographic relief, and convex longitudinal stream profiles, indicating transient Quaternary erosion. We interpret that hillslope adjustment and erosion of the Northern Range is buffered from climatic and tectonic forcings by dense tropical vegetation cover, thick saprolite, and porous regolith. Compared with longer-term, thermochronology exhumation rates, we document that an order of magnitude deceleration of and reversal in the locus of erosion and exhumation has occurred during the Pliocene to the Holocene. We suggest that these combined data sets highlight distinct syn- and post-collisional phases of Northern Range development that are related to a major change in relative Caribbean-South American plate motion from oblique convergence to transform motion. Oblique collision during the mid-Miocene likely caused relatively higher rates of and asymmetric exhumation of the Northern Range. Post-collisional mountain-scale tilting is likely caused by a combination of crustal extension in the Gulf of Paria and by deep subsurface lithospheric detachment processes that drives dynamic topography.

Kinematic constraints on the Rodinia to Gondwana transition

Earth’s plate tectonic history during the breakup of the supercontinent Pangea is well constrained from the seafloor spreading record, but evolving plate configurations during older supercontinent cycles are much less well understood. A relative paucity of available palaeomagnetic and geological data for deep time reconstructions necessitates innovative approaches to help discriminate between competing plate configurations. More difficult is tracing the journeys of individual continents during the amalgamation and breakup of supercontinents. Typically, deep-time reconstructions are built using absolute motions defined by palaeomagnetic data, and do not consider the kinematics of relative motions between plates, even for occasions where they are thought to be ‘plate-pairs’, either rifting apart leading to the formation of conjugate passive margins separated by a new ocean basin, or brought together by collision and orogenesis. Here, we use open-source software tools (GPlates/pyGPlates) to assess quantitative plate kinematics inherent within alternative reconstructions, such as rates of relative plate motion. We analyse the Rodinia-Gondwana transition during the Neoproterozoic, investigating the proposed Australia-Laurentia configurations during Rodinia, and the motion of India colliding with Gondwana. We find that earlier rifting times provide more optimal kinematic results. The AUSWUS and AUSMEX configurations with rifting at 800Ma are the most kinematically supported configurations for Australia and Laurentia (average rates of 57 and 64mm/a respectively), and angular rotation of ∼1.4°/Ma, compared to a SWEAT configuration (average spreading rate ∼76mm/a) and Missing-Link configuration (∼90mm/a). Later rifting, at, or after, 725Ma necessitates unreasonably high spreading rates of >130mm/a for AUSWUS and AUSMEX and ∼150mm/a for SWEAT and Missing-Link. Using motion paths and convergence rates, we create a kinematically reasonable (convergence below 70mm/a) tectonic model that is built upon a front-on collision of India with Gondwana, while also incorporating sinistral strike-slip motion against Australia and East Antarctica. We use this simple tectonic model to refine a global model for the breakup of western Rodinia and the transition to eastern Gondwana.

Shear Wave Splitting and Mantle Flow in Mexico: What Have we Learned?

A review is presented of the shear wave splitting studies of the upper mantle carried out in Mexico during the last decade. When a seismic wave enters an anisotropic medium it splits, which means that a fast and a slow wave are produced. Two parameters are used to quantify anisotropy. These are the fast polarization direction and the delay time between the fast and the slow wave. An example of the measurement technique is presented using an SKS phase because most observations are based on teleseismic data. Results of two studies using local S waves from intraslab earthquakes are also discussed. Key aspects of the interpretation of splitting measurements are explained. These include the depth localization of anisotropy, the relation-ship between olivine fabrics and mantle flow, the role of absolute plate motion, and the role of relative plate motions with a special focus on subduction zones. An important motivation for studying seismic anisotropy is that it makes it possible to constrain the characteristics of upper mantle flow and its relationship to tectonic processes. Mexico has many diverse tectonic environments, some of which are currently active, or were formerly active, and have left their imprint on seismic anisotropy. This has resulted in a wide variety of mechanisms for driving mantle flow. Broadly speaking, the discussion is organized into the following regions: Baja California peninsula, Western Mexican Basin and Range, northern and northeastern Mexico, the Middle America Trench, the Yucatán peninsula, and lowermost mantle anisotropy. Depending on the unique characteristics encountered within each region, the relationship between anisotropy and mantle flow is explored.

