KINEMATIC HISTORY OF THE MEADE THRUST BASED ON PROVENANCE OF THE BECHLER CONGLOMERATE AT RED MOUNTAIN, IDAHO, SEVIER THRUST BELT

With up to approximately 45 km of slip, the Meade thrust fault in sout heastern Idaho is one of the major thrusts in the Sevier thrust belt, yet its age of displacement has remained enigmatic because of the lack of a derivative synorogenic deposit. We propose that the synorogenic conglomerate produced by initial Meade thrusting crops out on Red Moun tain, in southeastern Idaho, approximately 1.5 km east of the present trace of the Meade thrust. This conglomerate historically has been con sidered part of the Ephraim Formation of the Lower Cretaceous Gannett Group, but we suggest that it is a conglomeratic facies of the Bechler Formation localized to the area of Red Mountain. Regional stratigraph ic considerations and paleontological dates from underlying and overly ing strata indicate that the Bechler conglomerate facies (BCF) is Apti an in age. The BCF is 850 m thick and consists of pebble- to boulder-c onglomerate, sandstone, and mudrock deposited by fluvial and mass flow processes on medial to distal parts of an alluvial fan. Paleocurrent data indicate an eastward, fan-shaped dispersal pattern. The BCF conta ins clasts of micritic limestone, chert-pebble conglomerate, and chert y litharenite that were derived from the Ephraim Formation. In additio n, the conglomerate contains abundant clasts of Ordovician, Carbonifer ous, and lower Mesozoic rocks that crop out on both the Meade thrust s heet and the Paris thrust sheet approximately 25-30 km to the west. Th e base of the BCF is marked by a local 27-degrees angular unconformity on top of the Ephraim Formation. The BCF contains intraformational, p rogressively rotated angular unconformities, internal growth folds, an d minor dip discontinuities that were produced by simultaneous folding and sediment accumulation on the proximal footwall of the Meade thrus t. Provenance modeling indicates that the BCF was derived from Mesozoi c strata that were exposed along the frontal part of the Meade thrust sheet and from Paleozoic strata that were coevally exposed in the hind ward located Paris thrust sheet. Topography on the Paris sheet was rej uvenated as it was carried passively over a major ramp in the Meade th rust. Provenance and structural data are combined to produce an increm ental, bulk-rock retrodeformation of initial Meade displacement. Clast s of Jurassic Twin Creek Limestone with pressure-solution cleavage in the lower part of the BCF indicate that an episode of layer-parallel s hortening occurred in the Meade hanging wall prior to Meade displaceme nt; this may have been related to emplacement of the Paris thrust shee t. The BCF recorded approximately 6 km of initial Meade-related shorte ning by thrust slip and fault-propagation folding during Aptian time. Illite crystallinity, systematic fracture sets in the Ephraim Formatio n and BCF, finite strain in Meade footwall and hanging wall rocks, and thermal models by previous workers indicate that the Meade thrust she et ultimately overrode the BCF and other footwall rocks, probably duri ng Albian-Cenomanian time. The BCF and the Meade thrust were folded du ring slip on thrust faults related to the northern Crawford thrust dur ing Coniacian time. A revised interpretation of Gannett Group provenan ce throughout the thrust belt suggests that the Ephraim Formation was derived from both the Paris thrust sheet and an older, hindward locate d thrust sheet. Bechler was derived from the Meade and Paris sheets.