The residual Bouguer anomaly map of the central and northwestern Missoula Valley demonstrates that the configuration of the bedrock in this region is complex. Dense gravity measurements in conjunction with inverse modeling resources allowed for the resolution of subsurface bedrock features less than 300 m². GPS provided sub-meter elevation control making the field data highly accurate.

To isolate the residual Bouguer field from the total field measurements, I evaluated the regional field using gravity anomaly measurements on bedrock extending well beyond the valley edges. I determined the density contrast between the bedrock and valley fill using measurements on representative samples and calculations on the Bouguer density values that minimize correlation between the gravity anomaly and topography.

Interpretations of gravity models indicate that extension of the Missoula Valley is due to normal displacement along two offset segments of the Ninemile Fault. Models show that a minor amount of slip also occurred along a fault that runs parallel to the northern segment of the Ninemile Fault. The bedrock on the southwestern flank of the Missoula Valley slopes gently over about four kilometers across the valley. It dips steeply from the center of the valley into the Ninemile Fault to the northeast.

A fault oriented perpendicular to the strike of the valley connects the northern segment of the Ninemile Fault to the segment that is offset to the southwest. This fault appears to continue southwest to the center of the valley. The gravity models also show a small, deep graben near the junction of the northern segment of the Ninemile Fault and the perpendicular fault. I attribute this graben to pull-apart extension caused by right-lateral movement on the offset segments of the Ninemile Fault.