SUMMARY A network of 27 GPS sites was implemented in Iran and northern Oman to measure displacements in this part of the Alpine–Himalayan mountain belt. We present and interpret the results of two surveys performed in 1999 September and 2001 October. GPS sites in Oman show northward motion of the Arabian Plate relative to Eurasia slower than the NUVEL‐1A estimates (e.g. 22 ± 2 mm yr−1 at N8°± 5°E instead of 30.5 mm yr−1 at N6°E at Bahrain longitude). We define a GPS Arabia–Eurasia Euler vector of 27.9°± 0.5°N, 19.5°± 1.4°E, 0.41°± 0.1° Myr−1. The Arabia–Eurasia convergence is accommodated differently in eastern and western Iran. East of 58°E, most of the shortening is accommodated by the Makran subduction zone (19.5 ± 2 mm yr−1) and less by the Kopet‐Dag (6.5 ± 2 mm yr−1). West of 58°E, the deformation is distributed in separate fold and thrust belts. At the longitude of Tehran, the Zagros and the Alborz mountain ranges accommodate 6.5 ± 2 mm yr−1 and 8 ± 2 mm yr−1 respectively. The right‐lateral displacement along the Main Recent Fault in the northern Zagros is about 3 ± 2 mm yr−1, smaller than what was generally expected. By contrast, large right‐lateral displacement takes place in northwestern Iran (up to 8 ± mm yr−1). The Central Iranian Block is characterized by coherent plate motion (internal deformation <2 mm yr−1). Sites east of 61°E show very low displacements relative to Eurasia. The kinematic contrast between eastern and western Iran is accommodated by strike‐slip motions along the Lut Block. To the south, the transition zone between Zagros and Makran is under transpression with right‐lateral displacements of 11 ± 2 mm yr−1.

We present the results of a dense seismological experiment in the western part of the Gulf of Corinth (Psathopyrgos-Aigion area), one of the most active rifts in the Aegean region for which we have precise tectonic information. The network included 51 digital stations that operated during July and August 1991, covering a surface of 40 x 40 km2.Among the 5000 recorded events with M L ranging between 1.0 and 3.0, we precisely located 774 events. We obtained 148 well-constrained focal mechanisms using P-wave first motions. Of these, 60 also have mechanisms obtained by combining the P-wave first motions with the S-wave polarization directions. The observed seismicity is mainly located between 6 and 11 km depth. Most of the fault-plane solutions correspond to E-W-striking normal faulting, in agreement with the geological evidence. Most of the well-determined mechanisms indicate a nodal plane dipping 10-25" due north and a steep south-dipping plane. A similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the Corinth Rift. We discuss this evidence and the inference of a deep detachment zone, a structure where the major faults seen at the surface appear to root. A large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone. This feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes.

Abstract.Between 1990 tectonic observations shows that these two earthquakes occurred on low-angle (_< 35 ø) north dipping normal faults located between 4.5 and 10 km depth in the inner part of the rift. Assuming that the deformation is concentrated in relatively narrow deforming zones, we use a simple model of a dislocation in an elastic half-space to study the implication of the localization. Using the geometry of the known seismogenic faults, our observations imply continuous aseismic deformation in the uppermost crust of the inner rift. This model predicts geodetic strain rates close to seismic strain rates in opposition to previous estimates. This is because our model takes into account the activity on low-angle normal faults in the inner rift and an effective seismogenic layer of 6-7 kin, about half that usually assumed.

[1] We compare the geologic histories, the deep structures, and the present-day kinematics of deformation of the Himalaya and the adjacent Tibetan Plateau with those of the Zagros and Iranian Plateau to test geodynamic processes of continental collision. Shortly after India and Arabia collided with Eurasia, horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from India's and Arabia's underlying crystalline basement. Subsequently, slip on thrust faults stacked slices of India's basement to build the Himalaya on India's northern margin. Such faulting has not yet developed in the Zagros, where collision is more recent and Arabia penetrates into Eurasia more slowly than India does, so that postcollision convergence with Eurasia is less. The greater elevation, thicker crust, and more marked heterogeneity of the upper mantle beneath the Tibetan than beneath the Iranian Plateau also reflect a more advanced stage of development. Moreover, while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus, normal faulting, suggesting horizontal extension and crustal thinning, occurs within Tibet but not in Iran. Hence, the balance of forces that built the high Tibetan Plateau must have changed, apparently some time since ∼15 Ma. Removal of Tibetan mantle lithosphere could have altered that balance. If mantle lithosphere beneath the Iranian Plateau has been removed, however, the change in force balance has been too small to initiate normal faulting. Low seismic wave speeds in the uppermost mantle just beneath the Moho of both plateaus suggest (to us) that lithosphere beneath both is thin, consistent with late Cenozoic removal of it, but alternative explanations might account for these low speeds. Despite its apparently thin, and hence presumably weak, mantle lithosphere, much of central Iran undergoes little deformation. It illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small. Whereas differences between the two regions clearly depend on the amount that Arabia and India have penetrated into Eurasia, which scales with both the dates of collision and rates of convergence, we see no differences in the operative processes that depend on the present-day rates of convergence.Citation: Hatzfeld, D., and P. Molnar (2010), Comparisons of the kinematics and deep structures of the Zagros and Himalaya and of the Iranian and Tibetan plateaus and geodynamic implications, Rev. Geophys., 48, RG2005,

