Rebecca Bendick

GPS measurements from the ladakh himalaya, India: preliminary tests of plate-like or continuous deformation in tibet

Observations of relative motion in a geodetic network in Ladakh, India, and across southern Tibet indicate slow shear on the Karakorum fault, rapid east-west extension across the whole of southern Tibet, and constant arc-normal convergence between India and southern Tibet along the Himalayan arc. Measurements of ten campaign-style and six permanent sites with global positioning system (GPS) precise geodesy provide these bounds on the style and rates of the large-scale deformation in the Tibet-Himalaya region. Divergence between sites at Leh, Ladakh, India, and Shiquanhe, western Tibet, as well as slow relative motion among sites within the Ladakh network, limit right-lateral slip parallel to the Karakorum fault to only 3.4 ± 5 mm/yr. This low rate concurs with a recent estimate of 3–4 mm/yr for Late Holocene time, but disagrees with the much higher rate of 30–35 mm/yr that has been used to argue for plate-like behavior of the Tibetan Plateau. Convergence between Ladakh and the Indian subcontinent at 18.8 ± 3 mm/yr at 224° ± 17° (1σ) differs little from estimates of convergence across the central segment of the Himalaya. Finally, lengthening of the baseline between Leh, Ladakh, and Lhasa (in southeastern Tibet) at 17.8 ± 1 mm/yr or between Leh and Bayi (farther to the southeast) at 18 ± 3 mm/yr, is consistent with an extrapolation of rates of east-west extension of the Tibetan Plateau based both on shorter GPS baselines (e.g., Lhasa-Simikot) and on diverging slip vectors of earthquakes in the Himalaya. We interpret these results to indicate that Tibet behaves more like a fl uid than like a plate

How perfect is the Himalayan arc

The Himalayan plate boundary, because it is entirely subaerial, is both the most dramatic and the most accessible to direct observation of all active convergent boundaries on Earth. The shape of this boundary can be described as a small circle of radius 1696 ± 55 km, centered at long 91.6° ± 1.6°E and lat 42.4° ± 2.1°N for the extent of the arc between long 77.2° and 92.1°E. The pole of this small circle is consistent whether seismicity, topography, or stress state is used to define the position of the tectonic boundary. The defined small circle also coincides with a peak in microseismicity, the maximum horizontal strain rate, and a peak in the vertical velocity field. This quantitative definition of a stable, curved tectonic boundary is a prerequisite to modeling the dynamics of curvature in convergent arcs and applying appropriate boundary conditions to other regional models.

Reconciling lithospheric deformation and lower crustal flow beneath central Tibet

A viscous region with deformable boundaries is used to model simultaneous crustal flow and lithospheric coupling in northern Tibet. This model suggests that (1) the deformation of northern Tibet is different from that of southern Tibet because of structural differences between the regions; (2) the viscosity contrast between the crust and the mantle lithosphere is relatively small beneath northern Tibet; and (3) crustal flow is compatible with crust-mantle coupling under these conditions.

Slip on an active wedge thrust from geodetic observations of the 8 October 2005 Kashmir earthquake

By combining global positioning system observations of surface displacements and the locations of aftershocks, we infer that the 8 October 2005 Kashmir earthquake occurred on multiple fault planes. Mean slip of ∼5.1 m occurred on a rupture between Bagh and Balakot with strike 331° and dip 29°. Additional slip occurred at depth on a NNE-dipping fault plane extending WNW from Balakot, and on an intersecting nearly fiat dislocation at ∼5 km depth, forming an active wedge thrust. Both the simple fault plane and the blind wedge accommodate convergence between Peshawar and Leh, Ladakh, accumulating at 7 ± 2 mm/yr, suggesting a 680 ± 150 yr recurrence interval for Kashmir 2005-like events.

Extreme localized exhumation at syntaxes initiated by subduction geometry

Some of the highest and most localized rates of lithospheric deformation in the world are observed at the transition between adjacent plate boundary subduction segments. The initiating perturbation of this deformation has long been attributed to vigorous erosional processes as observed at Nanga Parbat and Namche Barwa in the Himalaya and at Mount St. Elias in Alaska. However, an erosion-dominated mechanism ignores the 3-D geometry of curved subducting plates. Here we present an alternative explanation for rapid exhumation at these locations based on the 3-D thermomechanical evolution of collisions between plates with nonplanar geometries. Comparison of model predictions with existing data reproduces the defining characteristics of these mountains and offers an explanation for their spatial correlation with arc termini. These results demonstrate a “bottom-up” tectonic rather than “top-down” erosional initiation of feedbacks between erosion and tectonic deformation; hence, the importance of 3-D subduction geometry.

