LONGITUDINAL DISPLACEMENT TRANSFER SYSTEM LINKING BASIN AND RANGE FAULTS AND VOLCANIC RIFT ZONES ACROSS THE NORTHWESTERN BOUNDARY OF THE EASTERN SNAKE RIVER PLAIN, IDAHO

The northeast-trending transition between three active Basin and Range faults in east-central Idaho and the Eastern Snake River Plain (ESRP) is abrupt. It is a zone where the system of NNW-trending range-bounding faults to the north is juxtaposed to the low topographic expression of the volcanic providence to the south. The NNW-trending normal faults have trace lengths approaching 150 km and throws ranging between 5.5 to 2.5 km. The northern and central segments are seismically active, but the activity is reduced as the ESRP is approached toward the south. In the ESRP, only modest seismicity is recorded and deformation is associated with NNW-trending volcanic rift zones that result from basalt dike intrusion since the northeasterly passage of the Yellowstone Hotspot between 6 to 4 Ma. The age distribution of basalt volcanism along the southern termination of Basin and Range faults is inconsistent with active accommodation of normal fault slip into the ESRP either by brittle displacement or by along-strike dike emplacement. Geologic and geophysical relations in the Little Lost River basin, adjacent to the southern segment of the Lemhi fault suggest basin segmentation by ENE-trending transfer faults that control the polarity of the fault bound basin. The transfer fault intersects the Lemhi fault near the location of a 3.5 km wide left step where the range-front fault experiences a step that corresponds to a decrease in range height of 900 m and a major reduction of scarp elevations to the south. As the ESRP is approached, the half-graben bound fault is transferred from the eastern to western sides of the basin and the transfer fault served as a boundary that prevented basalt from reaching the basin segment farther north. The local development of the Lemhi transfer fault system may serve as a small analog for a transtensional basin system marking the boundary between Basin and Range structures and the ESRP. The age of basalt flows together with localized development of thick sedimentary accumulation along the ENE-trending boundary zone suggest that normal fault slip on Basin and Range faults was transferred to localized basaltic volcanic rift zones whose periods of activity migrated southward along the northwestern margin of the ESRP over the last several million years.