Description: Multimethod geochronology (U-Pb zircon; 40 Ar/ 39 Ar hornblende, biotite, feldspar; apatite fission track) on granitoids, gneisses, and Cenozoic intramontane basin clastics of the Gissar-Alai ranges, South Tian Shan collisional belt, west of the Talas-Fergana fault, elucidates a history of Neoproterozoic magmatism, late Paleozoic magmatism and metamorphism, and Mesozoic–Cenozoic thermal reactivation. Zircon-core and grain-interior U-Pb ages of ca. 2.7–2.4, 2.2–1.7, 1.1–0.85, and 0.85–0.74 Ga tie the early evolution of the Gissar-Alai ranges to that of the Tarim craton. At least part of the Gissar range crystalline basement—the Garm massif—shows U-Pb zircon crystallization ages of ca. 661–552 Ma (median ca. 609 Ma), again suggesting a Tarim craton connection. Tarim collided with the Middle Tian Shan block at ca. 310–305 Ma, completing the protracted formation of the South Tian Shan collisional belt. The massive Gissar range granitoids intruded later (ca. 305–270 Ma), contemporaneous with peak Barrovian-type metamorphism in the Garm massif rocks. Major- and trace-element compositions suggest that the Gissar granitoid melts have continental arc affinity. Zircon Hf and whole-rock Nd values of –2.1 to –6.9 and –2.7 to –7.2, respectively. and Hf-isotope crustal model and Nd-isotope depleted mantle model ages of ca. 1.0–1.2 and ca. 1.1–2.2 Ga, respectively, suggest significant input of Precambrian crust in the Gissar granitoid and Garm orthogneiss melts, consistent with the U-Pb ages of inherited and detrital zircons. The distinct ca. 661–552 Ma Garm gneiss crystallization ages and the ca. 1.0–2.2 Ga model ages (and the lack of 2.4–3.4 Ga model ages) tie the Garm gneisses and the reworked crust of the Gissar range to the northern rim—the Kuqa and Kolar sections—of the Tarim craton, suggesting a united Karakum-Tarim craton. Although about contemporaneous with widespread postcollisional magmatism in the entire Tian Shan, the large volume and short duration of the Gissar range magmatism, including crustal thickening and prograde metamorphism during Tarim craton–Middle Tian Shan block collision, and formation and closure of an oceanic back-arc basin (the Gissar basin), indicate its origin in a distinct setting. Combined, this likely resulted in midcrustal melting and upper-crustal batholith emplacement. Mafic dikes and pipes intruded at ca. 256–238 Ma (median ca. 241 Ma); the source region of the parental melts was within the asthenospheric mantle. The simplest interpretation for these basanites is that they were part of the Tarim flood basalt province; this would extend this province westward from the Tarim craton into the southwestern Tian Shan and imply that the relatively short-lived flood basalt event (ca. 290–270 Ma) was followed by much less voluminous but longer-lasting hotspot magmatism. The 40 Ar/ 39 Ar and detrital apatite fission-track dates outline post–Gissar-Alai range granitoid emplacement cooling, Cimmerian collision events at the southern margin of Asia, Late Cretaceous crustal extension and local magmatism, and early Cenozoic shortening and burial in the far field of the India-Asia collision.

Description: The over 7000m high peaks of the Trans-Alai and the on average over 4000m elevated Pamir plateau is in some way the mirrored equivalent of the Himalaya and Tibet plateau on the northwestern promontory of India-Eurasia collision. Current shortening across the Trans Alai of 13mm/a takes up about 1/3 of India–Eurasia convergence, only little less than across the Himalaya and the highest rate localized far away from a plate boundary. Accumulated Cenozoic shortening reaches also a magnitude similar to the adjacent Himalaya-Tibet system, yet was accommodated over about half the meridional width. There are other marked differences between the two systems. The Pamir is presumably thrusted over Eurasia rather than India and instead of a foreland basin another major orogen, the Tien Shan, stands between its frontal thrust and stable Eurasia. The Pamir and adjacent Hindu Kush feature vigorous intermediate depth (100–300km) mantle seismicity, an attribute absent beneath all other major continental orogens. We will report on a new earthquake data set collected during a field campaign between 2008 and 2010, when we operated a network of 40 seismic stations across the southern Tien Shan and Pamir mountain ranges in Kyrgyzstan and Tajikistan. This is the first modern, digital and dense seismic network in the region. From more than 6000 well located earthquakes approximately two third are crustal. To derive robust source mechanisms and additional constraint on event depths, we use full waveform inversion of our local temporary and regional permanent seismic station recordings for events with magnitudes 〉 3.5. We use relocated hypocenters and fault plane solutions to describe the deformation pattern in the crust. Seismicity clusters in several well defined known and unknown structures. The Main Pamir Thrust (MPT) on the northern perimeter of the Pamir is clearly outlined by a string of earthquakes with thrust mechanisms, with the eastern part of the Alai valley more active than the western part. The largest earthquake we recorded, an M6.7, occurred at MPT’s north-easternmost point, where the Alai valley closes and the Pamir collides with the Tien Shan. This is a region of significant structural complexity, where the MPT fans out in a series of northeast trending orographic features. The mainshock shows an almost pure thrust mechanism with one steeper (55°) and one more shallow (38°) nodal plane. The aftershock seismicity displays two lineaments forming a hockey stick like feature that tightly follows the orographic relief. The “blade” strikes approximately 85° in agreement with one nodal plane of the mainshock double couple. A cross section through the aftershocks reveals that the steeper, south-dipping nodal plane is the fault plane. The earthquake probably ruptured the very tip of the Pamir frontal thrust where it reaches the surface in the southern margin of the Alai valley. On the Pamir plateau deformation is mainly extensional with dextral components. One seismically active zone crosses the entire plateau from Lake Karakul to the Wakhan corridor of Afghanistan. High seismicity is also detected along the deeply incised valleys of the western Pamir.

