Rate Of Plate Motion Research Articles

Slab buckling as a driver for rapid oscillations in Indian plate motion and subduction rate

Plate tectonics is primarily driven by the constant gravitational pull of slabs where oceanic lithosphere sinks into the mantle at subduction zones. Under stable plate boundary configurations, changes in plate motion are then thought to occur gradually. Surprisingly, recent high-resolution Indian plate reconstructions revealed rapid (2–3 Million-year) plate velocity oscillations of ±50 %. We show, through numerical experiments, that the buckling of slabs in the mantle transition zone causes such oscillations. This buckling results from the deceleration of slabs as they sink into the lower mantle. The amplitude and period of buckling-associated oscillations depend on average subduction velocity and the available space in the mantle transition zone. The oscillations also affect the upper plate which may explain enigmatic observations of episodic deformation and fluid flow in subduction-related orogens. We infer that the slab pull that drives plate tectonics is generated in just the top few hundred kilometers of the mantle.

One tune, many tempos: Faults trade off slip in time and space to accommodate relative plate motions

Analysis of incremental slip rates from the four major strike-slip faults of the Marlborough fault system (MFS) of northern South Island, New Zealand, provides a first-ever record at the scale of an entire plate-boundary fault system of how relative plate motions are accommodated in time and space. This record, which spans the past 350–450 m of relative plate motion and ca. 12–14 ky, demonstrates that the fault system as a whole accommodates a steady plate-boundary slip rate, with the MFS faults “keeping up” with the overall rate of relative Pacific-Australia plate motion at relatively short displacement (10 s of meters) and time (102–103 yr) scales. These results affirm the often-assumed but until now unproven assumption that the relative plate-motion rate provides a robust basic constraint on both geodynamical models and analyses of system-level seismic hazard at these scales. In marked contrast, the incremental slip rates of each of the four main Marlborough faults are highly variable through time, marked by coordinated accelerations and decelerations spanning 4–6 earthquakes and several millennia as the faults trade off slip to accommodate a steady relative plate motion rate. These results suggest that (a) the weakest fault in the system will slip faster than average while adjacent mechanically complementary faults slip more slowly, and (b) that these patterns switch back and forth through time, likely reflecting reversible changes in the strength (i.e., resistance to shear) of the individual faults as they collectively accommodate relative plate motion. Interestingly, the periods of fast slip on the MFS faults exhibit ∼20–25 m of displacement, suggesting that these may record periods of fast slip on a weakened fault/ductile shear zone that continues until it uses up all locally stored elastic strain energy, thus potentially approaching local complete stress drop, albeit during a few tens of meters of rapid fault slip during multiple earthquakes, rather than during a single event. This hypothesis is consistent with typical earthquake stress drops of ∼1–10 MPa and estimates of depth-averaged crustal shear stress of a few 10 s of MPa, such as might be released in clusters of 4–6 earthquakes. These results emphasize the need to analyze the collective behavior of the entire fault systems, rather than just individual faults, to understand the mechanics of the system. Moreover, these patterns suggest a potential path forward for more accurate estimation of time-dependent seismic hazard, with the possible incorporation of current position of a fault within a fast- or slow/no-slip period into the probability analysis, as well as a means of potentially estimating crustal shear stress.

Paleogeography and high-precision geochronology of the Neoarchean Fortescue Group, Pilbara, Western Australia

While rates of Phanerozoic plate movements and magnetic field reversals have been well studied, little is known about such phenomena on early Earth. The ca. 2.8–2.7 Ga Fortescue Group on the Pilbara craton in Western Australia has been recognized as a well-preserved sequence of Archean rift volcanics thought to derive from a flood basalt province, and may have been moving rapidly across the globe at two different intervals in its depositional history. We present the results of a magnetostratigraphic study integrated with high-precision U-Pb ID-TIMS geochronology aiming to quantify rates of cratonic motion and provide a continuous time series for changes in Pilbara paleogeography during these two rapid intervals, at ∼2.77 and 2.72 Ga. We provide six new or updated high-quality paleomagnetic poles for inclusion in databases tracking Precambrian cratonic motion. During the craton’s largest geographic displacement at ∼2.77 Ga, we resolve a minimum drift rate of 23 ± 20 cm/a if there was substantial rotation of the Pilbara craton along with translational motion, and a more rapid minimum estimate of 64 ± 23 cm/a if the motion was dominated by translation; these estimates exceed both Mesoarchean and most modern rates of plate motion. We provide a new high-precision U-Pb zircon age of 2721.23 ± 0.88/0.88/6.9 Ma for the Tumbiana Formation stromatolite colony, which developed as the Pilbara craton drifted from 51.5 ± 7.0° to 32.1 ± 5.7° paleolatitude. Although the Fortescue Group has been considered an early prototype of large igneous provinces, it was emplaced over a longer duration than its Phanerozoic counterparts and does not fit at least one definition of a large igneous province (LIP). But as a potential prototype of LIP magmatism, the Fortescue succession chronicles eruptive dynamics, rapid paleogeographic changes, and a series of robustly determined magnetic field reversals during the Neoarchean.

