Michael A. Krol

Evidence for rapid displacement on Himalayan normal faults and the importance of tectonic denudation in the evolution of mountain ranges

East-striking, low-angle normal faults of the South Tibetan detachment system have played an important role in exposing the high-grade metamorphic core of the Himalayan orogen. In the Mount Everest region of southern Tibet, granites both pre- and postdate an important fault of the system, the Qomolangma detachment. New U-Pb and 40 Ar/ 39 Ar geochronologic data for these rocks constrain the age of brittle faulting to between 16.67 ± 0.04 and 16.37 ± 0.40 Ma, significantly expanding the known age range for extension in the central Himalaya (widely regarded as ca. 20–22 Ma). More importantly, they indicate an average displacement rate of ≥47 mm/yr and a consequent tectonic unroofing rate of ≥8.2 mm/yr. Such unroofing is faster than all but the highest estimates of combined physical and chemical erosion rates in mountainous regions, suggesting that large-displacement normal faulting can be an extremely efficient agent of mass redistribution in orogenic systems.

Constraints on India-Eurasia collision in the Arabian Sea region taken from the Indus Group, Ladakh Himalaya, India

Abstract The Indus Group is a Paleogene, syntectonic sequence from the Indus Suture Zone of the Ladakh Himalaya, India. Overlying several pre-collisional tectonic units, it constrains the timing and nature of India’s collision with Eurasia in the western Himalaya. Field and petrographic data now allow Mesozoic-Paleocene deep-water sediments underlying the Indus Group to be assigned to three pre-collisional units: the Jurutze Formation (the forearc basin to the Cretaceous-Paleocene Eurasian active margin), the Khalsi Flysch (a Eurasian forearc sequence recording collapse of the Indian continental margin and ophiolite obduction), and the Lamayuru Group (the Mesozoic passive margin of India). Cobbles of neritic limestone, deep-water radiolarian chert and mafic igneous rocks, derived from the south (i.e. from India), are recognized within the upper Khalsi Flysch and the unconformably overlying fluvial sandstones of the Chogdo Formation, the base of the Indus Group. The Chogdo Formation is the first unit to overlie all three pre-collisional units and constrains the age of India-Eurasia collision to being no younger than latest Ypresian time (>49 Ma), consistent with marine magnetic data suggesting initial collision in the Arabian Sea region at c. 55 Ma. The cutting of equatorial Tethyan circulation north of India at that time may have been a trigger to the major changes in global palaeoceanography seen at the Paleocene-Eocene boundary. New 40Ar/39Ar, apatite fission-track and illite crystallinity data from the Ladakh Batholith and Indus Group show that the batholith, representing the old active margin of Eurasia, experienced rapid Eocene cooling after collision, but was not significantly reheated when the Indus Group basin was inverted during north-directed Miocene thrusting (23–20 Ma). Subsequent erosion has preferentially removed 5–6 km (c. 200°C) over much of the exposed Indus Group, but only c. 2 km from the Ladakh Batholith. Reworking of this material into the Indus fan may complicate efforts to interpret palaeo-erosion patterns from the deep-sea sedimentary record.

Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by 40Ar/39Ar thermochronology

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Episodic unroofing of the Kohistan Batholith, Pakistan: Implications from K‐feldspar thermochronology

New 40Ar/39Ar and U-Pb mineral ages from plutonic rocks help constrain the thermal and tectonic evolution of the Kohistan island arc terrane following its collision with India in early Tertiary time. Kohistan has experienced a prolonged tectonomagmatic history extending from the Early Cretaceous through the Tertiary. Thermal histories derived from multi diffusion domain analyses of K-feldspar within the Kohistan batholith reveal rapid cooling events (70–140°C/m.y.) distributed through space and time. The cooling histories show a systematic variation along the length of the batholith suggesting that Kohistan experienced differential unroofing. An episode of rapid cooling in the middle Eocene is recognized in western Kohistan, whereas rapid cooling occurs substantially later, during the middle and late Miocene, in eastern Kohistan. Rapid cooling in western Kohistan might have been caused by postemplacement cooling of hot magma against cold country rocks at relatively shallow crustal levels. Within the NW region of eastern Kohistan, rapid cooling at 13–12 Ma is recorded in K-feldspars ∼60 km from the contact with the Nanga Parbat-Haramosh Massif (NPHM), whereas cooling does not occur until 11 Ma only 2.5 km from the contact. This temporal and spatial variation in cooling histories may record differential unroofing in response to the development and propagation of the NPHM structure beneath Kohistan. The NPHM has experienced rapid cooling and unroofing over the last 10 m.y., and our results are consistent with this mid–late Miocene event.

Sequential ductile to brittle reactivation of major fault zones along the accretionary margin of Gondwana in Central Argentina

Abstract Metamorphic and plutonic basement rocks and cover sequences of the Eastern Sierras Pampeans, Argentina, have undergone multiple episodes of fault reactivation. Faults take advantage of mid- to late Cambrian, NW-SE-striking, steeply east-dipping foliations in Vendian-aged accretionary prism metasedimentary rocks. Foliations in peraluminous schists, paragneisses and migmatites are deflected into late Cambrian amphibolite-grade high-strain zones. Greenschist-grade mylonite zones and thick retrogressed ultramylonite zones with mainly NNW strikes, easterly dips, and east-over-west movement, affect the metasedimentary rocks and Ordovician-aged intrusive rocks and are presumably related to early Devonian accretion of terranes to the west of Gondwana. pseudotachylyte veins occur in nearly all mylonite zones. Brittle deformation during Carboniferous to Triassic time produced major pull-apart basins located above terrane boundaries. Outcrop patterns of Triassic to Cretaceous sedimentary rocks are consistent with transtensional pull-apart basins followed by Andean transpressional deformation. The theoretical basis for fault reactivation and production of ‘short cuts’ is examined in the context of Tertiary to Recent basin inversion faults. The inversion faults follow the Palaeozoic trends and produce the present-day NNW-oriented, deep sedimentary basins and intervening ranges of basement rocks.