America Motion Research Articles

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.

Farallon plate dynamics prior to the Laramide orogeny: Numerical models of flat subduction

The Laramide orogeny (~80–50Ma) was an anomalous period of mountain-building in the western United States that occurred more than 1000km inboard of the Farallon Plate subduction margin. It is widely believed that this orogeny is coincident with a period of flat (subhorizontal) subduction. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat subduction are not well understood. Three proposed factors are: (1) a westward (trenchward) increase in North America motion, (2) an increased slab suction force owing to the presence of thick Colorado Plateau lithosphere, and (3) subduction of a low-density oceanic plateau. This study uses 2D upper mantle scale numerical models to investigate these factors. The models show that trenchward continental motion is the primary control on subduction geometry, with decreasing slab dip as velocity increases. However, this can only create low-angle subduction, as the Farallon Plate was old (>100Myr) and denser than the mantle. A transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau that includes an 18-km-thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer, and (2) a slab break-off at the landward side of the plateau. The break-off removes the dense frontal slab, and flat subduction develops as the buoyant plateau deflects the slab upward. The slab suction force has only a minor effect on slab flattening, but the thickness of the Colorado Plateau lithosphere controls the depth of the flat slab. With a continental velocity of 4cm/yr and a 400-km-wide oceanic plateau, flat subduction develops within 15Ma after plateau subduction. The flat slab underthrusts the continent at ~200km depth, eventually extending >1500km inboard of the trench.

Pliocene sinistral slip across the Adobe Hills, eastern California–western Nevada: Kinematics of fault slip transfer across the Mina deflection

The Adobe Hills region (California and Nevada, USA) is a faulted volcanic field located within the western Mina deflection, a right-stepping zone of faults that connects the northern Eastern California shear zone (ECSZ) to the south with the Walker Lane belt (WLB) to the north. New detailed geologic mapping, structural studies, and 40 Ar/ 39 Ar geochronology in the Adobe Hills allow us to calculate fault slip rates and test predictions for the kinematics of fault slip transfer into the Mina deflection. The Adobe Hills are dominated by Pliocene tuffaceous sandstone, basaltic lavas that yield 40 Ar/ 39 Ar ages between 3.13 ± 0.02 and 3.43 ± 0.01 Ma, and basaltic cinder cones. These Pliocene units unconformably overlie Middle Miocene latite ignimbrite that yields an 40 Ar/ 39 Ar age of 11.17 ± 0.04 Ma, and Quaternary tuffaceous sands, alluvium, and lacustrine deposits cap the sequence. Northwest-striking normal faults, west-northwest–striking dextral faults, and northeast-striking sinistral faults cut all units; the northeast-striking sinistral faults are the youngest and most well developed fault set. We calculate ∼0.1 mm/yr of approximately east-west horizontal extension and northwest dextral shear since the Pliocene. The prominent northeast-striking sinistral faults offset basalt ridgelines, normal fault–hanging-wall intersections, a channelized basalt flow, a basalt flow edge, and a basalt flow contact a net minimum of 921 ± 184 to 1318 ± 264 m across the Adobe Hills. These measured sinistral offsets yield a minimum Pliocene sinistral fault slip rate of 0.2–0.5 mm/yr; our preferred minimum slip rate is 0.4–0.5 mm/yr. The geometry and orientation of the prominent sinistral faults are consistent with simple shear/couple clockwise block rotation within a broad dextral shear zone. Vertical axis block rotation data are needed to test this interpretation. We propose that a set of faults subparallel to Sierra Nevada–North America motion and associated releasing steps, located west of the White Mountains fault zone and east of the Long Valley Caldera, transfer a portion of dextral Owens Valley fault slip northwestward onto the sinistral faults in the Adobe Hills. Dextral slip distributed across faults between the White Mountains fault zone and the Sierra Nevada and east of the Fish Lake Valley fault zone may account for the apparent discrepancy between summed long-term geologic slip rates and present-day geodetic rates across the northern ECSZ. Fault slip in the Adobe Hills is part of a regional pattern of initiation and renewal of dextral, sinistral, and normal fault slip during the Pliocene that extends from lat ∼40°N to ∼36°N within the ECSZ-WLB and along the western margin of the Basin and Range Province. This regional deformation episode may be related to changes in gravitational potential energy.