Variations in crustal magma volume flux along the Hawaii-Emperor seamount chain and their implications for plume–lithosphere interactions

The magma volume flux is reduced by a fracture zone (FZ) crossing the Laccadives-Maldives-Chagos Bank-Mascarene Plateau chain in the Indian Ocean and is negatively correlated with the plate motion rate along the Ninetyeast Ridge (NER). Whether the FZs in the Pacific decrease the crustal magma volume flux and whether the flux along the Hawaii-Emperor seamount chain (HESC) is also negatively correlated with the plate motion rate are both unclear. Focusing on these two issues, we estimate the variations in crustal magma volume flux along the HESC and discussed plume–lithosphere interactions. The present-day crustal thickness is mapped via gravity inversion based on Earth interior density modeling. The accretionary thickness is isolated by filtering the present-day crustal thickness. Then, according to the seamount age variation trend inferred from ages by rock dating, the crustal magma volume flux along the whole HESC is derived. All the FZs correspond well to flux peaks because these weak lithospheric zones provided potential conduits for magma upwelling and promoted magma accretion. However, for the Indian Ocean case, the Reunion plume was captured by the mid-ocean ridge before and after reaching the low-flux transform fault, so the transform fault represented a relatively stronger lithosphere than the nearby mid-ocean ridge. The crustal magma volume flux along the HESC does not correlate with the motion rate of the Pacific Plate because the lithosphere was old during seamount loading, which contrasts with the inverse correlation between these two parameters along the NER, which had a young lithosphere during seamount loading.

Jurassic true polar wander recorded by the Lhasa terrane on its northward journey from Gondwana to Eurasia

Paleomagnetic data constrain paleogeographic motion of rocks relative to the Earth's spin axis, which is a sum of plate motion relative to the mantle and true polar wander. Discerning between these effects is challenging for studies aiming to reconstruct paleo-plate motions from deformed orogenic terranes. Here, we study the paleolatitudinal drift history of the Lhasa terrane of southern Tibet that migrated from the northern Gondwana to the southern Eurasian margin between late Triassic and early Cretaceous time. Previous work identified a 180 Ma near-equatorial Lhasa latitude and assumed near-constant paleolatitudinal drift. Large-scale true polar wander at this time, however, which has been argued for in previous work, requires highly variable Lhasa plate motion rates relative to Gondwana. Here, we test whether the alternative interpretation of constant plate motion rates provides a better prediction of paleomagnetic data. To this end, we present a new paleomagnetic pole from ∼155 Ma volcanics (here dated by U/Pb zircon) of the Lhasa terrane. Our pole comprises site mean directions from 46 lavas, passes a fold test, and is supported by an extensive rock magnetic and microscopic analysis that reveals no evidence of remagnetization. Our results give an average direction of D±ΔDx=337.1°±2.6°, I±ΔIx=−13.8°±4.9°, and a corresponding paleopole position at λp=45.3°N, φp=295.3°E with K=69.8 and A95=2.5°, predicting a similar near-equatorial, paleolatitude as at 180 Ma. This paleolatitudinal standstill is consistent with predicted paleolatitudes from global APWPs that account for Jurassic TPW and assume a constant Lhasa-Gondwana (India) oceanic spreading rate of ∼8 cm/a during the 215-130 Ma opening of the Neotethys and closure of the Mesotethys oceans. Contemporaneous arc magmatism on the Lhasa terrane was previously interpreted to indicate that the Lhasa terrane was located above a subduction zone during its northward journey to Tibet: our results show that if our reconstruction is accurate, then trench migration rates also must have been ∼8 cm/a. Our results are consistent with the range of rates of global long-lived trench migration that have been reconstructed in other studies, but they are much higher than measured in most modern systems. Our results more completely document the Lhasa terrane's separation from Gondwana and provide a straightforward methodology of using paleomagnetic data to test plate kinematic scenarios, overcoming the problem of the unknown effect of true polar wander on paleomagnetic data from deformed terranes.

