Award Date

5-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Geoscience

Department

Geoscience

First Committee Member

Michael Wells, Chair

Second Committee Member

Terry Spell

Third Committee Member

Thomas Hoish

Fourth Committee Member

Sean Mulcahy

Graduate Faculty Representative

Clemense Heske

Number of Pages

362

Abstract

The Funeral Mountains of Death Valley National Park, CA, provide an opportunity to date metamorphism resulting from crustal shortening and subsequent episodic extensional events in the Sevier hinterland. It was not clear whether crustal shortening and thus peak temperature metamorphism in the hinterland of the Sevier-Laramide orogenic wedge have occurred whether in Late Jurassic, Early Cretaceous , Late Cretaceous or somewhere between. Particularly ambiguous is the timing of crustal shortening in the deep levels of the hinterland of the Sevier belt, now manifest in the metamorphic core complexes, and how and when these middle-to-lower crustal rocks were exhumed. A 6-point garnet and a whole rock Savillax isochron from middle greenschist facies pelitic schist of the southeastern Funeral Mountains core complex yields an age of 162.1 ± 5.8 Ma (2σ). Composite PT paths determined from growth-zoned garnets from the same samples show a nearly isothermal pressure increase of ∼2 kbar at ∼490°C, suggesting thrust burial at 162.1 ± 5.8 Ma. A second sample of Johnnie Formation from the comparatively higher metamorphic grade area to the northwest (East of Chloride Cliff) yielded an age of 172.9 ± 4.9 Ma (2σ) suggesting an increase of thrust burial age towards the higher grade rocks (northwest part of the core complex), consistent with paleo-depth interpretation and metamorphic grade. 40 Ar/39 Ar muscovite ages along footwall of the Boundary Canyon detachment fault and intra-core Chloride Cliff shear zone exhibit significant 40 Ar/39 Ar muscovite age differences. For samples from the immediate footwall of BCD, the pattern of ages decreasing toward the northwest is consistent with differences in depth of metamorphism, and for Late Cretaceous, top-to-northwest exhumation by motion along the precursor BCD; consistent with mesoscopic and microscopic kinematic studies. Samples from the footwall of the structurally-lower Chloride Cliff shear zone yield Tertiary 40 Ar/39 Ar muscovite ages (53 to 29 Ma) and interpreted to reflect a more youthful age of extensional ductile deformation confined along the CCSZ. (U-Th)/He analyses on detrital zircon (ZrHe) from quartzite samples collected along the footwall of the BCD along the same transect as the 40 Ar/39 Ar samples revealed inception of the Miocene BCD at ∼10-11 Ma. A slip rate of 8.5 ±2.0 km/Ma was determined excluding three analyses that significantly deviate from the regression line. This study indicate that the core rocks of the Funeral Mountains were buried during Late Jurassic, and then slowly exhumed, probably by erosion between 152 and 90 Ma, and then more rapidly exhumed initially by movement along the precursor of the BCD during Late Cretaceous. Following a hiatus of tectonic activity, exhumation resumed during late early Tertiary with deformation likely confined along discrete ductile shear zones. The latest period of motion along the BCD and thus the final exhumation of metamorphosed core rocks in the Funeral Mountains has initiated ∼11-10 Ma and likely ceased around ∼6 Ma, consistent with ages of motions of detachment faults and exhumation of footwall rocks in the surrounding mountain ranges.

Keywords

California – Funeral mountains; Cenozoic exhumation; Cenozoic geologic period; Core complex; Earth sciences; Geology, Structural; Mesozoic burial; Mesozoic exhumation; Mesozoic geologic period; Metamorphism; Orogeny; Plate tectonics; United States – Death Valley

Disciplines

Earth Sciences | Geology | Geomorphology | Tectonics and Structure

Language

English


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