Zircon (U‐Th)/He Thermochronometric Constraints on Himalayan Thrust Belt Exhumation, Bedrock Weathering, and Cenozoic Seawater Chemistry

Abstract

Shifts in global seawater 187Os/188Os and 87Sr/86Sr are often utilized as proxies to track global weathering processes responsible for CO2 fluctuations in Earth history, particularly climatic cooling during the Cenozoic. It has been proposed, however, that these isotopic records instead reflect the weathering of chemically distinctive Himalayan lithologies exposed at the surface. We present new zircon (U‐Th)/He thermochronometric and detrital zircon U‐Pb geochronologic evidence from the Himalaya of northwest India to explore these contrasting interpretations concerning the driving mechanisms responsible for these seawater records. Our data demonstrate in‐sequence southward thrust propagation with rapid exhumation of Lesser Himalayan strata enriched in labile 187Os and relatively less in radiogenic 87Sr at ∼16 Ma, which directly corresponds with coeval shifts in seawater 187Os/188Os and 87Sr/86Sr. Results presented here provide substantial evidence that the onset of exhumation of 187Os‐enriched Lesser Himalayan strata could have significantly impacted the marine 187Os/188Os record at 16 Ma. These results support the hypothesis that regional weathering of isotopically unique source rocks can drive seawater records independently from shifts in global‐scale weathering rates, hindering the utility of these records as reliable proxies to track global weathering processes and climate in deep geologic time.

Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya

“…Studies of sandstone petrography and conglomerate clast composition, along with Nd isotope geochemistry of conglomerate clasts from the Kangra reentrant (Figure ), suggest that the inner‐Lesser Himalayan Sequence (iLHS) was already eroding at ~11 Ma (Brozovic & Burbank, ; Meigs et al, ; Najman et al, ). This inferred age for the onset of erosional unroofing of the iLHS agrees well with Nd isotopic results from the central Himalaya in Nepal (Huyghe et al, , ; Robinson et al, ) and with zircon (U‐Th)/He thermochronological data from outer‐Lesser Himalayan Sequence (oLHS) and iLHS rocks of the northwestern Indian Himalaya (Colleps et al, ). However, based on detrital zircon U‐Pb geochronological and Hf isotopic data from Late Paleocene to mid‐Miocene Sub‐Himalayan foreland basin deposits in the Subathu salient (Figure ), Ravikant et al () proposed sediment input from the iLHS as early as Late Paleocene/Early Eocene.…”

“…However, based on detrital zircon U‐Pb geochronological and Hf isotopic data from Late Paleocene to mid‐Miocene Sub‐Himalayan foreland basin deposits in the Subathu salient (Figure ), Ravikant et al () proposed sediment input from the iLHS as early as Late Paleocene/Early Eocene. This age seems at odds with kinematic reconstructions of the Himalaya, which suggest that exhumation of the iLHS initiated with the growth of the LHD in the Late Miocene (e.g., DeCelles et al, , and references therein; Srivastava & Mitra, ; Webb, ; Webb et al, ), which is also suggested by Lesser Himalayan bedrock cooling age data (e.g., Colleps et al, ).…”

“…Although the timing of LHD formation is critical for shortening estimates across the Himalaya and for understanding the overall evolution of this mountain belt, so far, it has remained rather poorly documented. This may in part be related to the dominant clastic and carbonate lithologies of the Lesser Himalayan unit, which are less suitable for low‐temperature thermochronology (e.g., Bollinger et al, ; Colleps et al, ).…”

Footwall Rotation in a Regional Detachment Fault System: Evidence for Horizontal‐Axis Rotational Flow in the Miocene Searchlight Pluton, NV

“…Because our ages do not display such correlation (supporting information Figure S1), we expect that our samples were not affected by considerable radiation damage. Thus, the obtained average ZHe ages represent the time at which the samples cooled through an effective closure temperature range of ~180 to 200 °C (e.g., Colleps et al, ; Guenthner et al, ; Reiners et al, , ; Wolfe & Stockli, ). Furthermore, the lack of age‐eU correlations in all samples suggests that the samples moved rapidly through the partial retention zone (PRZ).…”

“…In this work, we use zircon (U‐Th)/He thermochronology (ZHe) to understand the thermal history of the Searchlight pluton and relate this history to possible mechanisms for its exhumation. We chose ZHe because of the moderate closure temperature of the system (180–200 °C to as low as ~130 °C; Reiners et al, , ; Wolfe & Stockli, ; Guenthner et al, ; Colleps et al, ). Specifically, we aimed to constrain the cooling history of this pluton—intruded at approximately 750 °C—as it was exhumed rapidly toward the surface.…”