Understanding earthquakes using the geological record: an introduction

Understanding earthquakes using the geological record: an introduction

Forearc origin for Coast Range Ophiolites inferred from osmium isotopes and highly siderophile elements

The Jurassic Coast Range Ophiolites, California, provide evidence for tectonic processes that formed the western margin of North America. Whether the ophiolites were formed in a mid-ocean ridge, back arc or forearc setting, however, remains unconstrained. To address this issue, a systematic grid sampling approach was taken to examine the mantle section of an archetypal ~161 Ma Coast Range Ophiolite at Point Sal, California. The Point Sal peridotites are serpentinized, with 187Re187Os evidence for a localized Late Miocene alteration event in grids closest to the lowermost bounding fault of the ophiolite. This event is interpreted to be related to tectonic changes and hydration-dehydration reactions acting on the serpentinites in response to changes in the azimuth and rate of Pacific-North American plate motion. Alteration and serpentinization have not systematically affected highly siderophile element (HSE) abundances, especially Os, Ir or Ru which, together with rare earth element abundances, record significant melt depletion (>20%) of a fertile mantle protolith (187Os/188Os = ~0.129) during ophiolite formation. Peridotites subsequently experienced localized and heterogenous addition of Re, Pt, Pd and radiogenic 187Os/188Os from Cl-rich, oxidized (QFM) fluids. Geochemical features of both Point Sal peridotites, as well as associated basaltic lavas, are consistent with high degrees of partial melting and variable addition of fluids as also observed in global forearc peridotites. A forearc origin for the Point Sal Ophiolite Complex supports models for the dominant formation process of 153–166 Ma Coast Range Ophiolite infant Arc crust terranes within a single westward facing arc along the Sierra continental-margin.

Reconciling seismic structures and Late Cretaceous kimberlite magmatism in northern Alberta, Canada

AbstractThe Late Cretaceous kimberlites in northern Alberta, Canada, intruded into the Paleoproterozoic crust and represent a nonconventional setting for the discovery of diamonds. Here, we examined the origin of kimberlite magmatism using a multidisciplinary approach. A new teleseismic survey reveals a low-velocity (−1%) corridor that connects two deep-rooted (>200 km) quasi-cylindrical anomalies underneath the Birch Mountains and Mountain Lake kimberlite fields. The radiometric data, including a new U-Pb perovskite age of 90.3 ± 2.6 Ma for the Mountain Lake intrusion, indicate a northeast-trending age progression in kimberlite magmatism, consistent with the (local) plate motion rate of North America constrained by global plate reconstructions. Taken together, these observations favor a deep stationary (relative to the lower mantle) source region for kimberlitic melt generation. Two competing models, mantle plume and slab subduction, can satisfy kinematic constraints and explain the exhumation of ultradeep diamonds. The plume hypothesis is less favorable due to the apparent age discrepancy between the oldest kimberlites (ca. 90 Ma) and the plume event (ca. 110 Ma). Alternatively, magma generation may have been facilitated by decompression of hydrous phases (e.g., wadsleyite and ringwoodite) within the mantle transition zone in response to thermal perturbations by a cold slab. The three-dimensional lithospheric structures largely controlled melt migration and intrusion processes during the Late Cretaceous kimberlite magmatism in northern Alberta.

A Comparison of Geodetic and Geologic Rates Prior to Large Strike‐Slip Earthquakes: A Diversity of Earthquake‐Cycle Behaviors?

