A GRAVITY INVESTIGATION OF THE TOBACCO ROOT BATHOLITH IN SOUTHWEST MONTANA

The Tobacco Root batholith, a satellite pluton of the larger Boulder batholith, is the easternmost of the late Cretaceous plutons of the North American Cordillera. It was emplaced into Archean metamorphic basement rocks during the Laramide orogeny (ca. 75 Ma).

A gravity survey was done to delineate the subsurface shape of the batholith and ultimately better understand its mode of emplacement. 232 gravity stations were made across the batholith and across the host rocks with 24 samples collected for density determination of the batholith and host rocks. Gravity station coordinates were obtained using differential GPS techniques, and the data were reduced using a standard crustal density of 2.67 g/cm³. The regional gravity was subtracted from a larger regional Bouguer anomaly map to produce a residual gravity map showing a negative anomaly that has a NW-SE trending, semi-elliptical pattern. Four NE-SW profiles across the gravity map indicate that the negative anomaly is greater and batholith is thicker (-51 mGal, 30 km) in the northwestern portion of the batholith than the southeast portion (-5 mGal, 3 km), which suggests a source to the northwest. A negative anomaly of -18 mGal is evident on the NE-SW profile located outside of the batholith outcrops on the northwest.

Each profile was modeled with 2D techniques to determine the subsurface shape of the batholith. Density contrasts used in modeling account for variation in composition from both the batholith and host rocks. The northwestern model outside of the outcrops of the batholith, strongly suggest that the batholith continues below the host rock outcrops, dipping towards the northwest parallel to foliation in the host rocks and it may connect with the Boulder batholith at depth. The models also imply the emplacement of the batholith was controlled buy coeval movement along large NW-trending faults.