Apatite and zircon (U‐Th)/He cooling ages are used to quantify the timing of exhumation associated with thrust faulting in the High Zagros fold‐thrust belt. Single‐grain cooling age data were collected from (1) Cambrian sandstone in various thrust sheets, (2) sandstone and basement clasts derived from structurally controlled salt plugs or fault‐bounded slices, and (3) syntectonic Neogene siliciclastics strata. In the northwestern (Kuhrang) and central (Kuh‐e Lajin) High Zagros, apatite (U‐Th)/He (AHe) ages range from ∼26.7 to ∼0.38 Ma. Most cooling and exhumation occurred in the early to middle Miocene, constrained by AHe ages ∼19–15 Ma from the High Zagros thrust sheet, localized faults, and reset cooling ages from Bakhtiyari deposits. In the southeastern High Zagros (Kuh‐e Dinar), cooling occurred later with AHe ages ranging from ∼16.5 to ∼0.79 Ma. Here most cooling and exhumation occurred in the late Miocene, constrained by AHe ages ∼12–8 Ma from the High Zagros fault, and exhumed Paleozoic clasts in synorogenic strata. Zircon (U‐Th)/He (ZHe) ages from bedrock samples across the High Zagros are reflective of the precollisional thermal history. The preservation of precollisional ZHe ages limits the pre‐Miocene maximum burial temperature of the exhumed strata to < 180°C, and indicate < 7–9 km of maximum exhumation in the central Zagros. This study shows that thrust activity in the High Zagros and continental suturing along the Zagros suture was underway by at least 19 Ma, and initiated no later than latest Oligocene to early Miocene time (∼23 Ma).
Partially exposed bedrock beneath Pleistocene glacial till in Erie County (north-central Ohio) displays unusual structural deformation in the Devonian Berea Sandstone, Bedford Shale, and Ohio Shale. These folded and faulted units are exposed in creeks as anticlines and synclines. Past studies of this area proposed Pleistocene ice movement and soft-sediment deformation during the Late Paleozoic as the deformation mechanisms, but these hypotheses cannot explain the extent of layer displacement or the contradiction between the southwest travel direction of the ice sheet and the structural sense of motion on the folded units. A new interpretation using field data and constructing geologic profiles explains the development of these structures. This study investigated 17 anticlines that trend in different directions. Four of these anticlines are tightly folded with steep or overturned flanks and thrust-faulted Ohio Shale in their cores. Structural analysis of these folds shows that the incompetent shaly units of the Plum Brook–Ohio–Bedford and competent Berea Sandstone were folded above the Delaware–Niagara carbonates as a result of the compressional stress during the Late Paleozoic. Development of these tight or overturned folds, and change in trend of the anticlines, is caused by unusual stratigraphic thickness variations in the Berea and Bedford units. Preserved and undeformed fine sedimentary structures, and sharply faulted beds, in the Berea and Bedford indicate that soft-sediment deformation was not the cause of the regional structural deformation. Finally, the absence of physical features of glacially deformed bedrock demonstrates that Pleistocene glacial ice shove was not the cause of deformed bedrock units in the study area.
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