AbstractComparison of preevent geodetic and geologic rates in three large‐magnitude (Mw = 7.6–7.9) strike‐slip earthquakes reveals a wide range of behaviors. Specifically, geodetic rates of 26–28 mm/yr for the North Anatolian fault along the 1999 MW = 7.6 Izmit rupture are ∼40% faster than Holocene geologic rates. In contrast, geodetic rates of ∼6–8 mm/yr along the Denali fault prior to the 2002 MW = 7.9 Denali earthquake are only approximately half as fast as the latest Pleistocene‐Holocene geologic rate of ∼12 mm/yr. In the third example where a sufficiently long pre‐earthquake geodetic time series exists, the geodetic and geologic rates along the 2001 MW = 7.8 Kokoxili rupture on the Kunlun fault are approximately equal at ∼11 mm/yr. These results are not readily explicable with extant earthquake‐cycle modeling, suggesting that they may instead be due to some combination of regional kinematic fault interactions, temporal variations in the strength of lithospheric‐scale shear zones, and/or variations in local relative plate motion rate. Whatever the exact causes of these variable behaviors, these observations indicate that either the ratio of geodetic to geologic rates before an earthquake may not be diagnostic of the time to the next earthquake, as predicted by many rheologically based geodynamic models of earthquake‐cycle behavior, or different behaviors characterize different fault systems in a manner that is not yet understood or predictable.

Slip Behavior of the Queen Charlotte Plate Boundary Before and After the 2012, MW 7.8 Haida Gwaii Earthquake: Evidence From Repeating Earthquakes

AbstractThe Queen Charlotte plate boundary, near Haida Gwaii, B.C., includes the dextral, strike‐slip, Queen Charlotte Fault (QCF) and the subduction interface between the downgoing Pacific and overriding North American plates. In this study, we present a comprehensive repeating earthquake catalog that represents an effective slip meter for both structures. The catalog comprises 712 individual earthquakes (0.3≤MW≤3.5) arranged into 224 repeating earthquake families on the basis of waveform similarity and source separation estimates from coda wave interferometry. We employ and extend existing relationships for repeating earthquake magnitudes and slips to provide cumulative slip histories for the QCF and subduction interface in six adjacent zones within the study area between 52.3°N and 53.8°N. We find evidence for creep on both faults; however, creep rates are significantly less than plate motion rates, which suggests partial locking of both faults. The QCF exhibits the highest degrees of locking south of 52.8°N, which indicates that the seismic hazard for a major strike‐slip earthquake is highest in the southern part of the study area. The 28 October 2012, MW 7.8 Haida Gwaii thrust earthquake occurred in our study area and altered the slip dynamics of the plate boundary. The QCF is observed to undergo accelerated, right‐lateral slip for 1–2 months following the earthquake. The subduction interface exhibits afterslip thrust motion that persists for the duration of the study period (i.e., 3 years and 2 months after the Haida Gwaii earthquake). Afterslip is greatest (5.7–8.4 cm/yr) on the periphery of the main rupture zone of the Haida Gwaii event.

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.

Timing of the initial collision between the Indian and Asian continents

There exist three mainstream opinions regarding the timing of the initial collision between the Indian and Eurasian continents, namely, 65±5, 45±5, and 30±5 Ma. Five criteria are proposed for determining which tectonic event was related to the initial collision between India and Asia: the rapid decrease in the rate of plate motion, the cessation of magmatic activity originating from the subduction of oceanic crust, the end of sedimentation of oceanic facies, the occurrence of intracontinental deformation, and the exchange of sediments sourced from two continents. These criteria are used to constrain the nature of these tectonic events. It is proposed that the 65±5 Ma tectonic event is consistent with some of the criteria, but the upshot of this model is that the magmatic activity originating from the Tethyan subduction since the Mesozoic restarted along the southern margin of the Asian continent in this time after a brief calm, implying that the subduction of the Neotethys slab was still taking place. The magmatic activity that occurred along the southern margin of the Asian continent had a 7-Myr break during 72–65 Ma, which in this study is interpreted as having resulted from tectonic transformation from subduction to transform faulting, indicating that the convergence between the Indian and Asian continents was once dominated by strike-slip motion. The 30±5 Ma tectonic event resulted in the uplift of the Tibetan Plateau, which was related to the late stage of the convergence between these two continents, namely, a hard collision. The 45±5 Ma tectonic event is in accordance with most of the criteria, corresponding to the initial collision between these two continents.

