Research Article| August 01, 1985 Hydrothermal systems and Tertiary low-angle normal faulting in the southwestern United States John M. Bartley; John M. Bartley 1Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27514 Search for other works by this author on: GSW Google Scholar Allen F. Glazner Allen F. Glazner 1Department of Geology, University of North Carolina, Chapel Hill, North Carolina 27514 Search for other works by this author on: GSW Google Scholar Geology (1985) 13 (8): 562–564. https://doi.org/10.1130/0091-7613(1985)13<562:HSATLN>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John M. Bartley, Allen F. Glazner; Hydrothermal systems and Tertiary low-angle normal faulting in the southwestern United States. Geology 1985;; 13 (8): 562–564. doi: https://doi.org/10.1130/0091-7613(1985)13<562:HSATLN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Many Tertiary normal faults in the southwestern United States dip gently (<30°). Some of these normal faults apparently were initiated with gentle dips, in conflict with the predictions of elementary dynamic analysis. Close correspondence between low-angle normal faulting and intense hydrothermal alteration in the Mojave Desert, western Arizona, and the Rio Grande rift is consistent with the hypothesis that hydrothermal systems play an important role in the genesis of these faults. We present a mechanical model to demonstrate that low-angle normal faults could form in response to stress trajectories that become reoriented in a periodically sealed geothermal system beneath a sloping surface. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The Mojave block of southern California has undergone significant late Cenozoic north-south contraction. This previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate-tectonic estimates of Pacific-North American plate motion, and for part of the Big Bend in the San Andreas fault. In the eastern Mojave block, contraction is superimposed on early Miocene crustal extension. In the western Mojave block, contractional folds and reverse faults have been mistaken for extensional structures. The three-dimensional complexity of the contractional structures may mean that rigid-block tectonic models of the region based primarily on paleomagnetic data are unreliable.
This field guide was created in coordination with the Geological Society of America Field Forum “Rethinking the Assembly and Evolution of Plutons: Field Tests and Perspectives,” held 7-14 October 2005 in the Sierra Nevada and White and Inyo ranges, California. The goal of this five-day field trip was to examine field relations and characteristics of plutons in the central Sierra Nevada and in the White and Inyo ranges as they relate to processes of pluton growth and emplacement and, more particularly, as they relate to the hypothesis that plutons are assembled slowly and incrementally.
Directly north of Ofotfjorden in northern Norway, pelitic schists within the Evenes and Bogen Groups contain the mineral assemblage garnet + biotite ± kyanite ± staurolite + white mica + quartz ± plagio clase. This assemblage implies metamorphic P-T minima of-540C and -4.8 kb. The rocks are thus at a higher grade than suggested by previous reports, which placed them in the greenschist facies. This indicates that several metamorphic allochthons in Ofoten, including rocks of the Narvik, Evenes, Bogen, and Niingen Groups, are all at kyanite grade, supporting recent interpretations which on structural grounds concluded that the metamorphic peak outlasted stacking of these allochthons. A proposed correlation of the Evenes Group with the Middle Ordovician-Lower Silurian Balsfjord Supergroup implies that this stacking and associated kyanite-grade metamorphism are post-early Silurian and are related to the Scandian phase of the Caledonian orogeny.
Magmatic and hydrothermal systems are intimately linked, significantly overlapping through time but persisting in different parts of a system. New preliminary U-Pb and trace element petrochronology from zircon and titanite demonstrate the protracted and episodic record of magmatic and hydrothermal processes in the Alta stock–Little Cottonwood stock plutonic and volcanic system. This system spans the upper ~11.5 km of the crust and includes a large composite pluton (e.g., Little Cottonwood stock), dike-like conduit (e.g., Alta stock), and surficial volcanic edifices (East Traverse and Park City volcanic units). A temperature–time path for the system was constructed using U-Pb and tetravalent cation thermometry to establish a record of >10 Myr of pluton emplacement, magma transport, volcanic eruption, and coeval hydrothermal circulation. Zircons from the Alta and Little Cottonwood stocks recorded a single population of apparent temperatures of ~625 ± 35 °C, while titanite apparent temperatures formed two distinct populations interpreted as magmatic (~725 ± 50 °C) and hydrothermal (~575 ± 50 °C). The spatial and temporal variations required episodic magma input, which overlapped in time with hydrothermal fluid flow in the structurally higher portions of the system. The hydrothermal system was itself episodic and migrated within the margin of the Alta stock and its aureole through time, and eventually focused at the contact of the Alta stock. First-order estimates of magma flux in this system suggest that the volcanic flux was 2–5× higher than the intrusive magma accumulation rate throughout its lifespan, consistent with intrusive volcanic systems around the world.
Dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear. Each class of model predicts a different incremental strain history that should be recorded in the footwall. In the Tauern Window (eastern Alps), postmylonitic structures in the footwall of the Brenner Line normal shear zone predominantly consist of closely spaced, steep, west down and east down microfaults. Formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear. In contrast, formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes. The field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed. However, both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion‐lined microfaults with the macroscopic west down and east down faults, obtaining density data for the inclusions, and correlating these data with previously obtained geochronologic data. The results indicate that the west down structures formed at depths of 10–20 km and temperatures >450°C in the mid to late Oligocene and that the east down structures formed at 2‐ to 10‐km depth and temperatures of 300 ± 50°C in the mid‐Miocene. These data support the hypothesis that a "rolling hinge" was present in the footwall of the Brenner Line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear. The depths at which west down and east down faulting occurred, coupled with the angle of dip of the Brenner Line, yield a minimum lateral displacement on the fault of 15–26 km. Approximately coeval ductile shearing and brittle faulting at depths of 15–20 km and temperatures in excess of 400°C may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation.
The Grant and Quinn Canyon ranges lie in a part of the eastern Great Basin commonly designated as the hinterland of the Mesozoic Sevier thrust belt. Published mapping of the northern and central Grant Range indicates that virtually all of the low-angle faults in that area are Cenozoic normal faults. However, our recent mapping in the southern Grant and northern Quinn Canyon ranges confirms the presence of Mesozoic thrust faults that have been overprinted by normal faulting.
The tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens. The classic central Cordilleran foreland fold‐and‐thrust belt in the United States (Late Jurassic to early Tertiary Sevier belt) and the more internal zones to the west (central Nevada thrust belt) provide data critical to understanding the development of internal and external parts of orogens. The Garden Valley thrust system, part of the central Nevada thrust belt, crops out in south‐central Nevada within a region generally considered to be the hinterland of the Jurassic to Eocene Sevier thrust belt. The thrust system consists of at least four principal thrust plates composed of strata as young as Pennsylvanian in age that are unconformably overlain by rocks as old as Oligocene, suggesting that contraction occurred between those times. New U/Pb dates on intrusions that postdate contraction, combined with new paleomagnetic data showing significant tilting of one area prior to intrusion, suggest that regionally these thrusts were active before ∼85–100 Ma. The thrust faults are characterized by long, relatively steeply dipping ramps and associated folds that are broad and open to close, upright and overturned. Although now fragmented by Cenozoic crustal extension, individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike. We correlate the structurally lowest thrust of the Garden Valley thrust system, the Golden Gate‐Mount Irish thrust, southward with the Gass Peak thrust of southern Nevada. This correlation carries the following regional implications. At least some of the slip across Jurassic to mid‐Cretaceous foreland thrusts in southern Nevada continues northward along the central Nevada thrust belt rather than northeastward into Utah. This continuation is consistent with age relations, which indicate that thrusts in the type Sevier belt in central Utah are synchronous with or younger than the youngest thrusts in southern Nevada. This in turn implies that geometrically similar Sevier belt thrusts in Utah must die out southward before they reach Nevada, that slip along the southern Nevada thrusts is partitioned between central Nevada and Utah thrusts, or that the Utah thrusts persist into southeastern Nevada but are located east of the longitude of the central Nevada thrust belt. As a result of overall cratonward migration of thrusting, the central Nevada thrust belt probably formed the Cordilleran foreland fold‐thrust belt early in the shortening event but later lay in the hinterland of the Sevier fold‐thrust belt of Idaho‐Wyoming‐Utah.
