Research Article| July 01, 1986 Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region John H. Stewart; John H. Stewart 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Thomas H. Anderson; Thomas H. Anderson 2University of Pittsburgh, Pittsburgh, Pennsylvania 15620 Search for other works by this author on: GSW Google Scholar Gordon B. Haxel; Gordon B. Haxel 3U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Leon T. Silver; Leon T. Silver 4California Institute of Technology, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar James E. Wright James E. Wright 5Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar Geology (1986) 14 (7): 567–570. https://doi.org/10.1130/0091-7613(1986)14<567:LTPOTS>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 H. Stewart, Thomas H. Anderson, Gordon B. Haxel, Leon T. Silver, James E. Wright; Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region. Geology 1986;; 14 (7): 567–570. doi: https://doi.org/10.1130/0091-7613(1986)14<567:LTPOTS>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 The Upper Triassic Chinle Formation of the Colorado Plateau contains voluminous volcanic detritus evidently derived from a source to the south. Volcanic rocks exposed in southern Arizona and northern Sonora have been assumed to represent this source terrane, but U-Pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the Chinle, and thus not a source for the volcanic detritus in the Chinle. Igneous rocks of known or possible Late Triassic age in Nevada, California, or northeastern Mexico are possible sources, but a clearly defined source terrane for the volcanic detritus in the Chinle has not been identified. Tectonic removal of the source terrane by rifting or strike-slip offset, though not proven, is a possibility. 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.
New thermochronologic and microstructural data from the Quitobaquito Hills provide insight into conditions of deformation in the middle crust of southwestern Arizona during Late Cretaceous to early Paleogene crustal shortening associated with low angle subduction. The mylonitic Quitobaquito shear zone juxtaposes Paleoproterozoic (~1.6 Ga) gneiss and schist structurally above Jurassic (175–170 Ma) meta-volcanic and meta-volcaniclastic rocks. The shear zone dips ~50° SSE and exhibits top-to-the-north, reverse-sense kinematics based on mineral (quartz) stretching lineations in outcrop and σ-type and domino-type fragmented porphyroclasts, mica fish, and S-C fabrics in thin section. The main shear zone is ~30 m thick and underlain by at least one footwall imbricate in the Jurassic sequence. Argon-argon and zircon (U-Th)/He thermochronologic data from the footwall and hanging wall of the shear zone reveal distinct time-temperature paths from approximately 70 to 55 Ma, interpreted to represent the timing of displacement on the shear zone. From ~55 Ma to present, both blocks of the shear zone experienced similar exhumation histories, including a period of moderate cooling during Eocene time. Quartz and feldspar recrystallization fabrics and deformation mechanisms bracket deformation temperature between 450–600°C, consistent with thermochronologic data and greenschist- to lower-amphibolite-facies mineral assemblages in the Jurassic metamorphic rocks. Local metamorphic foliations are parallel to the Quitobaquito shear zone and metamorphism of footwall rocks is interpreted as synkinematic with the shear zone. The timing and kinematics of the Quitobaquito shear zone indicate that it is an expression of Laramide contractional tectonism in southwestern Arizona. We did not find structural or thermochronological evidence to suggest that the shear zone is a reactivated strand of the postulated Jurassic Mojave-Sonora megashear.
Stable isotopes combined with pre-existing 40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.
At the request of the U.S. Bureau of Land Management, approximately 34,172 acres of the Orocopia Mountains Wilderness Study Area (CDCA-344) were evaluated for mineral resources (known) and mineral resource potential (undiscovered).In this report, the area studied is referred to as the "wilderness study area" or simply "the study area".Any reference to the Orocopia Mountains Wilderness Study Area refers only to that part of the wilderness study area for which a mineral survey was requested by the U.S. Bureau of Land Management.The Orocopia Mountains Wilderness Study Area is located in southeastern Riverside County, California, about 25 miles southeast of Indio.Fieldwork for this report was conducted between 1982 and 1986.In 1982, there were about 20 unpatented mining claims within or adjacent to the wilderness study area.Identified resources in the Orocopia Mountains Wilderness Study Area include low-grade talc deposits of moderate size and a small, low-grade, low-tonnage quartz-vein gold deposit.Active exploration for gold, including geochemical sampling and exploratory drilling, was underway in the late 1980's, and a tract with high resource potential for disseminated gold in crystalline rocks is delineated in the central part of the Orocopia Mountains Wilderness Study Area.One small area in the southeast corner of the wilderness study area has moderate resource potential for gold in quartz veins associated with propylitically altered mafic dikes cut by fault zones.Other small tracts within the wilderness study area have low resource potential for gold in quartz veins, moderate resource potential for gold in placer
The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.
The northern Aquarius Mountains volcanic field ([approximately]50km east of Kingman) covers an area of 400 km[sup 2], bounded by upper Trout Creek (S), the Truxton Valley (N), the Big Sandy Valley (W), and Cross Mountain (E). The volcanic sequence rests upon a pre-middle Eocene erosional surface. The lowest units is a 250 m-thick unit of rhyolitic pyroclastic breccias and airfall tuffs. Successively younger units are: basanite flows and cinder cones; hornblende latite flows and domes; porphyritic dacite flows, domes, and breccias; alkali basalt intrusions; and low-silica rhyolite domes and small high=silica rhyolite flows. Dacite is volumetrically dominant, and erupted primarily from vents in and around Cedar Basin (Penitentiary Mtn 7.5[prime] quad.). Other geologists have obtained K-Ar dates [approximately]24--20 Ma for the basanites and latites. The alkali basalts, latites, dacites, and rhyolites evidently constitute a genetically-related high-K to shoshonitic calcalkaline suite with chemistry typical of subduction-related magmatism: enrichment in LILE and LREE, and depletion of Nb and Ta relative to K and La and of Ti relative to Hf and Yb. Each rock type is unique and distinguishable in K/Rb and Rb/Sr. The basanites are primitive (mg=0.75--0.78), have intraplate affinities (La/Nb[<=]1), and show consistent and distinctive depletion of K relative to more » the other LILE. The presence of these basanites in an early Miocene volcanic sequence is unusual or unexpected, as they predate (by [approximately]10 m.y.) the regional eruption of asthenosphere-derived basalts associated with Basin-and-Range extension. « less