Geologically current plate motions

We describe best-fitting angular velocities and MORVEL, a new closure-enforced set of angular velocities for the geologically current motions of 25 tectonic plates that collectively occupy 97 per cent of Earth's surface. Seafloor spreading rates and fault azimuths are used to determine the motions of 19 plates bordered by mid-ocean ridges, including all the major plates. Six smaller plates with little or no connection to the mid-ocean ridges are linked to MORVEL with GPS station velocities and azimuthal data. By design, almost no kinematic information is exchanged between the geologically determined and geodetically constrained subsets of the global circuit—MORVEL thus averages motion over geological intervals for all the major plates. Plate geometry changes relative to NUVEL-1A include the incorporation of Nubia, Lwandle and Somalia plates for the former Africa plate, Capricorn, Australia and Macquarie plates for the former Australia plate, and Sur and South America plates for the former South America plate. MORVEL also includes Amur, Philippine Sea, Sundaland and Yangtze plates, making it more useful than NUVEL-1A for studies of deformation in Asia and the western Pacific. Seafloor spreading rates are estimated over the past 0.78 Myr for intermediate and fast spreading centres and since 3.16 Ma for slow and ultraslow spreading centres. Rates are adjusted downward by 0.6–2.6 mm yr−1 to compensate for the several kilometre width of magnetic reversal zones. Nearly all the NUVEL-1A angular velocities differ significantly from the MORVEL angular velocities. The many new data, revised plate geometries, and correction for outward displacement thus significantly modify our knowledge of geologically current plate motions. MORVEL indicates significantly slower 0.78-Myr-average motion across the Nazca–Antarctic and Nazca–Pacific boundaries than does NUVEL-1A, consistent with a progressive slowdown in the eastward component of Nazca plate motion since 3.16 Ma. It also indicates that motions across the Caribbean–North America and Caribbean–South America plate boundaries are twice as fast as given by NUVEL-1A. Summed, least-squares differences between angular velocities estimated from GPS and those for MORVEL, NUVEL-1 and NUVEL-1A are, respectively, 260 per cent larger for NUVEL-1 and 50 per cent larger for NUVEL-1A than for MORVEL, suggesting that MORVEL more accurately describes historically current plate motions. Significant differences between geological and GPS estimates of Nazca plate motion and Arabia–Eurasia and India–Eurasia motion are reduced but not eliminated when using MORVEL instead of NUVEL-1A, possibly indicating that changes have occurred in those plate motions since 3.16 Ma. The MORVEL and GPS estimates of Pacific–North America plate motion in western North America differ by only 2.6 ± 1.7 mm yr−1, ≈25 per cent smaller than for NUVEL-1A. The remaining difference for this plate pair, assuming there are no unrecognized systematic errors and no measurable change in Pacific–North America motion over the past 1–3 Myr, indicates deformation of one or more plates in the global circuit. Tests for closure of six three-plate circuits indicate that two, Pacific–Cocos–Nazca and Sur–Nubia–Antarctic, fail closure, with respective linear velocities of non-closure of 14 ± 5 and 3 ± 1 mm yr−1 (95 per cent confidence limits) at their triple junctions. We conclude that the rigid plate approximation continues to be tremendously useful, but—absent any unrecognized systematic errors—the plates deform measurably, possibly by thermal contraction and wide plate boundaries with deformation rates near or beneath the level of noise in plate kinematic data.

Stratified seismic anisotropy reveals past and present deformation beneath the East-central United States

Evolution of continental lithosphere during orogenies and the following periods of relative stability is poorly understood, largely because of the lack of relevant observational constraints. Measurements of seismic anisotropy provide such constraints, but due to limitations in the resolving power of available data sets and, more generally, of various data types, detailed mapping of lithospheric anisotropy has remained elusive. Here we apply surface-wave array analysis to data from the East-central U.S. and determine the layering of azimuthal anisotropy beneath the Grenville–Appalachian orogen in the entire lithosphere–asthenosphere depth range. Combined measurements of Rayleigh-wave phase velocities along 60 interstation paths constrain phase-velocity maps with statistically significant anisotropy. Distinct anisotropy patterns in three different period ranges point to the existence of three distinct layers beneath the orogen, with different anisotropic fabric within each. We invert phase-velocity maps and, alternatively, pairs of selected measured dispersion curves for anisotropic shear-velocity structure. The results confirm that three anisotropic layers with different fabric within each are present, two in the lithosphere (30–70 km; 70–150 km depths) and another in the asthenosphere beneath (> 150 km). Directions of fast wave propagation in the upper lithosphere are parallel to the Grenville and Appalachian fronts, suggesting that the region-scale anisotropy pattern reflects the pervasive deformation of the lower crust and uppermost mantle during the continental collisions. The fast-propagation azimuth within the lower lithosphere is different, parallel to the NNW direction of North America's motion after the orogeny (~ 160–125 Ma). This suggests that the lithosphere, 70-km thick by the end of the Appalachian orogeny, gradually thickened to the present 150-km while inheriting the fabric from the sheared asthenosphere below, as the plate moved NNW. Below 150 km, the fast-propagation direction is parallel to the present plate motion, indicating fabric due to recent asthenospheric flow. Anisotropy in narrower depth ranges beneath the region has been sampled previously. Published results (from observations of P n and SKS and waveform tomography) can be accounted for and reconciled by the three-layered model of anisotropy for the lithosphere–asthenosphere depth range constrained in this study. In particular, the anisotropy we detect in the asthenosphere can account for the magnitude of SKS-wave splitting, with the fast wave-propagation directions inferred from SKS and surface-wave data also consistent, both parallel to the current plate motion.

