GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 207-3
Presentation Time: 9:00 AM-6:30 PM


LUTZ, Brandon, PHILLIPS, Fred and AXEN, Gary, Department of Earth & Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801

How strain is partitioned across rheological layers of the lithosphere remains one of the most perplexing issues in intra-continental dynamics. Continuum numerical modeling approaches do not account for the inherent discontinuous nature of brittle faults, rely on scaling laboratory-derived flow laws, and require “weak seeds” to localize shear zones. Contrarily, discrete block or plate models assume internal rigidity and sharp, through-going boundaries that transect the entire crust or lithosphere. This seems at odds with shallow (<10-15 km) locking depth of faults in post-orogenic regions. Furthermore, a growing body of evidence from continental metamorphic core complexes suggests that crustal shear zones widen with depth into diffuse regions of distributed strain.

Here we present a 3D palinspastic reconstruction of intra-continental extension and transtension for the Death Valley region. The reconstruction utilizes a comprehensive dataset of offset features, Cenozoic basin architectures/timings, and detachment footwall thermal histories. Upper crustal fault block translations were reconstructed in map view. Crustal-scale cross sections that parallel tectonic transport were then balanced and restored (in Move). The balanced cross sections were further validated using a forward thermo-kinematic modeling simulation (Fetkin). Model results provide snapshots of crustal architecture and thermal state during extension and thinning. Paleo-crustal thickness is estimated using strain calculations and volume conservation. Lithospheric thermal states are estimated from melting column calculations using basalt geochemical data.

This integrated approach to geological modeling allows inferences to be made about the rheological evolution of the extending lithosphere. The typical problem of unconstrained time-dependent relationships among flow stress, strain rate, and temperature within ductile shear zones is somewhat mitigated. Structural components of detachment faults/ductile shear zones (e.g. fractures, faults cataclasites, mylonites) can be placed into paleo-rheological context. Their attitudes and positions are reconstructed in temperature-time space. Therefore, the mechanical behavior of a crustal-scale fault system can be modeled.