Description: Active tectonics in the Pamir mountains in central Asia, the westernmost part of the India-Eurasia collision zone, are controlled by ongoing convergence (about 20 mm/yr), causing substantial crustal shortening and compressional deformation. This leads to high seismicity rates throughout the region. Whereas seismic activity along the rim of the Pamir plateau is mostly compressional and concentrated along the Main Pamir Thrust, the distribution and focal mechanisms of earthquakes in its interior are more diffuse, with extensional events occurring along North-South trending rift zones (Kara Kul, Wachan). Seismicity in the south-western Pamir and in the Hindu Kush features frequent intermediate-depth earthquakes, reaching hypocentral depths of 300 km, which is rare for regions not obviously related to active subduction of oceanic lithosphere. These mantle earthquakes, which are not observed beneath the Himalayas and Tibet further east, form a rather well-defined Wadati-Benioff zone that was readily interpreted as subducted lithosphere present below the current collisional orogen. Earlier seismological studies showed the presence of a northward-dipping lithospheric slab under the Hindu Kush and a southward-dipping one beneath the Pamirs, with a small seismic gap in-between. Different hypotheses concerning the nature of these slabs (oceanic or continental lithosphere) and tectonic geometry in general (two slabs subducting in opposite directions or a single, hugely contorted slab) have been proposed in literature. Political instability in the region in the last two decades hampered on-site studies and field work, leaving many key issues poorly understood. In the framework of the multidisciplinary project TIPAGE (Tien Shan Pamir Geodynamic Programme), for the first time, new field campaigns collecting high quality data have been made possible. Local seismicity in the Pamir and Tien Shan mountain ranges (Tajikistan and Kyrgyzstan) is currently being recorded by a temporary installation of 40 seismic stations, 30 in Tajikistan and 10 in Kyrgyzstan, for a total time of two years starting summer 2008. In 2009, the configuration of the stations was changed from a 24-station North-South profile plus 16 additional stations distributed throughout the Pamirs to a 40-station 2D setup evenly covering the whole study region. Moreover, the first half of the data was retrieved, for which we will present preliminary results. The high density of seismic stations allows precise location of earthquake hypocenters and determination of source mechanisms for selected events. So far we detected some 10,000 events, a significant proportion of which are related to aftershocks of a Mw 6.6 earthquake that occurred in October 2008 in the border triangle of Kyrgyzstan, Tajikistan and China, directly beneath one of our stations. The hypocenter distribution of a selection of detected events provides a good indication on active faults in the region, thus enabling us to interpret ongoing tectonic activity.We will also present seismicity cross-sections through interesting subparts of the study region that will shed a new light on the complex geometry of mantle deformation.

Description: We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. A total of 9532 earthquakes were detected, located, and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algorithms, waveform cross-correlation, and multi-event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir-Hindu Kush intermediate-depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes: one beneath the Pamir and the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (approximately 10 km width), curviplanar arc that strikes east-west and dips south at its eastern end and then progressively turns by 90° to reach a north-south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate-depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20–25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east-west, yet bends northeast, toward the Pamir, at its eastern end. It may be divided vertically into upper and lower parts separated by a gap at approximately 150 km depth. In the upper part, events form a plane that is 15–25 km thick in cross section and dips sub-vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub-parallel planes. The observed geometry could be reconciled either with a model of two-sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitates a contortion and oversteepening of the latter.