A high-resolution model for Eurasia-North America plate kinematics since 20 Ma

SUMMARY We derive the first chronologically detailed model of Eurasia‐North America plate motion since 20 Ma from ship and airplane surveys of the well-expressed magnetic lineations along this slowly spreading plate boundary, including previously unavailable dense Russian magnetic data from the southern Reykjanes Ridge and northern Mid-Atlantic ridge near the Charlie Gibbs fracture zone. From more than 7000 crossings of 21 magnetic anomalies from Anomaly 1n (0.78 Ma) to Anomaly 6n (19.7 Ma), we estimate best-fitting finite rotations and realistic uncertainties. Linear regressions of total opening distances versus their reversal ages at different locations along the plate boundary show that reversal boundaries are shifted systematically outwards from the spreading axis with respect to their idealized locations, with the outward shift ranging from more than 5 km between Iceland and the Charlie Gibbs fracture zone to ∼2 km elsewhere. This outward displacement, which is a consequence of the finite zone of seafloor accretion, degrades estimates of the underlying plate motion and is thus removed for the ensuing kinematic analysis. The corrected plate motion rotations reveal surprising, previously unrecognized features in the relative motions of these two plates. Within the uncertainties, motion was steady from 20 to 8 Ma around a pole that was located ∼600 km north of the present pole, with seafloor spreading rates that changed by no more than 5 per cent (1 mm yr −1 ) along the Reykjanes Ridge during this period. Seafloor spreading rates decreased abruptly by 20 ± 2 per cent at 7.5‐6.5 Ma, coinciding with rapid southward migration of the pole of rotation and a5 ◦ ‐10 ◦ counter-clockwise change in the plate slip direction. Eurasia‐North America plate motion since 6.7 Ma has remained remarkably steady, with an apparently stationary axis of rotation and upper limit of ±2 per cent on any variations in the rate of angular rotation during this period. Based on the good agreement between seismotectonic constraints on present deformation in northeast Asia and directions of motion that are predicted by our 6.7 Ma to present pole, we hypothesize that motion has remained steady to the present and attempt to test this hypothesis with published GPS estimates for Eurasia‐North America motion. We find, however, that GPS estimates that are tied to recent versions of the international geodetic reference frame and rely principally on station velocities from Europe give implausible estimates of recent motion, with the most recently published GPS model predicting convergence along the southern Gakkel Ridge and in the Laptev Sea, where seafloor spreading occurs. An alternative GPS estimate that is not tied to the international terrestrial reference frame and employs GPS station velocities from northeastern Asia is marginally consistent with our 6.7‐0 Ma motion estimate.

Current deformation in the northern Canadian Cordillera inferred from GPS measurements