PT-symmetric spectral singularity and negative-frequency resonance

Vacuum consists of a bath of balanced and symmetric positive- and negative-frequency fluctuations. Media in relative motion or accelerated observers can break this symmetry and preferentially amplify negative-frequency modes as in quantum Cherenkov radiation and Unruh radiation. Here, we show the existence of a universal negative-frequency-momentum mirror symmetry in the relativistic Lorentzian transformation for electromagnetic waves. We show the connection of our discovered symmetry to parity-time ($\mathcal{PT}$) symmetry in moving media and the resulting spectral singularity in vacuum fluctuation-related effects. We prove that this spectral singularity can occur in the case of two metallic plates in relative motion interacting through positive- and negative-frequency plasmonic fluctuations (negative-frequency resonance). Our work paves the way for understanding the role of $\mathcal{PT}$-symmetric spectral singularities in amplifying fluctuations and motivates the search for $\mathcal{PT}$ symmetry in novel photonic systems.

Magnetic fabrics of arc plutons reveal a significant Late Jurassic to Early Cretaceous change in the relative plate motions of the Pacific Ocean basin and North America

Contrasting magnetic fabrics in five successively emplaced syntectonic plutons reveal temporal and spatial variations in tectonic strain in the oceanic terranes of the Blue Mountains province, northeastern Oregon, during the Late Jurassic to Early Cretaceous. The inferred strain regimes changed from: (1) thrusting and sinistral shearing at ca. 160 Ma, to (2) horizontal stretching at ca. 147 Ma (in the forearc–accretionary wedge Baker terrane), to (3) dextral transpression that started from ca. 140 Ma onward and was associated with progressive anticlockwise rotation of the principal horizontal shortening direction from ca. 130 Ma to ca. 126 Ma (in the Wallowa oceanic arc terrane). These progressive strain reorientations are interpreted in terms of an outboard Wallowa-­Baker terrane collision, lateral extrusion, docking of the amal­gamated Blue Mountains superterrane into a continental-margin reentrant, and onset of oroclinal bending, respectively. The changes in crustal strains are then interpreted as recording a progressive change in relative motions between the Pacific Ocean basin and North America and suggest a transition from Late Jurassic sinistral deformation to Early Cretaceous dextral terrane translations along the paleo-Pacific margin. We speculate that these events may have been linked along large portions of the North American Cordillera, from central California to Blue Mountains, and may have culminated in the onset of accretion in the Franciscan complex and voluminous plutonism in the Sierra Nevada magmatic arc. A similar plate-kinematic change is inferred to have occurred in British Columbia several tens of millions of years later (at ca. 100 Ma), implying that these kinematic transitions may have varied in space and time along the length of the Cordilleran orogen.

Examination of the kinematic structures in İzmir (Western Anatolia) with repeated GPS observations (2009, 2010 and 2011)

