Using restored cross sections to evaluate magma emplacement, White Horse Mountains, Eastern Nevada, U.S.A.

Abstract

New field observations and cross section restoration from the Jurassic White Horse pluton–host rock system, Goshute Range, eastern Nevada, USA, indicate a sequential variation of host rock rheology attending magma emplacement. The pluton intruded weakly to nondeformed Devonian–Mississippian limestone, argillite and quartzite at shallow crustal levels (ca. 7 km). The contact aureole is well exposed along the southern, eastern and northern margin of the intrusive body and is less than 1 km wide. Rocks outside of the aureole are sub-horizontal and do not contain a penetrative fabric or are gently folded (interlimb angles > 120°) about sub-vertical axial planes. Within the contact aureole, continuous and discontinuous spaced, axial planar foliations and harmonic to disharmonic, tight to isoclinal folds wrap around the eastern margin of the pluton. Folds verge toward and away from the pluton and rim anticlines, synclines, and monoclines with wavelength in excess of 250 m are preserved along the pluton margin. The spatial proximity of these ductile structures to the pluton and the apparent increase in intensity of structure development approaching the pluton is compatible with contraction within the aureole attending pluton emplacement. However, all of the above structures are truncated by the intrusive contact at various scales. Granodioritic dikes ranging in thickness from 1 m up to ∼ 10 m emanate from the intrusion and cut host rock structure at high angles and turn to propagate towards one another, parallel to the pluton margin and host rock anisotropy. Such features are interpreted to reflect the last stages of diking and brittle deformation that modified the pluton contact after emplacement-related folding of the carbonate rocks, but before final solidification of the pluton. Eight serial geologic cross sections were constructed and evaluated to place geometric constraints on the shape and growth of the White Horse intrusion. Based on line-length restoration of serial cross sections oriented perpendicular to the pluton contact, the aureole was shortened approximately 54% during emplacement. Extrapolating this shortening value over the exposed area of the pluton indicates that approximately 48% of the exposed pluton area may be accounted for by the restoration. Therefore, host rocks must have been displaced out of the map plane and likely downward based on geometrical constraints provided by the cross sections. The preferred emplacement model requires in-situ chamber dilation accommodated by ductile deflection of host rocks followed by fracturing and removal of the aureole rocks via cauldron subsidence and/or stoping.