Localization of gravity and topography: constraints on the tectonics and mantle dynamics of Venus

We develop a method for spatio-spectral localization of harmonic data on a sphere and use it to interpret recent high-resolution global estimates of the gravity and topography of Venus in the context of geodynamical models. Our approach applies equally to the simple spatial windowing of harmonic data and to variable-length-scale analyses, which are analogous to a wavelet transform in the Cartesian domain. Using the variable-length-scale approach, we calculate the localized RMS amplitudes of gravity and topography, as well as the spectral admittance between the two fields, as functions of position and wavelength. The observed admittances over 10 per cent of the surface of Venus (highland plateaus and tessera regions) are consistent with isostatic compensation of topography by variations in crustal thickness, while admittances over the remaining 90 per cent of the surface (rises, plains and lowlands) indicate that long-wavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere. The global average crustal thickness is less than 30 km, but can reach values as large as 40 km beneath tesserae and highland plateaus. We also note that an Earth-like radial viscosity structure cannot be rejected by the gravity and topography data and that, without a mechanical model of the lithosphere, admittance values cannot constrain the thickness of the thermal boundary layer of Venus. Modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae, the effective elastic plate thickness, T_e , was less than 20 km. Estimates of T_e at highland rises are consistently less than 30 km. Our inability to find regions with T_e > 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models.     

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

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.     

Large-scale velocity field and strain tensor in Iran inferred from GPS measurements: new insight for the present-day deformation pattern within NE Iran

A network of 26 GPS sites was implemented in Iran and Northern Oman to measure displacements in this part of the Arabia–Eurasia collision zone. We present the GPS velocity field obtained from three surveys performed in 1999 September, 2001 October and 2005 September and the deduced strain tensor. This study refines previous studies inferred from only the two first surveys. Improvements are significant in NE Iran. The present-day shortening rate across the mountain belts of NE Iran is estimated to 5 ± 1 mm yr⁻¹ at about N11°, 2 ± 1 mm yr⁻¹ of NS shortening across the eastern Kopet Dag and 3 ± 1 mm yr⁻¹ of NS shortening across Binalud and Kuh-e-Sorkh. Our GPS measurements emphasize the varying character of the Kopet Dag deformation between its southeastern part with prevailing thrusting at low rates and its northwestern part with dominant strike-slip activity at increasing rates. The principal axes of the horizontal strain tensor appears very homogeneous from the Zagros to the Alborz and the Kopet-Dag (N20°) and in eastern Iran (Makran and Lut block: N30°). Only NW Iran suffers a variable strain pattern which seems to wrap the Caspian basin. The strain tensor map underlines the existence of large homogeneous tectonic provinces in terms of style and amplitude of the deformation.     

Active tectonics of an apparently aseismic region: distributed active strike-slip faulting in the Hangay Mountains of central Mongolia

We identify and describe a series of east–west left-lateral strike-slip faults (named the Songino-Margats, the Hag Nuur, the Uliastay and the South Hangay fault systems) in the Hangay mountains of central Mongolia: an area that has little in the way of recorded seismicity and which is often considered as a rigid block within the India–Eurasia collision zone. The strike-slip faults of central Mongolia constitute a previously unrecognized hazard in this part of Mongolia. Each of the strike-slip faults show indications of late Quaternary activity in the form of aligned sequences of sag-ponds and pressure-ridges developed in alluvial deposits. Total bed-rock displacements of ∼3 km are measured on both the Songino-Margats and South Hangay fault systems. Bed-rock displacements of 11 km are observed across the Hag Nuur fault. Cumulative offset across the Uliastay fault systems are unknown but are unlikely to be large. We have no quantitative constraint on the age of faulting in the Hangay. The ≤20 km of cumulative slip on the Hangay faults might, at least in part, be inherited from earlier tectonic movements. Our observations show that, despite the almost complete absence of instrumentally recorded seismicity in the Hangay, this part of Mongolia is cut through by numerous distributed strike-slip faults that accommodate regional left-lateral shear between Siberia and China. Central Mongolia is thus an important component of the India–Eurasia collision that would be overlooked in models of the active tectonics based on the distribution of seismicity. We suggest that active faults such as those identified in the Hangay of Mongolia might exist in other, apparently aseismic, regions within continental collision zones.     

The 2004 May 28 Baladeh earthquake (Mw 6.2) in the Alborz, Iran: overthrusting the South Caspian Basin margin, partitioning of oblique convergence and the seismic hazard of Tehran

We use teleseismic waveform analysis and locally recorded aftershock data to investigate the source processes of the 2004 Baladeh earthquake, which is the only substantial earthquake to have occurred in the central Alborz mountains of Iran in the modern instrumental era. The earthquake involved slip at 10–30 km depth, with a south-dipping aftershock zone also restricted to the range 10–30 km, which is unusually deep for Iran. These observations are consistent with co-seismic slip on a south-dipping thrust that projects to the surface at the sharp topographic front on the north side of the Alborz. This line is often called the Khazar Fault, and is assumed to be a south-dipping thrust which bounds the north side of the Alborz range and the south side of the South Caspian Basin, though its actual structure and significance are not well understood. The lack of shallower aftershocks may be due to the thick pile of saturated, overpressured sediments in the South Caspian basin that are being overthrust by the Alborz. A well-determined earthquake slip vector, in a direction different from the overall shortening direction across the range determined by GPS, confirms a spatial separation ('partitioning') of left-lateral strike-slip and thrust faulting in the Alborz. These strike-slip and thrust fault systems do not intersect within the seismogenic layer on the north side, though they may do so on the south. The earthquake affected the capital, Tehran, and reveals a seismic threat posed by earthquakes north of the Alborz, located on south-dipping thrusts, as well as by earthquakes on the south side of the range, closer to the city.