The Western Anatolia and the Aegean Sea regions are one of the most significant seismically active and rapidly deforming fields in the world. Generally, seismic activities cause deformations and these deformations are monitored with Global Positioning System (GPS) /Global Navigation System (GNSS). In this context, GPS data were used to determine the deformation of İzmir and its surrounding to estimate the relative plate motions. In this study, the kinematic structures of the faults, which control the seismic hazard in İzmir and its surroundings, processing results of the three-year (2009, 2010 and 2011) episodic GPS observations and the estimation of displacements for 21 GPS stations were presented. The aim of this study is to examine interplate motion of the stations and their relations with the tectonic structures, seismicity and paleomagnetism and additionally, to interprete the motions of the study area relative to different block motions. Consequently, the mean motion of the study area was found approximately 25 mm/yr (towards the SSW) in the Eurasia fixed frame solution. The Aegean block fixed frame and the Anatolian block fixed frame solutions were computed relative to Euler vectors. In Aegean and Anatolian block solutions it was determined that the stations move separately, not as a group. In Euler pole solution, some stations are separated from each other and meanwhile some stations are grouped by considering the differences and similarities of the station motions. According to this solution three lines and two regions were described in the study area. The relations between seismicity and paleomagnetic studies and the kinematic structures determined in Anatolian block fixed frame and Euler pole solution were also investigated. When the Anatolian block fixed frame solution and the earthquakes occurred between the years 1973 and 2011 were evaluated together, it was found that in the high seismically active region especially near to Sığacık bay, the motions of GPS stations were different even though their locations were close to each other. As a result of this, the relationship between the vector directions and active tectonism was determined. Additionally, in the Euler pole solution directions of the motion were found to be coherent with the paleomagnetic results, particularly in Urla and its surroundings. Here, the block fixed frame and Euler pole solutions and additionally, relations of them with seismicity and tectonism were mentioned as difference from previous studies. Besides, in this study, high importance was given to locate each station in main geological formations of the study area.

Global plate boundary evolution and kinematics since the late Paleozoic

Many aspects of deep-time Earth System models, including mantle convection, paleoclimatology, paleobiogeography and the deep Earth carbon cycle, require high-resolution plate motion models that include the evolution of the mosaic of plate boundaries through time. We present the first continuous late Paleozoic to present-day global plate model with evolving plate boundaries, building on and extending two previously published models for the late Paleozoic (410–250Ma) and Mesozoic-Cenozoic (230–0Ma). We ensure continuity during the 250–230Ma transition period between the two models, update the absolute reference frame of the Mesozoic-Cenozoic model and add a new Paleozoic reconstruction for the Baltica-derived Alexander Terrane, now accreted to western North America. This 410–0Ma open access model provides a framework for deep-time whole Earth modelling and acts as a base for future extensions and refinement.We analyse the model in terms of the number of plates, predicted plate size distribution, plate and continental root mean square (RMS) speeds, plate velocities and trench migration through time. Overall model trends share many similarities to those for recent times, which we use as a first order benchmark against which to compare the model and identify targets for future model refinement. Except for during the period ~260–160Ma, the number of plates (16–46) and ratio of “large” plates (≥107.5km2) to smaller plates (~2.7–6.6) are fairly similar to present-day values (46 and 6.6, respectively), with lower values occurring during late Paleozoic assembly and growth of Pangea. This temporal pattern may also reflect difficulties in reconstructing small, now subducted oceanic plates further back in time, as well as whether a supercontinent is assembling or breaking up. During the ~260–160Ma timeframe the model reaches a minima in the number of plates, in contrast to what we would expect during initial Pangea breakup and thus highlighting the need for refinement of the relative plate motion model. Continental and plate RMS speeds show an overall increase backwards through time from ~200 to 365Ma, reaching a peak at 365Ma of >14 and >16cm/yr, respectively, compared to ~3 and ~5cm/yr, respectively, at present-day. The median value of trench motion remains close to, yet above 0cm/yr for most of the model timeframe, with a dominance in positive values reflecting a prevalence of trench retreat over advance. Trench advance speeds are excessive during the 370–160Ma period, reaching more than four times that observed at present-day. Extended periods of trench advance and global continental and plate RMS speeds that far exceed present-day values warrant further investigation. Future work should test whether alternative absolute reference frames or relative motions would mitigate these high speeds, while still being consistent with geologic and geophysical observations, or alternatively focus on identifying potential driving mechanisms to account for such rapid motions.

