The Sage Hen Flat Pluton: Emplacement of a Pluton with Minimal Wall Rock Deformation

Ellen Nelson, University of Wisconsin– Madison
Claire Ruggles,
Basil Tikoff, University of Wisconsin– Madison
Sven Morgan, University of Michigan-Dearborn
Thomas Shipley, Temple University
Richard Law, Virginia Tech
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Abstract

The Jurassic Sage Hen Flat pluton intruded greenschist facies Precambrian-Cambrian sedimentary rocks of the White-Inyo Mountains, California, without pronounced deformation of the wallrocks. The lack of wallrock deformation surrounding a pluton is unusual in the White-Inyo Mountains, in which the wallrocks surrounding other plutons are greatly attenuated, fractured, or otherwise deformed. We utilize a variety of data types – field-based observations, microstructural observations, magnetic fabric orientations, and gravity data complemented by forward models – to present a new geometry for the Sage Hen Flat pluton and hypothesize a potential emplacement history. Small wallrock screens are present throughout the granite and wallrock overlies the granite on outward dipping contacts, suggesting that the current exposure level is near the original roof of the intrusion. Thin section observations suggest the pluton developed through multiple pulses of magmatic intrusion and experienced little internal deformation after solidification. Anisotropy of magnetic susceptibility measurements document magnetic fabrics throughout the pluton. Magnetic foliations exhibit a concentric pattern, with steeply dipping foliations near the pluton margins and shallow foliations near the center of the body suggesting magma initially intruded near center and was subsequently pushed outward. The gravity dataset documents a -3 mGal gravity anomaly spatially coincident with the Sage Hen Flat pluton. Forward modelling of this dataset was used to compare four potential geometries for the pluton: 1) a thin sheet; 2) a stock; 3) a sheet emplaced between east-dipping faults; and 4) a stock emplaced between east dipping faults. The predicted gravity for the thin-sheet model best fits the gravity data and geology. Together, these datasets provide evidence for the Sage Hen Flat pluton emplacing in multiple increments from a centralized conduit as a thin sheet, accommodated by faulting and subsequent translation of the wallrock.

Session

Deformation in the upper crust