The relationship between surface kinematics and deformation of the whole lithosphere

The variation of mechanical properties with depth in the lithosphere determines the relationship between surface deformation and whole-lithosphere deformation, hence between surface deformation and whole-lithosphere dynamics. Where viscosity (or elastic strength) is a continuous function with depth, surface deformation can be used to constrain both force balance and rheological parameters. Where viscosity is discontinuous, but the upper crust and mantle lithosphere have comparable maximum values, surface deformation can be used to approximate force balance and rheological parameters, but tradeoffs mean that estimates of stress and viscosity are effective equivalent values rather than actual values. Where viscosity is both discontinuous and differs by much more than an order of magnitude between the upper crust and mantle lithosphere, information about both force balance and rheology are absent from the surface deformation, so surface observations alone are insufficient to estimate either the dynamic or mechanical state of the lithosphere.

Do weak global stresses synchronize earthquakes

Insofar as slip in an earthquake is related to the strain accumulated near a fault since a previous earthquake, and this process repeats many times, the earthquake cycle approximates an autonomous oscillator. Its asymmetric slow accumulation of strain and rapid release is quite unlike the harmonic motion of a pendulum and need not be time predictable, but still resembles a class of repeating systems known as integrate-and-fire oscillators, whose behavior has been shown to demonstrate a remarkable ability to synchronize to either external or self-organized forcing. Given sufficient time and even very weak physical coupling, the phases of sets of such oscillators, with similar though not necessarily identical period, approach each other. Topological and time series analyses presented here demonstrate that earthquakes worldwide show evidence of such synchronization. Though numerous studies demonstrate that the composite temporal distribution of major earthquakes in the instrumental record is indistinguishable from random, the additional consideration of event renewal interval serves to identify earthquake groupings suggestive of synchronization that are absent in synthetic catalogs. We envisage the weak forces responsible for clustering originate from lithospheric strain induced by seismicity itself, by finite strains over teleseismic distances, or by other sources of lithospheric loading such as Earths variable rotation. For example, quasi-periodic maxima in rotational deceleration are accompanied by increased global seismicity at multidecadal intervals.

Kinematic evidence for the effect of changing plate boundary conditions on the tectonics of the northern U.S. Rockies

We derive surface velocities from GPS sites in the interior Northwest U.S. relative to a fixed North American reference frame to investigate surface tectonic kinematics from the Snake River Plain (SRP) to the Canadian border. The Centennial Tectonic Belt (CTB) on the northern margin of the SRP exhibits west-directed extensional velocity gradients and strain distributions similar to the main Basin and Range Province (BRP) suggesting that the CTB is part of the BRP. North of the CTB, however, the vergence of velocities relative to North America switches from westward to eastward along with a concomitant rotation of the principal stress axes based on available seismic focal mechanisms, revealing paired extension in the northern Rockies and shortening across the Rocky Mountain Front. This change in orientation of surface velocities suggests that the change in the boundary conditions on the western margin of North America influences the direction of gravitational collapse of Laramide thickened crust. Throughout the study region, fault slip rate estimates calculated from the new geodetic velocity field are consistently larger than previously reported fault slip rates determined from limited geomorphic and paleoseismic studies.

Present-day kinematics at the India-Asia collision zone: COMMENT and REPLY COMMENT

The recent publication, “Present-day kinematics at the India-Asia collision zone” ([Meade, 2007][1]), attempts to address an ongoing controversy about the dynamics of the Tibetan Plateau by finding block models for the region that correctly reproduce the kinematics from many global positioning

Kinematics and dynamics of the Pamir, Central Asia: Quantifying surface deformation and force balance in an intracontinental subduction zone

Kinematic and dynamic models quantify deformation and force balance in the Pamir, a region undergoing the rare and poorly understood process of intracontinental subduction. We constrain a detailed kinematic model with 506 recent GPS velocities and Quaternary fault slip rates and show that the Pamir is organized like the Himalaya and Tibet, with regions of 1) localized strain rate ≥100e-9/yr along the Pamir Frontal Thrust System (the subduction interface), similar to the Himalaya, and 2) distributed north-south compression and east-west extension, similar to Tibet. Through standard thin viscous sheet methods we demonstrate that the lithospheric force balance in the Pamir is a combination of stresses caused by gravitational potential energy and India-Eurasia convergence accommodated at a subduction interface, in this case the Pamir Frontal Thrust System. We find that strain rate and deviatoric stress patterns near the Pamir Frontal Thrust System are characteristic of a mature subduction zone, despite its initiation in continental lithosphere. Although the Pamir and Tibet are kinematically and dynamically similar, the Pamir is stiffer overall than Tibet, perhaps due to the presence of the highly arcuate, geometrically stiffened continental slab at depth.