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Thrust Front (thrust + front)

Distribution by Scientific Domains

Earth and Environmental Science	100%

Selected Abstracts

Syntectonic infiltration by meteoric waters along the Sevier thrust front, southwest Montana

GEOFLUIDS (ELECTRONIC), Issue 4 2006
A. C. RYGEL
Abstract Structural, petrographic, and isotopic data for calcite veins and carbonate host-rocks from the Sevier thrust front of SW Montana record syntectonic infiltration by H2O-rich fluids with meteoric oxygen isotope compositions. Multiple generations of calcite veins record protracted fluid flow associated with regional Cretaceous contraction and subsequent Eocene extension. Vein mineralization occurred during single and multiple mineralization events, at times under elevated fluid pressures. Low salinity (Tm = ,0.6°C to +3.6°C, as NaCl equivalent salinities) and low temperature (estimated 50,80°C for Cretaceous veins, 60,80°C for Eocene veins) fluids interacted with wall-rock carbonates at shallow depths (3,4 km in the Cretaceous, 2,3 km in the Eocene) during deformation. Shear and extensional veins of all ages show significant intra- and inter-vein variation in ,18O and ,13C. Carbonate host-rocks have a mean ,18OV-SMOW value of +22.2 ± 3, (1,), and both the Cretaceous veins and Eocene veins have ,18O ranging from values similar to those of the host-rocks to as low as +5 to +6,. The variation in vein ,13CV-PDB of ,1 to approximately +6, is attributed to original stratigraphic variation and C isotope exchange with hydrocarbons. Using the estimated temperature ranges for vein formation, fluid (as H2O) ,18O calculated from Cretaceous vein compositions for the Tendoy and Four Eyes Canyon thrust sheets are ,18.5 to ,12.5,. For the Eocene veins within the Four Eyes Canyon thrust sheet, calculated H2O ,18O values are ,16.3 to ,13.5,. Fluid,rock exchange was localized along fractures and was likely coincident with hydrocarbon migration. Paleotemperature determinations and stable isotope data for veins are consistent with the infiltration of the foreland thrust sheets by meteoric waters, throughout both Sevier orogenesis and subsequent orogenic collapse. The cessation of the Sevier orogeny was coincident with an evolving paleogeographic landscape associated with the retreat of the Western Interior Seaway and the emergence of the thrust front and foreland basin. Meteoric waters penetrated the foreland carbonate thrust sheets of the Sevier orogeny utilizing an evolving mesoscopic fracture network, which was kinematically related to regional thrust structures. The uncertainty in the temperature estimates for the Cretaceous and Eocene vein formation prevents a more detailed assessment of the temporal evolution in meteoric water ,18O related to changing paleogeography. Meteoric water-influenced ,18O values calculated here for Cretaceous to Eocene vein-forming fluids are similar to those previously proposed for surface waters in the Eocene, and those observed for modern-day precipitation, in this part of the Idaho-Montana thrust belt. [source]

Forebulge migration in the Cretaceous Western Interior basin of the central United States

BASIN RESEARCH, Issue 1 2002
T. White
ABSTRACT This study combines stratigraphic evidence with geodynamic modelling to demonstrate that a forebulge played an identifiable role in Cenomanian,Turonian erosion and sediment accumulation in the North American Western Interior basin. The early to middle Turonian forebulge migrated progressively eastwards, and by the upper middle Turonian acted as a ,backstop' against which barrier islands formed in the axial basin. This paper focuses on the progressive migration of an unconformity on the forebulge. The lengthwise orogen-parallel orientation and time-transgressive orogen-normal migration of the forebulge unconformity are characteristics that differentiate it from unconformities developed on reactivated basement structures. We present a conceptual model in which the unconformity formed as the seafloor was uplifted by forebulge-related flexure to a water depth at which submarine bypass and erosion occurred. A numerical model that describes forebulge migration in response to load dispersal by erosion of the orogenic front and sedimentation into the foredeep indicates that the distance from the thrust front to the forebulge is within reasonable bounds established using a flexural rigidity of 3×1024 Nm. We identify architecturally similar, coeval unconformities from Montana to New Mexico, and interpret the similar distance from the thrust front to a point where each unconformity dissipates as indicative of a uniform lithospheric flexural response along the orogenic front. Here we ascribe cratonward (west-to-east) forebulge migration to erosional load redistribution, whereas orogen-parallel (north,south) stratigraphic climb of the forebulge unconformities developed in response to depocentre migration. Inherited lithospheric inhomogeneities may have allowed the forebulge in central Colorado to crest farther from the orogen than to the north and south. [source]