Geologic reconstructions support non-lithostatic pressure in the geologic record

Andrew Zuza, University of Nevada Reno
Dominik Vlaha, Peking University
Victor Guevara, Amherst College
Peter Haproff, Pomona College
Alex Webb, Freie Universität Berlin
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Abstract

The assumption of lithostatic pressure during crustal metamorphism underpins many reconstructions of global orogens, allowing metamorphic pressures to be converted directly to burial depth. This paradigm is supported by rock-deformation experiments and seismic stress-drop observations that show crustal rocks are too weak to sustain non-lithostatic pressures exceeding a few tens of MPa. However, direct tests of whether metamorphic pressures honor lithostatic conditions are rare because there are few well-constrained crustal sections with reliable palinspastic reconstructions of burial. Theoretical work and emerging observations suggest that dynamic pressure can significantly exceed lithostatic values.

Here, we summarize two case studies—the western US and NW Himalaya—where robust geologic reconstructions of upper-crust deformation imply preservation of non-lithostatic pressure. For both, we leverage well-defined Neoproterozoic–Paleozoic passive-margin stratigraphy and field relationships mapped over decades to reconstruct burial depths to compare against multi-method barometry. First, in eastern Nevada, pressures estimated from the Neoproterozoic–Cambrian rocks flanking exhumed metamorphic core complexes are 6–8+ kbar (22–29 km burial depths, assuming 2.8 g/cm3). However, reconstructions from various groups suggest these rocks were buried no deeper than 12–15 km. In places, one can walk from a ~7 kbar sample through ~8 km of stratigraphic section to the paleo-erosional surface. A Carlin-type gold deposit in this section requires shallow depths (<5 km), which further rules out wholesale burial of the section. Peak-temperature estimates define a warm but continuous geotherm inconsistent with deep burial.

Second, we investigated the Paleogene Tethyan Himalaya fold-thrust belt in Himachal Pradesh, northwestern India, the structurally highest part of the Himalayan orogen. Here a ~10 km thick Tethyan passive margin section was modestly shortened via south-directed thrusts. Basal strata consistently yield elevated pressure-temperature estimates of 7–8 kbar and ~650°C. New analyses reproduced these pressures and petrochronology constrains metamorphism during Cenozoic Himalayan orogeny. Field mapping and palinspastic reconstructions limit burial to 10–15 km depth. We mapped continuous stratigraphy from the metamorphic samples up to the Cretaceous paleo-surface over ~10 km vertical distance. Peak-temperature estimates define a warm, continuous geotherm that is inconsistent with deep burial and does not imply major structural discontinuities.

In both cases, robust field-based reconstructions support preservation of non-lithostatic pressure, with overpressure values doubling lithostatic expectations. We discuss potential causal mechanisms for such overpressure, but regardless of the exact driver, preservation of non-lithostatic pressure may require reassessment of orogenic reconstructions. At minimum, field-based reconstructions provide a critical framework against which analytical datasets such as barometric estimates can be evaluated.

Session

Deformation in the upper crust