Alexander C. Robinson

Tectonic evolution of the northeastern Pamir: Constraints from the northern portion of the Cenozoic Kongur Shan extensional system, western China

The late Cenozoic Kongur Shan extensional system lies along the northeastern margin of the Pamir at the western end of the Himalayan-Tibetan orogen, accommodating east-west extension in the Pamir. At the northern end of the extensional system, the Kongur Shan normal fault juxtaposes medium- to high-grade metamorphic rocks in both its hanging wall and footwall, which record several Mesozoic to Cenozoic tectonic events. Schists within the hanging wall preserve a Buchan metamorphic sequence, dated as Late Triassic to Early Jurassic (230–200 Ma) from monazite inclusions in garnet. Metamorphic ages overlap with U-Pb zircon ages from local granite bodies and are interpreted to be the result of regional arc magmatism created by subduction of the Paleo-Tethys ocean. The northern portion of the footwall of the extensional system exposes an upper-amphibolite-facies unit (~650 °C, 8 kbar), which structurally overlies a lowgrade metagraywacke unit. The high-grade unit records late Early Cretaceous crustal thickening at ca. 125–110 Ma, followed by emplacement over the low-grade metagraywacke along a north-northeast–directed thrust prior to ca. 100 Ma. Together these results indicate signifi cant middle Cretaceous crustal thickening and shortening in the northern Pamir prior to the Indo-Asian collision. A third Late Miocene (ca. 9 Ma) amphibolite-facies metamorphic event (~650–700 °C, 8 kbar) is recorded in footwall gneisses of the Kongur Shan massif. North of the Kongur Shan massif, rapid cooling in the footwall beginning at 7–8 Ma is interpreted to date the initiation of exhumation along the Kongur Shan normal fault. A minimum of 34 km of east-west extension is inferred along the Kongur Shan massif based on the magnitude of exhumation since the Late Miocene (~29 km) and the present dip of the Kongur Shan normal fault (~40°). Field observations and interpretation of satellite images along the southernmost segment of the Kongur Shan extensional system indicate that the magnitude of late Cenozoic east-west extension decreases signifi cantly toward the south. This observation is inconsistent with models in which east-west extension in the Pamir is driven by northward propagation of the right-slip Karakoram fault, suggesting instead that extension is driven by vertical extrusion due to topographic collapse, radial thrusting along the Main Pamir Thrust, or oroclinal bending of the entire Pamir region.

Cenozoic evolution of the eastern Pamir: Implications for strain-accommodation mechanisms at the western end of the Himalayan-Tibetan orogen

Detailed field mapping, geochronologic and thermochronologic analyses, and petrologic investigations conducted along the southern segment of the late Cenozoic Kongur Shan extensional system provide new information on the Cenozoic tectonic evolution of the eastern Pamir at the western end of the Himalayan-Tibetan orogen. Field relations and cooling-age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east-west extension along the southern Kongur Shan extensional system, from 20 km or less along the Muztaghata massif in the north, to l3 km along the Tashkorgan fault in the south. These results, in conjunction with previously published work on the northern segment of the Kongur Shan extensional system, show a general southward decrease in east-west extension along the entire length of the extensional system, consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the Pamir. Petrologic data, 40 Ar/ 39 Ar cooling ages, and monazite Th-Pb ages from schists and gneisses in the footwall of the southern Kongur Shan normal fault along the Muztaghata massif record two tectonic events that immediately preceded late Miocene initiation of east-west extension: (1) high-grade schists and gneisses experienced upper amphibolite facies metamorphic conditions (9–10 kbar, 700–750 °C) dated as late Oligocene to middle Miocene by in situ ion-microprobe analyses of monazite inclusions in garnet; and (2) high-grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late Miocene with 40 Ar/ 39 Ar biotite cooling ages of 7.5–9 Ma. Rapid exhumation was accommodated in part by the east-west–striking, south-dipping, Shenti normal fault. Field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the Central Pamir gneiss domes, and the antiformal Muztaghata massif is the eastward continuation of the Sares dome of the Central Pamir. These observations suggest that the antiformal gneiss domes of the Central Pamir have not been offset across the Karakorum right-slip fault from the Qiangtang anticlinorium in Tibet. Instead, we propose that the development of the Central Pamir gneiss domes may have been related to Oligocene-Miocene northward underthrusting and thickening of crust beneath the Pamir.

Cretaceous shortening and exhumation history of the South Pamir terrane

National Science Foundation (NSF) [EAR-1450899]; NSF [EAR-1419748, EAR-0929777, 1338583]; American Philosophical Society; American Association of Petroleum Geologists; Geological Society of America; Exxon Mobil Corporation

Kinematic Evolution of the Eastern Chalk Draw Fault during Basin and Range Extension: Evaluating the Role Played by Preexisting Structural Fabrics in Along-Strike Displacement Patterns

Basin and Range extensional structures in Trans-Pecos Texas have been interpreted to be strongly influenced by preexisting structures, including igneous centers in the Big Bend region. These igneous centers are interpreted to have deflected propagating faults, as they are rheologically strong relative to the surrounding rocks. We conducted detailed field mapping and analysis of fracture and fault data along the eastern segment of the Chalk Draw Fault in Big Bend National Park to evaluate its kinematic evolution and interaction with Paleogene plutons. Our primary results document the following key relationships: (1) the Rosillos Laccolith and other Paleogene plutons in the study area are pervasively fractured, dominantly by northwest-striking mode I fractures, which generally do not extend into adjacent sedimentary rock sequences; (2) throw on the eastern Chalk Draw Fault is strongly asymmetric, decreasing rapidly as it enters the Rosillos Laccolith; and (3) analysis of fault-slip data in the region documents dominantly N 52° E–directed regional extension along the northwest-striking segment of the Chalk Draw Fault and secondary structures. We interpret these relationships to indicate that the Rosillos Laccolith developed pervasive mode I fractures with a dominant northwest-southeast orientation during cooling under Basin and Range extensional stresses but that this occurred prior to extensional faulting within the region. We suggest that the highly fractured nature of the Rosillos Laccolith resulted in it being rheologically weaker than the surrounding Cretaceous sedimentary rocks, which resulted in (1) hindering southward propagation of the Chalk Draw Fault by dissipating stress at the fault tip within the highly fractured laccolith and (2) the strongly asymmetric distribution of slip due to a strong rheological contrast during fault propagation.