GEOPHYSICAL EVIDENCE FOR THE TECTONIC DEVELOPMENT OF THE SWAN RANGE AND ADJACENT BASINS, NORTHWESTERN MONTANA

Reprocessing of five 2D seismic reflection profiles has revealed crustal-scale reflections in the Swan Range and adjacent valleys of northwestern Montana. Modern processing techniques such as pre-stack time and depth migration have removed image distortions that may have led to a past interpretation of large duplex structures of Phanerozoic rocks at the core of the Swan Range. The pre-orogenesis location of Belt metasedimentary rocks comprising the Swan Range is west of the present day Purcell Anticlinorium. Strong reflections— interpreted as tholeiitic sills — outline an eastward thinning wedge of Belt rocks. Stratal dip in the Swan Range is due primarily to inversion of the eastern margin of the Belt Basin atop a relatively flat detachment surface and subordinately to rotation along the South Fork Fault. This geometry requires Belt sedimentation upon a sloped cratonic surface west of the anticlinorium. The Purcell Anticlinorium represents a fault-bend fold over a ramp of autochthonous crystalline basement that projected the detachment into the Belt section before flattening above an east tapering wedge of autochthonous Belt rocks. The westward increase in basement depth is required to fit the entire Belt section between the basement surface and Cambrian sea level datum. Depth to bedrock reflections in the valleys do not agree with previous gravity based estimates largely due to inflated density contrast values used between fill and Belt bedrock. Sub-horizontal reflectivity above transparent Belt rocks continues to a depth of ~3.3 km in the Kalispell Valley — ~1.1 km of which we interpret to be preserved Paleozoic rocks. Gravity models for 3.3 km of purely Cenozoic fill do not appear to explain the residual gravity field in the Kalispell Valley. The absence of reflections dipping or thickening towards the Swan Fault suggest a change in basin style from half graben in the Swan Valley to full or asymmetric graben in the Kalispell Valley. This transition is controlled by at least two major normal faults striking perpendicular to the master Swan fault south of the basin and an additional normal fault on the northern edge. Gravity data supports an antithetic fault that forms the western boundary of a four sided sub-basin of preserved Paleozoic rocks in the Kalispell Valley.