Provenance of bentonite layers in the Palaeocene strata of the Central Basin, Svalbard: implications for magmatism and rifting events around the onset of the North Atlantic Igneous Province

A fold-and-thrust belt developed between Greenland and Svalbard during the Palaeogene, with an associated foreland basin forming in what is now Spitsbergen. This Central Basin is comprised of the Van Mijenfjorden Group, a ~2.3km thick sandstone-shale dominated succession that contains prominent and laterally continuous bentonite layers in the lower formations. These altered tephra layers can be used as stratigraphic markers that connect the basin development with regional explosive volcanism and changes to relative plate motions. We sampled and analysed bentonites from nine borehole cores across the Central Basin. Each layer shows evidence of alteration, with mobile elements such as alkali and alkali earth metals particularly disrupted. However, immobile elements including rare earth elements (REE) and preserved igneous minerals retain a magmatic signature, allowing for comparisons with potential volcanic sources to be made. The majority of bentonites are both evolved and strongly alkaline, with chemical signatures that are much closer to the continental rift events around Ellesmere Island and North Greenland than to the early activity of the North Atlantic Igneous Province (NAIP). There is a clear difference between tephra layers in the mid Palaeocene versus late Palaeocene strata. The early bentonites have a REE signature comparable to the volcanics of the Kap Washington Group exposed in North Greenland. The later bentonites have likely come from volcanic centres in the Nares Strait that are also the source of abundant volcaniclastic sediments in the Judge Daly Promontory, Ellesmere Island. These findings suggest that a mid to late Palaeocene change in locus of volcanic provenance may reflect changes in relative plate motions related to the formation of the West Spitsbergen fold-and-thrust belt and the emplacement of the NAIP. However, the lack of bentonites matching NAIP sources suggests that explosive volcanism was of insufficient magnitude to lead to deposition in the Central Basin at this time.

Acceleration of regional plate subduction beneath Kanto, Japan, after the 2011 Tohoku‐oki earthquake

AbstractTwo oceanic plates (the Pacific (PA) and Philippine Sea (PH)) subduct beneath the land plate, and they forms deeper (PH‐PA) and shallower (land‐PH) plate boundaries beneath Kanto. Remarkably enhanced seismicity was observed in the densely populated area after the 2011 Tohoku‐oki earthquake, which cannot simply be explained by the southern expansion of the postseismic slip of the earthquake. We examine interplate repeating earthquakes to constrain the relative plate motion across aseismically slipping faults. The repeater slip rates show creep rates in the deeper and shallower boundaries, respectively, increased to 2.4–6.6 times and 1.3–6.2 times the pre‐Tohoku‐oki rates. In addition, the repeater slip directions show no change larger than 4° in the deeper boundary. The interplate creep rates and slip directions suggest that regional movements of both the two plates had accelerated. They probably caused the seismicity increase, frequent slow slips on the shallower boundary, and enhanced probability of larger earthquakes.

Seamounts and oceanic igneous features in the NE Atlantic: a link between plate motions and mantle dynamics

Abstract A new regional compilation of seamount-like oceanic igneous features (SOIFs) in the NE Atlantic points to three distinct oceanic areas of abundant seamount clusters. Seamounts on oceanic crust dated 54–50 Ma are formed on smooth oceanic basement, which resulted from high spreading rates and magmatic productivity enhanced by higher than usual mantle plume activity. Late Eocene–Early Miocene SOIF clusters are located close to newly formed tectonic features on rough oceanic crust in the Irminger, Iceland and Norway basins, reflecting an unstable tectonic regime prone to local readjustments of mid-ocean ridge and fracture zone segments accompanied by extra igneous activity. A SOIF population observed on Mid-Miocene–Present rough oceanic basement in the Greenland and Lofoten basins, and on conjugate Kolbeinsey Ridge flanks, coincides with an increase in spreading rate and magmatic productivity. We suggest that both tectonic/kinematic and magmatic triggers produced Mid-Miocene–Present SOIFs, but the Early Miocene westwards ridge relocation may have played a role in delaying SOIF formation south of the Jan Mayen Fracture Zone. We conclude that Iceland plume episodic activity combined with regional changes in relative plate motion led to local mid-ocean ridge readjustments, which enhanced the likelihood of seamount formation. Supplementary material: Figures detailing NE Atlantic seamounts and SOIF distribution, and the location of earthquake epicentres are available at https://doi.org/10.6084/m9.figshare.c.3459729

