Cretaceous shortening and exhumation history of the South Pamir terrane

Abstract

Despite Miocene extension and exhumation of middle to lower crust in a series of gneiss domes and interpreted Cenozoic delamination of the lower crust, the crust in the modern Pamir Mountains is among the thickest in the world. Cenozoic shortening, crustal thickening, and prograde metamorphism in the Pamir have been associated with India-Asia collision. However, new mapping in the South Pamir terrane indicates relatively minor, distributed shortening since the Jurassic, which occurs in a thrust belt overprinted by late Cenozoic transpression. The thrust belt connects with the Rushan-Pshart suture zone, a Mesozoic terrane boundary. New detrital zircon U-Pb and detrital zircon fission track ages of synorogenic clastic rocks exposed in the footwall of thrust faults in the South Pamir thrust belt provide maximum deposition ages (76-112 Ma), which are interpreted to document Cretaceous shortening prior to India-Asia collision. Furthermore, zircon (U-Th)/He and apatite (U-Th)/He data from the South Pamir terrane generally record cooling ages of ca. 102-44 Ma, suggesting limited Cenozoic exhumation. These results (1) are consistent with widespread Cretaceous deformation throughout the Pamir-Tibet orogen with limited Cenozoic upper crustal shortening in the South Pamir terrane, (2) together with previous studies, allow for the possibility that the upper crust of the Pamir orogen was characterized by net extension during the Cenozoic rather than net shortening, and (3) are consistent with models that relate Cenozoic crustal thickening to the insertion of Indian lower crust beneath the Pamir. Lower crustal thickening of the South Pamir terrane is difficult to reconcile with the prograde metamorphic history of gneiss domes in the South Pamir terrane and may require a relatively shallow (<15-20 km) shear zone separating lower crustal contraction from upper crustal extension.

Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral, NW Pakistan): Insights from phase equilibria modelling, and garnet–monazite petrochronology

“…stage 2; our study; Hildebrand et al., ). Similar deformation of the Eurasian margin at this time has been documented in the Northern and Southern Pamir farther east and north (Chapman, Robinson, et al., ; Robinson, Manning, Harrison, Zhang, & Wang, ) and in the Karakoram Metamorphic Complex (Fraser et al., ; Palin et al., ; Streule, Phillips, Searle, Waters, & Horstwood, ; Zhuang et al., ) as inferred from the presence of (a) staurolite‐ and sillimanite‐grade rocks dated to 105–108 Ma, and (b) strongly deformed Permian to Jurassic sediments unconformably (angular) overlain by the Lower Cretaceous Tupop and Reshun conglomerates (Gaetani et al., ; Zanchi & Gaetani, ).…”

“…Cawood et al., ; Hyndman, Currie, & Mazzotti, ) beneath the south Eurasian active margin (Figure b). We also suggest that crustal shortening and thickening of the south Eurasian margin (far from the suture) was probably driven by low angle to flat subduction of the Neotethyan ocean during the Early to Mid‐Cretaceous (Figure b; as already proposed by Chapman, Robinson, et al., ; Chapman, Scoggin, et al., ; Kapp et al., ; Robinson, ; Schwab et al., ; Van Hinsbergen et al., ), prior to the obduction of the Kohistan–Ladakh arc at either 85–75 Ma ago (Searle et al., ; Treloar et al., ) or 50–40 Ma (Bouilhol et al., ). Moreover, the hot thermal structure evidenced in this study could also explain the deformation propagation within the Eurasian active margin through significant rheological weakening (as reported for other accretionary orogens; for example, Cawood et al., ) over the Mesozoic.…”

“…While there is no extension or exhumation evidence in the Chitral region and the South Pamir in general during this time, Chapman, Robinson, et al. () and Chapman, Scoggin, et al. () reported a shift in the isotopic nature of Late Cretaceous ( c .…”

“…() reported a shift in the isotopic nature of Late Cretaceous ( c . 75 Ma) and Eocene magmatic rocks in the Pamir which may reflect lithospheric extension, potentially related to the roll back of the Tethyan downgoing slab (Figure c; Chapman, Robinson, et al., ; Chapman, Scoggin, et al., ). Such a slab retreat would be also consistent with the large decrease in magmatic activity during this time throughout the Eurasian margin (Karakoram–Pamir–Qiantang) despite ongoing oceanic subduction.…”

“…This allows the direct study of the tectonic evolution of metamorphic and magmatic rocks residing at mid‐crustal conditions, and the crustal foundering that affected its root zone during this time (Hacker et al., ; Kooijman, Smit, Ratschbacher, & Kylander‐Clark, ), especially in the Central Pamir and Karakoram blocks (Fraser, Searle, Parrish, & Noble, ; Mahéo, Blichert‐Toft, Pin, Guillot, & Pêcher, ; Palin, Searle, Waters, Horstwood, & Parrish, ; Rolland, Mahéo, Guillot, & Pêcher, ; Rolland, Villa, Guillot, Mahéo, & Pêcher, ). Only a few studies, however, have examined the detailed timing of deformation and metamorphism in the South Pamir, including the Hindu Kush region, which has been shown to preserve a potentially more complex and longer history of magmatism and metamorphism than the adjacent Karakoram block (Aminov et al., ; Chapman, Robinson, et al., ; Chapman, Scoggin, et al., ; Faisal, Larson, Camacho, & Coutand, ; Faisal, Larson, Cottle, & Lamming, ; Faisal, Larson, King, & Cottle, ; Hildebrand, ; Hildebrand, Noble, Searle, Waters, & Parrish, ; Hildebrand, Searle, Shakirullah, Khan, & Van Heijst, ).…”