New geologic mapping, structural, and geochronologic analyses of the pre-Carboniferous ultramafic, mafic, and metasedimentary rocks exposed on the southwest margin of the Precordillera terrane, western Argentina, show that the ultramafic and mafic rocks belong to at least four distinct tectonic units, not one ophiolite pseudostratigraphy, as previously thought. One unit comprises gabbro, microgabbro, diabase, and minor plagiogranite that are interpreted as the mafic crustal section of an ophiolite pseudostratigraphy. Another unit consists of serpentinized peridotite, ultramafic cumulate, layered gabbro, and quartzofeldspathic gneiss that experienced granulite facies metamorphism in a deep continental crust environment. Other mafic rocks include highly altered basaltic flows interlayered with low-grade clastic metasedimentary rocks and diabase and microgabbro sills that intrude the metasedimentary rocks.The pre-Carboniferous rock units were juxtaposed along synmetamorphic, ductile, top-to-the-east shear zones in the Early to Middle Devonian. The intense deformational fabrics formed during the juxtaposition event were overprinted by strong Permian folding and top-to-the-west brittle thrust faulting, Late Permian to middle Mesozoic extension, and Late Tertiary west-vergent folding and top-to-the-west brittle thrust faulting. A speculative tectonic scenario for the juxtaposition of the pre-Carboniferous in the Early to Middle Devonian places the deep continental crust at the base of the upper plate (eastern Chilenia) of a west-dipping subduction zone, the mafic ophiolite crustal section in an ocean basin between the western Precordillera terrane and eastern Chilenia, and the mafic sills and flows on the extended western margin of the Precordillera terrane. Early to Middle Devonian closure of the ocean basin resulted in juxtaposition of the pre-Carboniferous rock units.

Natural fractures form preferred pathways for basinal fl uid fl ow and associated heat and mass transport. In gas sandstone reservoirs with low matrix permeability, fractures provide fl ow pathways between organic-rich source and reservoir layers during gas charge, and between matrix pores, hydraulic fractures, and the well bore during production. While the formation of natural fractures has previously been associated with gas generation and pore-fl uid pressure increase through a process referred to as natural hydraulic fracturing, other driving mechanisms such as stress changes by tectonic or exhumation processes remained viable alternatives. To test whether these mechanisms contributed to fracture development, we investigated the spatial and temporal distribution of fracture formation and its relationship to gas generation, migration, and charge in sandstone of the Cretaceous Mesaverde Group across the entire production interval on a basinwide scale. Using fl uid inclusion microthermometry of crack-seal fracture cement formed concurrently with fracture opening, we observed temperature trends that, when compared with temperature evolution models of the formation, date fracture formation between 41 and 6 Ma in the northern and between 39 and 6 Ma in the southern Piceance Basin. The onset of fracture formation 20-30 m.y. prior to maximum burial eliminates changes in stress state asso ciated with exhumation as a mechanism for triggering the onset of fracture formation. Instead, calculated paleo-pore-fl uid pressures of 40-90 MPa (5800-13,000 psi) during fracture opening and the presence of methane-rich inclusions in fracture cement suggest that fracture formation was aided by high pore-fl uid pres-For permission to copy, contact

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