Late Neogene changes in North America and Antarctica absolute plate motions inferred from the Mid‐Atlantic and Southwest Indian Ridges spreading histories

AbstractReconstructions of absolute plate motions underpin our understanding of the plate torque balance, but are challenging due to difficulties in inferring well‐dated rates and directions of plate movements from hot spot tracks. Useful information about plate dynamics can be inferred from rapid absolute plate motion changes, as these are linked only to the torque(s) that changed. Here we infer late Neogene changes in the absolute motions of North America and possibly Antarctica from changes in the easier‐to‐determine relative plate motions recorded along the Arctic, northern Mid‐Atlantic and Southwest Indian Ridges. We show that Eurasia/North America and Nubia/North America motions changed by the same amount between 8 and 5 Ma, as may have Nubia/Antarctica and Somalia/Antarctica plate motions. By considering additional, independent constraints on Somalia/India plate motion, we argue that a scenario in which North America and Antarctica absolute motions changed is the simplest one that explains the observed changes in relative motions. We speculate that these changes are linked to the late Neogene dynamics of the Pacific plate.

InSAR observations of strain accumulation and fault creep along the Chaman Fault system, Pakistan and Afghanistan

AbstractWe use 2004–2011 Envisat synthetic aperture radar imagery and InSAR time series methods to estimate the contemporary rates of strain accumulation in the Chaman Fault system in Pakistan and Afghanistan. At 29 N we find long‐term slip rates of 16 ± 2.3 mm/yr for the Ghazaband Fault and of 8 ± 3.1 mm/yr for the Chaman Fault. This makes the Ghazaband Fault one of the most hazardous faults of the plate boundary zone. We further identify a 340 km long segment displaying aseismic surface creep along the Chaman Fault, with maximum surface creep rate of 8.1 ± 2 mm/yr. The observation that the Chaman Fault accommodates only 30% of the relative plate motion between India and Eurasia implies that the remainder is accommodated south and east of the Katawaz block microplate.

Slip history of the La Cruz fault: Development of a late Miocene transform in response to increased rift obliquity in the northern Gulf of California

The Gulf of California rift has accommodated oblique divergence of the Pacific and North America plates in northwestern México since Miocene time. Due to its infancy, its rifted margins preserve a rare onshore record of early continental break-up processes and an opportunity to investigate the role of rift obliquity in strain localization. We map rift-related structures and syn-tectonic basins on southern Isla Tiburón, a proximal onshore exposure of the rifted North America margin. We integrate analysis and geochronology of syn-tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity. On southern Isla Tiburón, an early phase of extension initiated between ~19–17Ma and ~12.2Ma. Subsequently, these normal faults and related basins were cut by the La Cruz strike-slip fault and buried by deposits of the La Cruz basin, an elongate, fault-controlled trough coextensive with the La Cruz fault. Crosscutting relationships show that the NW-striking La Cruz fault accrued 5±2km of dextral slip ~8–4Ma. The La Cruz fault and parallel Tiburón transform were kinematically linked to detachment faulting that accommodated latest Miocene to Pliocene oblique opening of the offshore Upper Tiburón pull-apart basin. The onset of strike-slip faulting on Isla Tiburón was synchronous with the ~8–6Ma onset of transform faulting and basin formation along >1000km of the reconstructed Pacific-North America plate boundary. This transition coincides with the commencement of a clockwise azimuthal shift in Pacific-North America relative plate motion that increased the obliquity of the Gulf of California rift and formed the Gulf of California shear zone. The record from the proto-Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic rift basins.

