Investigation of granitic batholiths based on seismic and gravity data: The Boulder batholith of Montana and the Coast Plutonic Complex
University of Wyoming, Laramie
ix, 193 p., Illus.
Seismic reflection profiling combined with gravity data allow a more exact determination of the geometry of the controversial Boulder batholith of Montana, reveal a laminated structure in the lower crust beneath the batholith, and identify the Moho at a depth of 38 km. The batholith has inward-dipping contacts, the dip being 40-50$\sp\circ$ on the west side, on the basis of seismic data, and the depth to the batholith floor is constrained between 11-18 km, indicating a great volume for the batholith. The Boulder batholith was emplaced between 80 and 70 Ma during an eastward thrusting in the fold and thrust belt. Seismic reflection profiles determined the position of the Lombard thrust 4 km east of the margin of the batholith at a depth of 6.5 km, dipping at 20$\sp\circ$ to the north-west. A presumed basal decollement of the thrust system might coincide with the batholith floor and may correspond to the top of the lower crustal layering at a depth of 18 km. Low-velocity zones found inside the Boulder batholith by the tomographic processing of refraction arrivals from large-offset seismic experiments were interpreted as zones of hydrothermal alteration associated with porphyry copper deposits. Seismic refraction tomography along a profile crossing the Coast Plutonic Complex of southern Alaska revealed its large-scale internal velocity structure to a depth of 15-20 km, and confirmed the Coast Shear zone as a main crustal boundary. In the area of the Coast Plutonic Complex, the P-wave velocity model suggests massive underplating and magmatic differentiation resulting in formation of granitoid plutons and complexes of high-temperature metamorphic rocks. Granitic rocks apparently prevail in the uppermost 5-10 km of the crust, and may extend even deeper in two zones of lower seismic velocities.
Minerals Data and Information Rescue in Alaska (MDIRA)