Collaborative Research: Resolving Conflicting Thermobarometry and Stratigraphy in the Tethyan Himalaya: is Non-lithostatic Pressure During Orogenesis Preserved at Crustal Scales?

Amherst College, Amherst MA

Investigators

Abstract

Metamorphic rocks exposed in mountain belts record crustal burial and heating during mountain building. The metamorphic pressures recorded by exhumed rocks are commonly assumed to reflect their burial depth. Such pressures are in turn used to reconstruct the evolution of mountain belts and refine our knowledge of fundamental plate-tectonic processes. However, recent studies suggest that some rocks record pressures that do not reflect their true burial depths (i.e., “non-lithostatic pressure”), thus potentially complicating our understanding of mountain building events. The Tethyan Himalaya in northwestern India is a unique location to test the controversial concept of non-lithostatic pressure because previous structural, sedimentological, and paleontological observations regarding burial depths conflict with recorded metamorphic pressures. The preservation of non-lithostatic pressure in the Tethyan Himalaya will be tested via an integrated field and laboratory study to reveal the geometric, temperature, and pressure relationships and assess the viability of non-lithostatic pressure conditions recorded in the Himalayan orogen. This project serves national interests by: (1) making fundamental advances in science through testing and challenging the “lithostatic pressure” paradigm in the Earth Sciences; (2) training graduate and undergraduate students in STEM; (3) supporting three early-career faculty to establish globally competitive research programs; (4) international collaboration with researchers and institutions; and (5) development of a publicly accessible field-trip guide that will explore the geology of the Himalaya of northwestern India and focus on how scientific interpretations evolve with new observations. Whether pressures recorded by metamorphic rocks accurately reflect burial depths or can represent non-lithostatic pressure conditions—so called tectonic overpressure—is controversial. Numerical models and theory suggest that non-lithostatic pressure conditions are possible, and even likely in several tectonic settings, but direct, unambiguous observations of this phenomenon are rare. The research team has identified a unique study site in the western Himalaya where decades of sedimentological investigations of the Tethyan stratigraphic section are inconsistent with metamorphic pressure estimates from basal strata that imply ~28+ km burial. Existing kinematic models from field studies do not suggest such burial depths of the section, and thus the discrepancy between pressure estimates and palinspastic reconstructions requires more systematic investigation. We hypothesize that metamorphic pressures from the basal Tethyan strata reflect non-lithostatic pressure conditions at the regional scale (10’s of km). This will be tested via a systematic research plan involving geologic mapping, quantitative microstructural analysis, metamorphic thermobarometry and petrochronology, detrital zircon geochronology, and low-temperature thermochronology. Whether or not non-lithostatic pressure is preserved in the geological record, and the length-scales over which it is preserved, has been identified by the geoscience community as an important research goal in the field of tectonics. Empirical support of regional-scale, non-lithostatic pressures would require a paradigm shift in metamorphic petrology and tectonics, with profound implications for understanding the evolution of fault zones, orogens, and plate-scale dynamics. If the research finds that non-lithostatic pressure is not required to explain the geological discrepancies in the western Tethyan Himalaya, this study will still provide important geologic relationships to evaluate and advance models of Himalayan tectonics. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.