Paleomagnetic constraints on the Mesozoic drift of the Lhasa terrane (Tibet) from Gondwana to Eurasia

Abstract The Mesozoic plate tectonic history of Gondwana-derived crustal blocks of the Tibetan Plateau is hotly debated, but so far, paleomagnetic constraints quantifying their paleolatitude drift history remain sparse. Here, we compile existing data published mainly in Chinese literature and provide a new, high-quality, well-dated paleomagnetic pole from the ca. 180 Ma Sangri Group volcanic rocks of the Lhasa terrane that yields a paleolatitude of 3.7°S ± 3.4°. This new pole confirms a trend in the data that suggests that Lhasa drifted away from Gondwana in Late Triassic time, instead of Permian time as widely perceived. A total northward drift of ∼4500 km between ca. 220 and ca. 130 Ma yields an average south-north plate motion rate of 5 cm/yr. Our results are consistent with either an Indian or an Australian provenance of Lhasa.

Ocean Basin Evolution and Global-Scale Plate Reorganization Events Since Pangea Breakup

We present a revised global plate motion model with continuously closing plate boundaries ranging from the Triassic at 230 Ma to the present day, assess differences among alternative absolute plate motion models, and review global tectonic events. Relatively high mean absolute plate motion rates of approximately 9–10 cm yr−1 between 140 and 120 Ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time. An event at ∼100 Ma is most clearly expressed in the Indian Ocean and may reflect the initiation of Andean-style subduction along southern continental Eurasia, whereas an acceleration at ∼80 Ma of mean rates from 6 to 8 cm yr−1 reflects the initial northward acceleration of India and simultaneous speedups of plates in the Pacific. An event at ∼50 Ma expressed in relative, and some absolute, plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4–5 cm yr−1 indicates that an increase in collisional forces (such as the India–Eurasia collision) and ridge subduction events in the Pacific (such as the Izanagi–Pacific Ridge) play a significant role in modulating plate velocities.

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.

Continuous subsidence in the Thingvellir rift graben, Iceland: Geodetic observations since 1967 compared to rheological models of plate spreading

AbstractNorth America‐Eurasia relative plate motion across the Mid‐Atlantic Ridge in south Iceland is partitioned between overlapping ridge segments, the Western Volcanic Zone (WVZ) and the Eastern Volcanic Zone. The Thingvellir graben, a 4.7 km wide graben, lies along the central axis of the WVZ and has subsided >35 m during the Holocene. An ~8 km long leveling profile across the graben indicates a subsidence rate of ~1 mm yr−1 from 1990 to 2007, relative to the first (westernmost) benchmark. Modeled GPS velocities from 1994 to 2003 estimate a spreading rate of 6.7 ± 0.5 mm yr−1 or 35% of the full plate motion rate and up to 6.0 mm yr−1 subsidence. The combined geodetic observations show that the deformation zone is 10 times wider than the graben width. We utilize these geodetic observations to test the effects of ridge thermal structure on the kinematics across divergent boundaries. We apply a nonlinear rheology, thermomechanical model implemented in a finite element model. A 700°C isotherm is applied for the brittle to ductile transition in the crust, representing a dry olivine rheology. We adjust the depth of this isotherm to solve for the best fit model. The best fit model indicates that the 700°C isotherm is at 8 km depth below the ridge axis, which results in an average thermal gradient of 87.5°C km−1 in the upper crust. The thermomechanical model predicts a subsidence rate of 4 mm yr−1, comparable to our geodetic observations.

A geodetic plate motion and Global Strain Rate Model

We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ∼2.5 in areas with dense data coverage. We determined 6739 velocities from time series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15,772 velocities from 233 (mostly) published studies onto our core solution to obtain 22,511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25° longitude by 0.2° latitude in dimension). The remainder of the Earth's surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS-derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid-body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self-consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no-net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.

Seismic anisotropy predicted for 2-D plate-driven flow in the Lau back–arc basin

The Lau back–arc basin and spreading centers exhibit significant along-strike variations in plate motion rates and distance between the spreading centers and the Tonga trench. The mantle processes are expected to be complex in this region and seismic anisotropy offers a key constraint. Linked geodynamic and mineral alignment models of the mantle wedge predict the scale of anisotropic variation and underlying assumptions can be tested by comparison to observed anisotropy. Three cross-basin transects allow us to isolate the effects of trench-spreading center distance as well as rates of convergence and spreading. Lattice preferred orientations (LPO) corresponding to each transect’s 2-D flow field are calculated for peridotite polycrystals. Predicted P-wave anisotropy of several percent and shear-wave split times up to 3.5s are determined, with measurable differences predicted between transects. Variation in the 2-D estimates of flow driven by each transect’s plate kinematics will induce along-strike flow in the mantle wedge. We find that the rate of such flow would likely not alter the subduction/spreading induced LPO sufficiently to explain the reported trench/arc parallel fast seismic directions in the Lau Basin; additional geodynamic factors must play a role.