International audienceCrustal receiver functions computed from the records of 45 temporary seismological stations installed on a 620-km long profile across central Zagros provide the first direct evidence for crustal thickening in this mountain belt. Due to a rather short 14-km average station spacing, the migrated section computed from radial receiver functions displays the Moho depth variations across the belt with good spatial resolution. From the coast of the Persian Gulf to 25 km southwest of the Main Zagros Thrust (MZT), the Moho is almost horizontal with slight depth variations around 45 km. Crustal thickness then increases abruptly to a maximum of ~70 km beneath the Sanandaj-Sirjan metamorphic zone, between 50 and 90 km northeast of the surface exposure of the MZT. Further northeast, the Moho depth decreases to ~42 km beneath the Urumieh-Dokhtar magmatic assemblage and the southern part of the Central Iranian micro-continent. The region of thickest crust is located ~75 km to the northeast of the Bouguer anomaly low at –220 mgals. Gravity modelling shows that the measured Moho depth variations can be reconciled with gravity observations by assuming that the crust of Zagros underthrusts the crust of central Iran along the MZT considered as a crustal-scale structure. This hypothesis is compatible with shortening estimates by balanced cross-sections of the Zagros folded belt, as well as with structural and petrological studies of the metamorphic Sanandaj-Sirjan zone

Seismic imaging of the lithospheric structure of the Zagros mountain belt (Iran).

[1] Combined P and S receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the Aegean region are used to map the geometry of the subducted African and the overriding Aegean plates. We image the Moho of the subducting African plate at depths ranging from 40 km beneath southern Crete and the western Peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern Greece. However, the dip of the Moho of the subducting African plate is shallower beneath the Peloponnesus than beneath Crete and Rhodes and flattens out beneath the northern Aegean. Observed P-to-S conversions at stations located in the forearc indicate a reversed velocity contrast at the Moho boundary of the Aegean plate, whereas this boundary is observed as a normal velocity contrast by the S-to-P conversions. Our modeling suggests that the presence of a large amount of serpentinite (more than 30%) in the forearc mantle wedge, which generally occurs in the subduction zones, may be the reason for the reverse sign of the P-to-S conversion coefficient. Moho depths for the Aegean plate show that the southern part of the Aegean (crustal thickness of 20-22 km) has been strongly influenced by extension, while the northern Aegean Sea, which at present undergoes the highest crustal deformation, shows a relatively thicker crust (25-28 km). This may imply a recent initiation of the present kinematics in the Aegean. Western Greece (crustal thickness of 32-40 km) is unaffected by the recent extension but underwent crustal thickening during the Hellenides Mountains building event. The depths of the Aegean Moho beneath the margin of the Peloponnesus and Crete (25-28 and 25-33 km, respectively) show that these areas are also likely to be affected by the Aegean extension, even though the Cyclades (crustal thickness of 26-30 km) were not significantly involved in this episode. The Aegean lithosphere-asthenosphere boundary (LAB) mapped with S receiver functions is about 150 km deep beneath mainland Greece, whereas the LAB of the subducted African plate dips from 100 km beneath Crete and the southern Aegean Sea to about 225 km under the volcanic arc. This implies a thickness of 60-65 km for the subducted African lithosphere, suggesting that the Aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the Cyclades.

International audienceMeasurements on either side of the Kazerun fault system in the Zagros Mountain Belt, Iran, show that the accommodation of the convergence of the Arabian and Eurasian Plates differs across the region. In northwest Zagros, the deformation is partitioned as 3–6 mm yr−1 of shortening perpendicular to the axis of the mountain belt, and 4–6 mm yr−1 of dextral strike-slip motion on northwest–southeast trending faults. No individual strike-slip fault seems to slip at a rate higher than ~2 mm yr−1. In southeast Zagros, the deformation is pure shortening of 8 ± 2 mm yr−1 occurring perpendicular to the simple folded belt and restricted to the Persian Gulf shore. The fact that most of the deformation is located in front of the simple folded belt, close to the Persian Gulf, while seismicity is more widely spread across the mountain belt, confirms the decoupling of the surface sedimentary layers from the seismogenic basement. A comparison with the folding and topography corroborates a southwestward propagation of the surface deformation. The difference in deformation between the two regions suggests that right-lateral shear cumulates on the north–south trending Kazerun strike-slip fault system to 6 ± 2 mm yr−1