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Vejmelek, L.Y., 1996

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Vejmelek, L.Y., 1996, Investigation of granitic batholiths based on seismic and gravity data: The Boulder batholith of Montana and the Coast Plutonic Complex: University of Wyoming, Laramie, Ph.D. dissertation, ix, 193 p., illust.


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 degrees on the west side, on the basis of seismic data, and the depth to the batholith floor is constrained between 11 and 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 degrees to the northwest. 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.

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