Continuous and campaign‐style Global Positioning System (GPS) measurements provide new constraints on the first‐order current deformation pattern of the northern Cordillera of NW Canada and eastern Alaska. The Yakutat block is currently colliding with North America in the corner of the Gulf of Alaska. Our data infer that relative Yakutat–North America motion is accommodated across the eastern boundary by right‐lateral motion (∼40 mm/a), mainly on the Fairweather fault, and minor shortening (∼6 mm/a). To the north, collision is taken up by shortening (∼31 mm/a) mainly on the Chugach–St. Elias fault system, with westward extrusion and possible counterclockwise rotation of the Yakutat block and Alaskan fore arc facilitated by ∼23 mm/a right‐lateral motion that is shared by several faults. The seismicity pattern indicates that plate boundary deformation is far reaching, producing strain throughout eastern Alaska, Yukon, and western Northwest Territories. Our GPS data confirm the transfer of strain from the Yakutat collision zone and enable the extent of plate boundary deformation to be mapped, thereby defining the western edge of “stable” North America. GPS sites in SW Yukon show motion of 3–10 mm/a to the NE, confirming that strain is transferred at least 400 km from the Yakutat collision. Continued transfer north toward the Mackenzie Delta and NE to the Mackenzie Mountains, indicated by current seismicity, is not yet well resolved by the campaign GPS data, which are hampered by coseismic and postseismic effects of the 2002 Mw 7.9 earthquake on the Denali fault in eastern Alaska.

New constraints on relative motion between the Pacific Plate and Baja California microplate (Mexico) from GPS measurements

SUMMARY We present a new surface velocity field for Baja California using GPS data to test the rigidity of this microplate, calculate its motion in a global reference frame, determine its relative motion with respect to the North American and the Pacific plates, and compare those results to our estimate for Pacific‐North America motion. Determination of Pacific Plate motion is improved by the inclusion of four sites from the South Pacific Sea Level and Climate Monitoring Project. These analyses reveal that Baja California moves as a quasi-rigid block but at a slower rate in the same direction, as the Pacific Plate relative to North America. This is consistent with seismic activity along the western edge of Baja California (the Baja California shear zone), and may reflect resistance to motion of the eastern edge of the Pacific Plate caused by the ‘big bend’ of the San Andreas fault and the Transverse Ranges in southern California.

Fault locking, block rotation and crustal deformation in the Pacific Northwest

SummaryWe interpret Global Positioning System (GPS) measurements in the northwestern United States and adjacent parts of western Canada to describe relative motions of crustal blocks, locking on faults and permanent deformation associated with convergence between the Juan de Fuca and North American plates. To estimate angular velocities of the oceanic Juan de Fuca and Explorer plates and several continental crustal blocks, we invert the GPS velocities together with seafloor spreading rates, earthquake slip vector azimuths and fault slip azimuths and rates. We also determine the degree to which faults are either creeping aseismically or, alternatively, locked on the block-bounding faults. The Cascadia subduction thrust is locked mainly offshore, except in central Oregon, where locking extends inland. Most of Oregon and southwest Washington rotate clockwise relative to North America at rates of 0.4–1.0 ° Myr−1. No shear or extension along the Cascades volcanic arc has occurred at the mm/yr level during the past decade, suggesting that the shear deformation extending northward from the Walker Lane and eastern California shear zone south of Oregon is largely accommodated by block rotation in Oregon. The general agreement of vertical axis rotation rates derived from GPS velocities with those estimated from palaeomagnetic declination anomalies suggests that the rotations have been relatively steady for 10–15 Ma. Additional permanent dextral shear is indicated within the Oregon Coast Range near the coast. Block rotations in the Pacific Northwest do not result in net westward flux of crustal material—the crust is simply spinning and not escaping. On Vancouver Island, where the convergence obliquity is less than in Oregon and Washington, the contractional strain at the coast is more aligned with Juan de Fuca—North America motion. GPS velocities are fit significantly better when Vancouver Island and the southern Coast Mountains move relative to North America in a block-like fashion. The relative motions of the Oregon, western Washington and Vancouver Island crustal blocks indicate that the rate of permanent shortening, the type that causes upper plate earthquakes, across the Puget Sound region is 4.4 ± 0.3 mm yr−1. This shortening is likely distributed over several faults but GPS data alone cannot determine the partitioning of slip on them. The transition from predominantly shear deformation within the continent south of the Mendocino Triple Junction to predominantly block rotations north of it is similar to changes in tectonic style at other transitions from shear to subduction. This similarity suggests that crustal block rotations are enhanced in the vicinity of subduction zones possibly due to lower resisting stress.

