The geochemical and temporal evolution of the continental lithosphere and its relationship to continental‐scale faulting: The Karakoram Fault, eastern Karakoram, NW Himalayas

Abstract

New laser ablation multicollector–inductively coupled plasma–mass spectrometry and isotope dilution‐thermal ionization mass spectrometry U‐Pb ages, coupled with Sm‐Nd isotope and geochemical analysis, define the temporal and geochemical evolution of the continental lithosphere in the eastern Karakoram, India, NW Himalaya. Our analysis demonstrates that magmatism occurred between ~108 and 69 Ma and ~22 and 13 Ma. The new age data, coupled with geochemical examination of the granitoids, confirm a parallel evolution with the western Karakoram in Pakistan and supports a model of regional continental crustal thickening and related metamorphism. Middle to Late Cretaceous magmatism immediately adjacent to the Karakoram fault suggests that crustal melting and associated metamorphism are unrelated to shearing along the fault. Miocene leucogranite magmatism occurred almost exactly concomitant with the emplacement of the Baltoro batholith in Pakistan. These trans‐Karakoram leucogranites also display similar geochemical evolution trends. Our new data clearly link the leucogranites along the fault to the regional Baltoro batholith and related metamorphic complexes to the west. This supports previous work suggesting that magmatism and metamorphism were not syn‐kinematic with continental‐scale faulting. The data demonstrate that the Karakoram fault could not have accommodated lateral offset in this region prior to ~16 Ma, limiting the long‐term averaged slip rate to a maximum of ~10 mm/yr.

Fault weakening across the frictional‐viscous transition zone, Karakoram Fault Zone, NW Himalaya

“…1 m and 10 m, respectively ( Wells and Coppersmith , ). Thus, the 120 km offset accumulated on the KFZ over 15 Ma [ Phillips et al ., ; Phillips et al ., , Searle et al ., ] could be produced by M7 earthquakes with 125 year average recurrence intervals or M8 earthquakes with 1250 year average recurrence intervals. Brown et al .…”

“…This suggestion was considered as evidence that the KFZ has a high slip rate over geological timescales and is lithospheric in scale, allowing it to accommodate eastward extrusion of Tibet in a plate‐like manner [ Lacassin et al ., , ; Rolland and Pêcher , ]. However, this interpretation has been contested on the grounds that not only do peak metamorphism [ Streule et al ., ] and anatexis [ Phillips et al ., ] predate the fault zone, but also are widespread in occurrence away from the KFZ and hence were not produced by the KFZ [ Phillips et al ., ; Phillips and Searle , ; Searle et al ., , ; Searle and Phillips , ]. The potential for the KFZ to have deformed with a reduced coefficient of friction documented in this study makes it unlikely that the fault could have generated significant shear heating, particularly given its low slip rate [ LeLoup et al ., ].…”

“…The Arganglas fault strand is located at the NE margin of the Nubra‐Siachen leucogranite (Figure ). Here a <1 km wide, 35 km long band of marbles and metapelites of the Saser Formation is bounded by the Nubra‐Siachen leucogranite to the SW and the 107.2 Ma Arganglas diorite to the NE [ Phillips et al ., ; Phillips and Searle , ]. Deformation is focused within the weaker Saser Formation, with the Arganglas diorite undeformed at distances of a few hundred meters from the fault strand.…”

“…[] for a review). Offsets of geological markers [ Murphy et al ., ; Wang et al ., , ; Zhang et al ., ] including the Baltoro‐Nubra leucogranite batholith [ Phillips et al ., , ; Searle and Phillips , ; Searle et al ., ], the Shyok suture zone [ Phillips et al ., ], the Triassic‐Jurassic Aghil formation [ Robinson , ], and the antecedent Indus river [ Gaudemer et al ., ; Searle , ; Valli et al ., ] are in the range 40–150 km. Offsets of 250–565 km have also been proposed for differing interpretations of the same structures [ Lacassin et al ., ; Valli et al ., ].…”

Low effective fault strength due to frictional-viscous flow in phyllonites, Karakoram Fault Zone, NW India

“…The Karakoram terrane in this region consists of the predominantly amphibolite grade Eastern Karakoram Metamorphic Complex (EKMC), including the Nubra and Saser formations and Pangong Transpressional Zone, along with dioritic and leucogranitic intrusions of the Karakoram batholith (Fig. 2b, Streule et al, 2009;Phillips et al, 2013). These two intrusive suites were emplaced during an Early-Late Cretaceous phase of subduction-related magmatism and a mid-Miocene episode of crustal anatexis following the India-Asia collision .…”