Ridge‐spotting: A new test for Pacific absolute plate motion models

AbstractRelative plate motions provide high‐resolution descriptions of motions of plates relative to other plates. Yet geodynamically, motions of plates relative to the mantle are required since such motions can be attributed to forces (e.g., slab pull and ridge push) acting upon the plates. Various reference frames have been proposed, such as the hot spot reference frame, to link plate motions to a mantle framework. Unfortunately, both accuracy and precision of absolute plate motion models lag behind those of relative plate motion models. Consequently, it is paramount to use relative plate motions in improving our understanding of absolute plate motions. A new technique called “ridge‐spotting” combines absolute and relative plate motions and examines the viability of proposed absolute plate motion models. We test the method on six published Pacific absolute plate motions models, including fixed and moving hot spot models as well as a geodynamically derived model. Ridge‐spotting reconstructs the Pacific‐Farallon and Pacific‐Antarctica ridge systems over the last 80 Myr. All six absolute plate motion models predict large amounts of northward migration and monotonic clockwise rotation for the Pacific‐Farallon ridge. A geodynamic implication of our ridge migration predictions is that the suggestion that the Pacific‐Farallon ridge may have been pinned by a large mantle upwelling is not supported. Unexpected or erratic ridge behaviors may be tied to limitations in the models themselves or (for Indo‐Atlantic models) discrepancies in the plate circuits used to project models into the Pacific realm. Ridge‐spotting is promising and will be extended to include more plates and other ocean basins.

Tectonic evolution of Western Tethys from Jurassic to present day: coupling geological and geophysical data with seismic tomography models

ABSTRACTThe geodynamic evolution of the Western Tethys is characterized by multiple phases of rifting, seafloor spreading, subduction, and collisional events. Regional reconstructions are highly dependent on the kinematic history of the major plates bounding the Atlantic and Tethyan tectonic domains, as well as small micro-plates resulted from the fragmentation of northern Gondwanaland. The complexity of tectonic events in this area leads to major discrepancies between competing models about the timing, location, and polarity of subduction zones, for both the Cenozoic evolution and earlier phases. We focus on unravelling the Mesozoic evolution of the Western Tethys. We first reassessed kinematic models for the Early Jurassic–Late Cretaceous opening of the central, north central, and north Atlantic and used these as boundary conditions on the kinematic reconstructions of the Tethyan realm. We combined reconstructions of rifting and early seafloor spreading in northern Pangea that incorporate quantitative estimates of continental extension, and suggest a transtensional motion of Iberia relative to Europe in Early Cretaceous time to fit within the refined plate configuration of Central North Atlantic. We combined this regional framework with a recently published model for the motion of smaller blocks within the Western Tethys; from this model, we created synthetic isochrons for extinct oceanic basins and built evolving topological plate boundaries based on the new rigid plate model to derive a self-consistent and time-dependant model for the last 200 million years. We then examined the consistency of subduction history implied by the kinematic reconstructions, by comparing reconstructed plate boundary configurations to mantle velocity structure imaged by a range of seismic tomography models. Our results show that a satisfactory match can be made between Cenozoic subduction events in the Western Tethys region and observed shallow tomographic high-velocity material. However, the match is less clear for older subducted material. Correlations between surface reconstructions and deep Earth structure suggest that mid-deep mantle seismic features under present day Northeast-Central and Northwest Africa-Arabia may correspond with the Mesozoic subduction systems in the Vardar Ocean, Alpine Tethys, and Western Neotethys, respectively. These correlations support a model with intra-oceanic subduction of the Vardar Ocean from Middle Jurassic to Early Cretaceous, and mid-Early Cretaceous initiation of oceanic subduction in the Ligurian-Piemont Ocean. The results from the plate-tomography comparison suggest the existence of oceanic subduction in the Alpine Tethys Oceans in late early Cretaceous time. We investigated the uncertainties in the tectonic model in terms of absolute and relative plate motion and surface velocities and showed that choice of absolute reference frame can partially account for the lateral offset between the Vardar subduction zone and associated slab material in deep mantle; additional mismatches may be attributable to the limitations of our methodology, such as the assumption that slabs sink vertically.