The nature of the plate interface and driving force of interseismic deformation in the New Zealand plate‐boundary zone, revealed by the continuous GPS velocity field

AbstractNew Zealand straddles the boundary between the Australian and Pacific plates. Cenozoic relative plate motion has resulted in a complex pattern of faulting and block rotation, with displacements on individual faults up to hundreds of kilometers. However, over periods of several years, GPS measurements show a remarkably smooth pattern of velocities. We show here using a new method of back slip analysis, that almost the entire plate‐boundary continuous GPS velocity field can be predicted within measurement error from a simple model of elastic distortion due to deep slip on a single plate interface (megathrust in the Hikurangi and Putsegur subduction zones or fault through continental lithosphere beneath the Southern Alps) at the relative plate motion rates. This suggests that the main driving force of plate‐boundary deformation is slip on the deeper moving part of the plate interface, without buried creep in localized shear zones beneath individual surface faults. The depth at which this deep slip terminates (locking point line) determines the width of deformation. Along the Hikurangi margin, there is also clockwise rotation of ~150 km long segment of the fore arc (Wairoa domain) at 4.5° ± 1 Ma, relative to the Australian Plate, about a pole in western North Island; model residuals in the velocity field are mainly a result of incomplete averaging of the cycle of slow slip events on the plate interface, downdip of the locking point.

Relative Motion Between the Rivera and North American Plates Determined from the Slip Directions of Earthquakes

So far, the direction and rate of relative motion between the Rivera and the North American plates (RIV-NAM) has been determined by the combination of two Euler poles: Rivera (RIV), with respect to Pacific (PAC), and PAC with respect to North America. Here, we estimate the relative motion of this plate pair (RIV-NAM) assuming that the horizontal projection of the direction of slip of the earthquakes occurring on the RIV-NAM boundaries reflect their relative plate motion. A catalog of earthquakes for which focal mechanisms are reported since 1976 is used in the analysis. Earthquakes were considered in the three segments of the RIV-NAM plate boundary: the subduction zone of the Rivera plate beneath the Jalisco block, the Tres Marias Escarpment and the events associated with the Tamayo Fracture Zone. The best fitting Euler pole is determined using a grid search of 64 potential poles. The slip direction predicted for each grid point is compared to the slip direction of the focal mechanisms of the earthquakes on the plate boundary. The best fitting Euler pole, determined in a root mean square sense (RMS), is located at 21.8°N, 107.6°W. A rate of rotation of 5.3°/year is estimated assuming the seismic earthquake cycle of the 1932 and 1995 great earthquakes represents a lower bound of the rate of plate motion in the subduction zone. The best fitting Euler pole shows that the subduction of the Rivera plate takes place in a direction perpendicular to the trench with a relative velocity of 4.3 cm/year, offshore Manzanillo. The rate of relative motion RIV-NAM decreases from SE to NW. North of approximately 21°N, the subduction of the Rivera plate becomes oblique to the trench and the relative velocity between the two plates decreases to an average of 1.9 cm/year. This slow rate of convergence may explain the rapid decrease of seismicity in the trench and the apparent absence of large earthquakes in this region. In the Tres Marias Escarpment, our best-fitting pole suggests that subduction stops, giving way to high-angle reverse faulting perpendicular to the Tres Marias Escarpment, in agreement with the reverse faulting earthquakes occurring here. To the north of 22.5°N, the slip predicted by the best-fitting pole suggests right-lateral faulting in a direction parallel to the Tamayo Fracture Zone, at a very low velocity (0.5–1.0 cm/year). The best fitting Euler pole determined here lies very close to the RIV-NAM plate boundary in the vicinity of the Tamayo Fracture Zone. This location of our best fitting Euler pole explains the low relative plate velocity, the relatively low level of seismic activity and the presence of a broad zone of deformation that accommodates the RIV-NAM motion.