Deformation of Jamaica and motion of the Gonâve microplate from GPS and seismic data

SUMMARY We use velocities from 20 GPS sites on the island of Jamaica and seismic data from the Jamaican Seismic Network to quantify motion of the proposed Gonave microplate and study deformation and earthquake hazard in Jamaica. All 20 Jamaican GPS sites move significantly relative to both the Caribbean and North American plates, thereby confirming the existence of the Gonave microplate. In a Caribbean Plate reference frame, the fastest site velocities in Jamaica are 8 ± 1m m yr −1 , representing a minimum estimate for the rate of Gonave-Caribbean Plate motion. We estimate a firm upper limit of 13 ± 1m m yr −1 for Gonave-Caribbean Plate motion assuming that additional elastic or permanent deformation occurs north of Jamaica beyond the reach of our GPS network. The range of estimates for Gonave-Caribbean Plate motion exceeds the 3- 7m m yr −1 range of geologically derived rates for post-10 Ma fault slip in Jamaica, suggesting that motion has either increased relative to the long-term deformation rate or that estimates of the long-term deformation rate are too slow. Minimum and maximum rates for Gonave-North America Plate motion are 6 ± 1m m yr −1 and 11 ± 1m m yr −1 based on the differences between the estimated Gonave-Caribbean rates and a recent GPS-based estimate of 19.3 ± 1m m yr −1 of Caribbean-North American Plate motion in Jamaica. These are slower than 15- 20 mm yr −1 published estimates of the long-term seafloor spreading rate across the Cayman spreading centre, which records Gonave-North American Plate motion. Our observations are thus consistent with a model in which Gonave-Caribbean Plate motion has increased through time while Gonave-North America Plate motion has decreased, possibly reflecting a progressive transfer of slip from plate boundary structures that accommodate Gonave-North America motion to faults that carry Gonave-Caribbean Plate motion. In Jamaica, GPS site directions in both the Caribbean and North American Plate reference frames are parallel to the Caribbean-North America Plate slip direction and are only 10-20 ◦ different from the azimuths of the island's major strike-slip faults. The mean P- and T-axes for 48 earthquakes since 1941 have near-horizontal plunges and are oriented 45 ◦ from the predicted plate slip direction. The kinematic and seismic data thus indicate that deformation on the island is dominated by left-lateral shear along largely E-W-trending strike-slip faults. Relative to the Caribbean Plate, the Jamaican GPS velocities exhibit a nearly monotonic increase in site velocities from south to north along a transect orthogonal to the island's major E-W faults. This velocity gradient likely reflects the accumulation of elastic strain due to frictional locking of the major E-W faults. Velocities however also increase monotonically from the WSW to ENE along a transect orthogonal to the island's numerous NNW-striking faults, consistent with the accumulation of significant elastic shortening along those faults and supportive of an interpretation of those faults as restraining bends. Seismic hazard assessment based on the GPS-derived deformation budget, likely maximum fault rupture lengths, and the historical seismic record suggests approximate repeat times of one to several centuries for earthquakes of maximum magnitudes of M w = 7.0-7.3. The strain that has accumulated since the destructive 1692 earthquake near the capital city of Kingston is presently sufficient to release a M w = 7.2 earthquake in eastern Jamaica.

Northwest Basin and Range tectonic deformation observed with the Global Positioning System, 1999–2003

We use geodetic velocities obtained with the Global Positioning System (GPS) to quantify tectonic deformation of the northwest Basin and Range province of the western United States. The results are based on GPS data collected in 1999 and 2003 across five new quasi‐linear networks in northern Nevada, northeast California, and southeast Oregon. The velocities show ∼3 mm/yr westward movement of northern Nevada with respect to stable North America. West of longitude 119°W the velocities increase and turn northwest, parallel to Sierra Nevada/Great Valley microplate motion, and similar to velocities previously obtained to the south. The observations are explained by a kinematic model with three domains that rotate around Euler poles in eastern Oregon and western Idaho. Northeast California experiences internal dextral shear deformation (11.2 ± 3.6 nstrain/yr) subparallel to Pacific/North America motion. Relative motions of the domains imply 2–5 mm/yr approximately east‐west extension in northwest Nevada and 1–4 mm/yr approximately north‐south contraction near the California/Oregon border. The northward decreasing approximately east‐west extension in northwest Nevada is consistent with the northern termination of Basin and Range deformation, faulting and characteristic topography. No significant extension is detected in the Oregon Basin and Range. The Oregon Cascade arc moves north at ∼3.5 mm/yr and is possibly influenced by the approximately eastward motion of the Juan de Fuca plate. These results disagree with secular northwest trenchward motion of the Oregon forearc inferred from paleomagnetic rotations. South of latitude 43°, however, trenchward motion exists and is consistent with block rotations, approximately east‐west Basin and Range extension, and northwest Sierra Nevada translation.

