SPEAKING WITH FORKED (GLACIER) TONGUE IN THE SAWTOOTH RANGE AND STANLEY BASIN OF CENTRAL IDAHO: WERE THE GLACIERS TELLING US SOMETHING ABOUT FAULT HISTORY?

Unique and consistent patterns of split glacier lobes mantling the central portion of the Sawtooth fault suggest interaction between glaciation and basin structure governed by fault history. The Sawtooth normal fault separates the Sawtooth Range from its adjoining basin, with Holocene scarps marking 60 km of the range front. Fault scarp patterns and basin characteristics suggest three distinct portions of the fault—a more active central portion with 5-8 m scarps and northern and southern portions with 1-2 m scarps. Mountain glacier lobes extended up to 20 km from the range crest into the adjoining basin.

Moraines of multiple glaciers that flowed into the central basin display a unique forked pattern indicating that each glacier split into separate sub-lobes. Generally, this occurred within 2 km of their termini. As moraines of multiple ages display this pattern, it appears to have been a persistent behavior through at least the last and penultimate glaciations. No bedrock outliers have been mapped at the surface in the main parts of the basin. In the southern portion of the fault system, glaciers extended several km into the basin and constructed nested, single moraine lobes, indicating that they remained as single lobes.

Why did the glacier lobes in the central portion split into sub-lobes? The simplest interpretation is that the glaciers were split by low bedrock hills that may still protrude slightly from the basin, consistent with gravity patterns that suggest a deep, sediment-filled southern basin and a shallower central basin with variable structure. This would imply that the currently greater fault activity in the central portion is a relatively recent development. In the southern portion, the consistent, relatively low gravity signatures suggest a deep basin that has experienced longer-term faulting, with the scarp patterns and lacustrine disturbance history suggesting lesser recent activity. Such a pattern would be consistent with other major range-bounding faults north of the Snake River Plain.