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Tectonic forcing of stratigraphic architecture is likely in foreland basins. Tectonic driving forces are increasingly being invoked to explain stratigraphic patterns in the Cretaceous Western Interior Seaway Basin of North America, yet the evidence is largely circumstantial, and the details of driving forces remain elusive. In this paper I show direct stratigraphic evidence for syndepositional growth of a structural arch with at least 50 m of relief during accumulation of the upper Turonian Ferron Sandstone in south-central Utah, United States. Progressive growth of the arch was superimposed on several high-frequency stratal cycles that were driven by a more regionally extensive process (geodynamic or eustatic) and that produced laterally amalgamated sandstone bodies in a depositional strike-parallel orientation (north-south). All of this stratigraphy was then truncated by a more or less planar erosion surface (sequence boundary) that can be traced physically over at least 67 km north-south. This surface was later tilted northward, such that the upper member of the Ferron Sandstone thins progressively southward from 50 to 10 m over 67 km. The facies juxtapositions revealed by the Ferron Sandstone could, if seen in exposure of limited lateral extent, be wrongly interpreted as recording regionally extensive relative sea-level drops and potentially used in error as evidence for substantial eustatic sea-level falls during the Turonian. The folding and tilting documented in this study can be clearly attributed to geodynamic and/or tectonic driving forces, likely related to migration of a forebulge.
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During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ 18 O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ 18 O magmatic fluids caused δ 18 O depletions in the gneisses within the migmatite zone. These low δ 18 O fluids were derived from the young (<4 Ma), relatively low δ 18 O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40 Ar/ 39 Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.
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