Distribution of the Pacific/North America motion in the Queen Charlotte Islands‐S. Alaska plate boundary zone

We present GPS data that constrain the distribution of the relative Pacific/North America motion across the Queen Charlotte Islands‐Alaska Panhandle margin (NW North America). Velocities from a network of 22 campaign and permanent sites indicate that the Pacific/North America transpressive motion is mostly accommodated along the locked Queen Charlotte‐Fairweather Fault. A significant portion (6–7 mm/yr) of the relative plate motion is taken up by distributed dextral shear across a ∼200 km wide region of the margin. Two models have been proposed to describe how the Pacific/North America convergence is accommodated off the Queen Charlotte Islands: Internal shortening vs. underthrusting of the Pacific plate. Although the GPS data cannot discriminate between the models, they provide strong constraints on the convergence distribution. The significant non‐transient motion of GPS sites along the central British Columbia‐southern Alaska margin has implications for seismic hazard and tectonic evolution models of the Canadian Cordillera.

An integrated global model of present-day plate motions and plate boundary deformation

In this paper we present a global model (GSRM-1) of both horizontal velocities on the Earth's surface and horizontal strain rates for almost all deforming plate boundary zones. A model strain rate field is obtained jointly with a global velocity field in the process of solving for a global velocity gradient tensor field. In our model we perform a least-squares fit between model velocities and observed geodetic velocities, as well as between model strain rates and observed geological strain rates. Model velocities and strain rates are interpolated over a spherical Earth using bi-cubic Bessel splines. We include 3000 geodetic velocities from 50 different, mostly published, studies. Geological strain rates are obtained for central Asia only and they are inferred from Quaternary fault slip rates. For all areas where no geological information is included a priori constraints are placed on the style and direction (but not magnitude) of the model strain rate field. These constraints are taken from a seismic strain rate field inferred from (normalized) focal mechanisms of shallow earthquakes. We present a global solution of the second invariant of the model strain rate field as well as strain rate solutions for a few selected plate boundary zones. Generally, the strain rate tensor field is consistent with geological and seismological data. With the assumption of plate rigidity for all areas other than the plate boundary zones we also present relative angular velocities for most plate pairs. We find that in general there is a good agreement between the present-day plate motions we obtain and long-term plate motions, but a few significant differences exist. The rotation rates for the Indian, Arabian and Nubian plates relative to Eurasia are 30, 13 and 50 per cent slower than the NUVEL-1A estimate, respectively, and the rotation rate for the Nazca Plate relative to South America is 17 per cent slower. On the other hand, Caribbean–North America motion is 76 per cent faster than the NUVEL-1A estimate. While crustal blocks in the India–Eurasia collision zone move significantly and self-consistently with respect to bounding plates, only a very small motion is predicted between the Nubian and Somalian plates. By integrating plate boundary zone deformation with the traditional modelling of angular velocities of rigid plates we have obtained a model that has already been proven valuable in, for instance, redefining a no-net-rotation model of surface motions and by confirming a global correlation between seismicity rates and tectonic moment rates along subduction zones and within zones of continental deformation.

Neotectonic modelling of the western part of the Africa–Eurasia plate boundary: from the Mid-Atlantic ridge to Algeria

In this work we use the thin-shell approximation to model the neotectonics of the western part of the Africa–Eurasia plate boundary, extending from the Mid-Atlantic ridge to Tell Atlas (northern Algeria). Models assume a nonlinear rheology and include laterally variable heat flow, elevation, and crust and lithospheric mantle thickness. Including the Mid-Atlantic ridge permits us to evaluate the effects of ridge push and to analyse the influence of the North America motion on the area of the Africa–Eurasia plate boundary. Ridge push forces were included in a self-consistent manner and have been shown to exert negligible effects in the neotectonics of the Iberian Peninsula and northwestern Africa. Different models were computed with systematic variation of the fault friction coefficient. Model quality was scored by comparing predictions of anelastic strain rates, vertically integrated stresses and velocity fields to data on seismic strain rate computed from earthquake magnitude, most compressive horizontal principal stress direction, and seafloor spreading rates on the Mid-Atlantic ridge. The best model scores were obtained with fault friction coefficients as low as 0.06–0.1. The velocity boundary condition representing spreading on the Mid-Atlantic ridge is shown to produce concentrated deformation along the ridge and to have negligible effect in the interior of the plates. However, this condition is shown to be necessary to properly reproduce the observed directions of maximum horizontal compression on the Mid-Atlantic ridge. The maximum fault slip rates predicted by the model are obtained along the Mid-Atlantic ridge, Terceira ridge and Tell Atlas front. Relatively high slip rates are also obtained in the area between the Gloria fault and the Gulf of Cadiz. We infer from our modelling a significant long-term seismic hazard for the Gloria fault, and interpret the absence of seismicity on this fault as possibly due to transient elastic strain accumulation. The present study has also permitted better understanding of the geometry of the Africa–Eurasia plate boundary from the Azores triple junction to the Algerian Basin. The different deformational styles seem to be related to the different types of lithosphere, oceanic or continental, in contact at the plate boundary.

