Tracking the Growth of the Himalayan Fold‐and‐Thrust Belt From Lower Miocene Foreland Basin Strata: Dumri Formation, Western Nepal

Abstract

New data from the lower Miocene Dumri Formation of western Nepal document exhumation of the Himalayan fold‐thrust belt and provenance of the Neogene foreland basin system. We employ U‐Pb zircon, Th‐Pb monazite, 40Ar/39Ar white mica, and zircon fission track chronometers to detrital minerals to constrain provenance, timing, and rate of exhumation of Himalayan source regions. Clusters of Proterozoic–early Paleozoic (900–400 Ma) Th‐Pb monazite and 40Ar/39Ar white mica detrital ages provide evidence for erosion of a Greater Himalayan sequence protolith unaffected by high‐grade Eohimalayan metamorphism. A small population of ~40 Ma cooling ages in detrital white mica grains shows exhumation of low‐grade metamorphic Tethyan Himalayan sequence through the ~350 °C closure temperature along the Tethyan Frontal thrust (proto‐South Tibetan detachment) during the late Eocene. Dumri Formation detritus shows a ~12 Myr time difference between cooling of its source rocks through the ~350 and ~240 °C closure temperatures as recorded by ~40–38 Ma youngest peak cooling ages in 40Ar/39Ar detrital white mica and ~28–24 Ma youngest populations in detrital zircon fission track. Exhumation between circa 40 and 28 Ma is consistent with slip and exhumation along the Main Central Thrust. Combined with similar data from northwestern India, our study suggests west‐to‐east spatially variable exhumation rates along strike of the Main Central Thrust. Our data also show an increase in exhumation during middle Miocene–Pliocene time, which is consistent with growth of the Lesser Himalaya duplex.

The geologic interpretation of the detrital thermochronology record within a stratigraphic framework, with examples from the European Alps, Taiwan and the Himalayas

“…Detrital mica Ar-Ar data from the Siwaliks (White et al, 2002;Szulc et al, 2006;Najman et al, 2009;Stickroth et al, 2019), summarized in the lag-time diagram of Fig. 17d, confirm all of the major provenance changes previously detected by AFT and ZFT data (J, K and L in Fig.…”

“…Because of the fast exhumation characterizing parts of the Himalaya, thermochronologic systems such as mica Ar-Ar, ZFT and AFT, when applied to bedrock typically yield very young ages on the order of few millions of years (e.g., Zeitler et al, 2001;Herman et al, 2010;Gemignani et al, 2018), and do not address earlier geologic events. Many scientists have thus applied a detrital thermochronologic approach to reveal the exhumation history of the Himalaya on longer timescales, based on the thermochronologic analysis of the sedimentary successions either accreted on the southern side of the orogen, or accumulated in the Himalayan foreland basin and in the Indus and Bengal fans, largely fed by Himalayan erosion (e.g., Cerveny et al, 1988;Corrigan and Crowley, 1990;White et al, 2002;Szulc et al, 2006;van der Beek et al, 2006;Zhuang et al, 2015;Clift, 2017;Najman et al, 2019;Stickroth et al, 2019). The Himalayan region is therefore an ideal site not only to illustrate the detrital thermochronologic age trends defined by different thermochronologic systems in a large source-tosink system, but also to compare the detrital thermochronologic record in distal and more proximal sediment sinks.…”

“…Starting from the pioneering work of Cerveny et al (1988), the Neogene successions of the Sub-Himalaya have been characterized by detrital AFT, ZFT and mica Ar-Ar thermochronologic data with progressively more detail (e.g., White et al, 2002;Bernet et al, 2006;Szulc et al, 2006;van der Beek et al, 2006;Najman et al, 2009;Chirouze et al, 2012;Stickroth et al, 2019). Cerveny et al (1988) applied a detrital ZFT approach to the Indus River modern sands and to the Neogene successions deposited by the ancestral Indus River over the past 18 Ma.…”

“…Figure shows a synthesis of detrital thermochronology data sets from sedimentary successions of the Sub-Himalaya (b-to-d;White et al, 2002;Bernet et al, 2006;Szulc et al, 2006;Najman et al, 2009;Chirouze et al, 2012;Stickroth et al, 2019) and the Bengal Fan (e-to-g; Corrigan and Crowley, 1990;Najman et al, 2019)…”

“…Detrital ZFT data from the Siwaliks (Bernet et al, 2006;Chirouze et al, 2012;Stickroth et al, 2019) are summarized in the lag-time diagram of Fig. 17c.…”