Extreme multi-millennial slip rate variations on the Garlock fault, California: Strain super-cycles, potentially time-variable fault strength, and implications for system-level earthquake occurrence

Abstract Pronounced variations in fault slip rate revealed by new measurements along the Garlock fault have basic implications for understanding how faults store and release strain energy in large earthquakes. Specifically, dating of a series of 26.0 + 3.5 / − 2.5 m fault offsets with a newly developed infrared-stimulated luminescence method shows that the fault was slipping at > 14.0 + 2.2 / − 1.8 mm / yr , approximately twice as fast as the long-term average rate, during a previously documented cluster of four earthquakes 0.5–2.0 ka. This elevated late Holocene rate must be balanced by periods of slow or no slip such as that during the ca. 3300-yr-long seismic lull preceding the cluster. Moreover, whereas a comparison of paleoseismic data and stress modeling results suggests that individual Garlock earthquakes may be triggered by periods of rapid San Andreas fault slip or very large-slip events, the “on–off” behavior of the Garlock suggests a longer-term mechanism that may involve changes in the rate of elastic strain accumulation on the fault over millennial time scales. This inference is consistent with most models of the geodetic velocity field, which yield slip-deficit rates that are much slower than the average latest Pleistocene-early Holocene (post-8–13 ka) Garlock slip rate of 6.5 ± 1.5 mm / yr . These observations indicate the occurrence of millennia-long strain “super-cycles” on the Garlock fault that may be associated with temporal changes in elastic strain accumulation rate, which may in turn be controlled by variations in relative strength of the various faults in the Garlock-San Andreas-Eastern California Shear Zone fault system and/or changes in relative plate motion rates.

Late Miocene extension in coastal Sonora, México: Implications for the evolution of dextral shear in the proto-Gulf of California oblique rift

The timing, kinematics, and processes responsible for the rapid transition from subduction to oblique rifting and the localization of the Pacific-North America plate boundary in the Gulf of California are not well understood. Well exposed volcanic rocks deposited between ~15 and 10Ma in the Sierra Bacha (coastal Sonora, México) preserve a record of late Miocene deformation on the eastern rifted margin of the Gulf of California and offer new insights into the timing and kinematic evolution of oblique rifting. Detailed geologic mapping, fault kinematic analysis, U–Pb and 40Ar/39Ar geochronology, and paleomagnetic data reveal that the >2km-thick composite volcanic section is cut by a series of southwest-dipping, domino-style normal faults and uniformly tilted down-to-the-northeast. Palinspastic cross-section restoration suggests that the region experienced ca. 55–60% northeast-southwest-directed extension between ~11.7 and ~10–9Ma. Fault kinematic data reflect relatively minor dextral transtension either following or during the later stages of extension. Paleomagnetic results indicating modest clockwise vertical-axis block rotation suggest that dextral shear was concentrated in the southwest of the study area near the modern coastline.These results support an emerging model in which dextral strain was not ubiquitous across Sonora and did not initiate immediately following the ~12.5Ma transition from subduction to oblique rifting. Instead, strain east of the Baja California microplate at this latitude evolved from extension-dominated transtension prior to ~8Ma to dextral shear-dominated transtension by ~7–6Ma. The onset of dextral shear in coastal Sonora likely resulted from an increase in rift obliquity due to a change in relative plate motion direction at ~8Ma. The increase in rift obliquity and resultant onset of significant strike-slip faulting played a crucial role in facilitating subsequent plate boundary localization and marine incursion in the northern Gulf of California by ~6Ma and continental rupture at ~2–1Ma.