Laser ablation-multi collector-inductively coupled mass spectrometry U-Pb geochronology, detailed field mapping and stratigraphic data offer improved insights into the timing and style of Laramide deformation and basin development in the Little Hatchet Mountains, southwestern New Mexico, USA, a key locality in the 'southern Laramide province.' The Laramide synorogenic section in the northern Little Hatchet Mountains comprises upper Campanian to Maastrichtian strata consisting of the Ringbone and Skunk Ranch formations, with a preserved maximum thickness of >2400 m, and the correlative Hidalgo Formation with a total thickness >1700 m. The Ringbone Formation and superjacent Skunk Ranch Formation are each generally composed of (1) a basal conglomerate member; (2) a middle member consisting of lacustrine shale, limestone, sandstone, and interbedded ash-fall tuffs; and (3) an upper sandstone and conglomerate member. Basaltic andesite flows are intercalated with the upper member of the Ringbone Formation and the middle member of the Skunk Ranch Formation. The Hidalgo Formation, which crops out in the northern part of the range, is dominantly composed of basaltic andesite breccias and flows equivalent to those of the Ringbone and Skunk Ranch formations. The Laramide section was deposited in an intermontane basin partitioned across intrabasinal thrust structures, which controlled growth-stratal development. U-Pb zircon ages from five tuffs indicate that the age range of the Laramide sedimentary succession is ca. 75-70 Ma. U-Pb detrital-zircon age data (n = 356 analyses) from the Ringbone Formation and a Lower Cretaceous unit indicate sediment contribution from uplifted Lower and Upper Cretaceous rocks adjacent to the basin and the contemporary Tarahumara magmatic arc in nearby northern Sonora, Mexico. The new ages, combined with published data, indicate that uplift, basin development, and magmatism in the region proceeded diachronously northeastwards as the subducting Farallon slab flattened under northern Mexico and southern New Mexico from Campanian to Palaeogene time.
The Cenozoic structural geology of Asia comprises a continental-scale region of interacting strike-slip, thrust, and extensional faults (Figure 1a and 1b; e.g., Tapponnier & Molnar, 1977; Taylor & Yin, 2009). Cenozoic faulting is largely dictated by the India-Asia continental collision, the gravitational spreading of the Tibetan Plateau, and subduction along the eastern margin of Eurasia (e.g., A. Yin, 2010). In the circum-Ordos region of North China, more than 2,000 km from the nearest active plate boundary, Cenozoic deformation has produced intracontinental rifts that define the periphery of the Ordos block (Figure 1c). Along the eastern margin of the Ordos block is the ∼1,000 km long late Miocene-Quaternary Shanxi Rift characterized by NE-SW-striking basins and uplifts linked by ∼ N-S-striking accommodation zones (e.g., X. Xu & Ma, 1992) (Figure 2). Rifting is attributed to NW-SE extension associated with the propagation of left-lateral strike-slip faults emanating from the northern Tibetan Plateau (e.g., Peltzer et al., 1985; Tapponnier & Molnar, 1977), and the timing and mode of extension appears similar to other late Miocene and younger rift systems in the southern Himalayan-Tibetan orogen and the interior of Asia (e.g., Baikal Rift; A. Yin, 2000) (Figure 1a). The left-stepping en-echelon and sigmoid-shaped geometry of the Shanxi Rift suggests a transtensional origin, which is broadly attributed to the counterclockwise rotation of the Ordos block relative to the adjacent Alxa block and North China Plain (e.g.,
The Middle–Late Jurassic to earliest Cretaceous fold belts of the Yanshanian orogen in North China remain enigmatic with respect to their coeval deformation histories and possible relationship to the contemporaneous Cordilleran-style margin of eastern Asia. We present geological mapping, structural data, and a >400-km-long, strike-perpendicular balanced cross section for the Taihang-Luliangshan fold belt exposed in the late Cenozoic central Shanxi Rift. The northeast-southwest–trending Taihang-Luliangshan fold belt consists of long-wavelength folds (∼35–110 km) with ∼1–9 km of structural relief cored by Archean and Paleoproterozoic metamorphic and igneous basement rocks. The fold belt accommodated ≥11 km of northwest-southeast shortening between the Taihangshan fault, bounding the North China Plain, in the east and the Ordos Basin in the west. Geological mapping in the Xizhoushan, a northeast-southwest–oriented range within the larger Taihangshan mountain belt, reveals two major basement-cored folds: (1) the Xizhou syncline, with an axial trace that extends for ∼100 km and is characterized by a steep to overturned forelimb consistent with a southeast sense of vergence, and (2) the Hutuo River anticline, which exposes Archean–Paleoproterozoic rocks in its core that are unconformably overlain by shallowly dipping (<∼20°) Lower Paleozoic rocks. In the Luliangshan, Mesozoic structures include the Luliang anticline, the largest recognized anticline in the region, the Ningjing syncline, which preserves a complete section of Paleozoic to Upper Jurassic strata, and the Wuzhai anticline; together, these folds are characterized by a wavelength of ∼45–50 km. Shortening in the Taihang-Luliangshan fold belt is estimated to have occurred between ca. 160 Ma and 135 Ma, based on the age of the youngest deformed Upper Jurassic rocks in the Ningjing syncline, previously published low-temperature thermochronology, and regional correlations to better-studied Yanshanian fold belts. The timing of basement-involved deformation in the Taihang-Luliangshan fold belt, which formed >1000 km from the nearest plate margin, corresponds with the termination of arc magmatism along the eastern margin of Asia, implying a potential linkage to the kinematics of the westward-subducting Izanagi (paleo-Pacific) plate.
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