GPS‐determination of along‐strike variation in Cascadia margin kinematics: Implications for relative plate motion, subduction zone coupling, and permanent deformation

High‐precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along‐strike variation in Cascadia margin kinematics. These results constrain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of magnitude longer than the decades‐long history of seismic monitoring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station velocities along the coast. After correction for a simple elastic dislocation model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3–7 mm/yr) to northern and central Oregon (∼9 mm/yr), consistent with oblique convergence and geologic constraints on permanent deformation. The margin‐parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north‐westward velocities reflect the influence of Pacific‐North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin.

Evidence for a deep asthenosphere beneath North America from western United States SKS splits

Research Article| April 01, 2001 Evidence for a deep asthenosphere beneath North America from western United States SKS splits Derek L. Schutt; Derek L. Schutt 1Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Search for other works by this author on: GSW Google Scholar Eugene D. Humphreys Eugene D. Humphreys 1Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (4): 291–294. https://doi.org/10.1130/0091-7613(2001)029<0291:EFADAB>2.0.CO;2 Article history received: 24 Jul 2000 rev-recd: 08 Dec 2000 accepted: 13 Dec 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Derek L. Schutt, Eugene D. Humphreys; Evidence for a deep asthenosphere beneath North America from western United States SKS splits. Geology 2001;; 29 (4): 291–294. doi: https://doi.org/10.1130/0091-7613(2001)029<0291:EFADAB>2.0.CO;2 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 The uniform southwesterly trend of fast-traveling split SKS waves that traverse the upper mantle beneath the Yellowstone swell provides good evidence for both southwesterly motion of North America over a relatively stable deep Earth interior, and young strain accommodation within the Yellowstone swell mantle via dislocation creep of olivine. These results contrast with the many SKS splits recorded across the western United States, the splitting behavior of which often is very complex at individual sites, and the orientations of which, as a set, cannot be attributed simply to plate motion or to small-scale convection beneath North America lithosphere. These data suggest that, away from Yellowstone, (1) mantle fabric is complex, and (2) North America motion is accommodated by strain at depths greater than those typically associated with asthenosphere. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Upper plate contraction north of the migrating Mendocino triple junction, northern California: Implications for partitioning of strain

Geologic measurement of permanent contraction across the Cascadia subduction margin constrains one component of the tectonic deformation along the convergent plate boundary, the component critical for the seismic hazard assessment of crustal faults. A comprehensive survey of active faults in onshore subduction margin rocks at the southern end of the Cascadia subduction zone indicates that these thrust faults accommodate ∼10 mm/yr of convergence oriented 020°–045°. Seismotectonic models of subduction zones typically assign this upper plate strain to the estimate of aseismic slip on the megathrust. Geodetic models include this permanent crustal strain within estimates of elastic strain accumulation on the megathrust. Both types of models underestimate the seismic hazard associated with crustal faults. Subtracting the observed contraction from the plate convergence rate (40–50 mm/yr; directed 040°–055°) leaves 30–40 mm/yr of convergence to be partitioned between slip on the megathrust, contraction within the southern Juan de Fuca plate, and crustal contraction outside the subduction complex rocks. This simple estimate of slip partitioning neglects the discrepancy between the plate convergence and contraction directions in the vicinity of the Mendocino triple junction.The San Andreas and Cascadia limbs of the Mendocino triple junction are not collinear. The eastern edge of the broad San Andreas boundary is ∼85 km east of the Cascadia subduction boundary, and across this zone the Pacific plate converges directly with the North America plate. The skewed orientation of crustal structures just north of the leading edge of the Pacific plate suggests that they are deforming in a hybrid stress field resulting from both Juan de Fuca‐North America motion and Pacific‐North America motion. The composite convergence direction (50 mm/yr; directed 023°) is consistent with the compressive stress axis (020°) inferred from focal mechanisms of crustal earthquakes in the Humboldt region. Deformation in such a hybrid stress field implies that the crustal faults are being loaded from two major tectonic sources. The slip on crustal faults north of the Mendocino triple junction may consume 4–5 mm/yr of Pacific‐Humboldt convergence. The remaining 17–18 mm/yr of convergence may be consumed as distributed shortening expressed in the high rates of uplift in the Cape Mendocino region or as northward translation of the continental margin, north of the triple junction.

