“CHIPPING” AWAY AT MAGMA EMPLACEMENT PROCESSES: THE DUCTILE TO BRITTLE TRANSITION IN THE WHITE HORSE PLUTON CONTACT AUREOLE

The ~160 Ma White Horse pluton intruded Paleozoic carbonate sedimentary rocks of the Sevier hinterland in northeastern Nevada. The exposed 16 square km area of pluton has an apparent circular shape with a jagged contact and tight to isoclinal, upright to overturned, cylindrically and disharmonically folded host rocks; some folds wrap 180 degrees around the pluton. Detailed mapping and cross section analysis indicate that folding was emplacement related and followed by stoping. Restored cross sections through the pluton/host rock contact suggest that aureole rocks underwent as much as 48 % shortening during magma emplacement. Restoring 48% shortening around the exposed pluton/host rock contact may only account for ~45% of the area taken up by the pluton within the confines of the exposed aureole. Stoped blocks are conspicuously absent from the pluton. Host rock structures (i.e. bedding, foliation, and fold axes) vary from concordant to discordant to the pluton/host rock contact. The contact may dip 14 to 45 degrees inward, moderately (30 to 40 degrees) outward, or stand almost vertical. Fluctuations in the contact geometry occur over lateral distances of < 1000 m and are constrained within 500 m of topographic relief. Over short distances thin (< 40 m) apophyses (dikes) cross cut host rock structures but then turn abruptly 90 degreees to parallel host rock anisotropy before ending at blunt tips. These small apophyses may represent a snapshot of the magmas last attempt at stoping by “chipping” away at the margins of the magma chamber. The stoping process has created a complex contact geometry that is often parallel to the host rock anisotropy. The transition from ductile folding to brittle truncation of host rock structure may be the result of a strain rate increase caused by fluctuation in magmatic overpressure because host rock creep rates could not keep pace with magma input rates.