New kinematic models for Pacific‐North America Motion from 3 Ma to Present, II: Evidence for a “Baja California Shear Zone”

We use new models for present‐day Pacific‐North America motion to evaluate the tectonics of offshore regions west of the Californias. Vandenburg in coastal Alta California moves at the Pacific plate velocity within uncertainties (∼1 mm/yr) after correcting for strain accumulation on the San Andreas and San Gregorio‐Hosgri faults with a model that includes a viscoelastic lower crust. Modeled and measured velocities at coastal sites in Baja California south of the Agua Blanca fault, a region that most previous models consider Pacific plate, differ by 3–8 mm/yr, with coastal sites moving slower that the Pacific plate. We interpret these discrepancies in terms of strain accumulation on known on‐shore faults, combined with right lateral slip at a rate of 3–4 mm/yr on additional faults offshore peninsular Baja California in the Pacific. Offshore seismicity, offset Quaternary features along the west coast of Baja California, and a discrepancy between the magnetically determined spreading rate in the Gulf Rise and the total plate rate from a geological model provide independent evidence for a “Baja California shear zone.”

Motion of the Rivera plate since 10 Ma relative to the Pacific and North American plates and the mantle

To better understand the influence of Rivera plate kinematics on the geodynamic evolution of western Mexico, we use more than 1400 crossings of seafloor spreading magnetic lineations along the Pacific–Rivera rise and northern Mathematician ridge to solve for rotations of the Rivera plate relative to the underlying mantle and the Pacific and North American plates at 14 times since 9.9 Ma. Our comparison of magnetic anomaly crossings from the undeformed Pacific plate to their counterparts on the Rivera plate indicates that significant areas of the Rivera plate have deformed since 9.9 Ma. Dextral shear along the southern edge of the plate from 3.3–2.2 Ma during a regional plate boundary reorganization deformed the Rivera plate farther into its interior than previously recognized. In addition, seafloor located north of two rupture zones within the Rivera plate sutured to North America after 1.5 Ma. Anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the Rivera plate. Finite rotations that best reconstruct Pacific plate anomaly crossings onto their undeformed counterparts on the Rivera plate yield stage spreading rates that decrease gradually by 10% between 10 and 3.6 Ma, decrease rapidly by 20% after ∼3.6 Ma, and recover after 1 Ma. The slowdown in Pacific–Rivera seafloor spreading at 3.6 Ma coincided with the onset of dextral shear across the then-incipient southern boundary of the Rivera plate with the Pacific plate. The available evidence indicates that the Rivera plate has been an independent microplate since at least 10 Ma, contrary to published assertions that it fragmented from the Cocos plate at ∼5 Ma. Motion of the Rivera plate relative to North America has changed significantly since 10 Ma, in concert with significant changes in Pacific–Rivera motion. A significant and robust feature of Rivera–North America motion not previously recognized is the cessation of margin-normal convergence and thus subduction from 2.6 to 1.0 Ma along the entire plate boundary, followed by a resumption of trench-normal subduction along the southern half of the Rivera–North America plate boundary after 1.0 Ma. Motion of the Rivera plate relative to the underlying mantle since 10 Ma has oscillated between periods of landward motion and seaward motion. The evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion.

GPS geodetic constraints on Caribbean‐North America Plate Motion

We describe a model for Caribbean plate motion based on GPS velocities of four sites in the plate interior and two azimuths of the Swan Islands transform fault. The data are well fit by a single angular velocity, with average misfits approximately equal to the 1.5–3.0 mm yr−1 velocity uncertainties. The new model predicts Caribbean‐North America motion ∼65% faster than predicted by NUVEL‐1A, averaging 18–20±3 mm yr−1 (2σ) at various locations along the plate boundary. The data are best fit by a rotation pole that predicts obliquely convergent motion along the plate boundary east of Cuba, but are fit poorly by a suite of previously published models that predict strike‐slip motion in this region. The data suggest an approximate upper bound of 4–6 mm yr−1 for internal deformation of the Caribbean plate, although rigorous estimates await more precise and additional velocities from sites in the plate interior.