Research Article| December 01, 1988 Proterozoic polydeformation in basement rocks of the Needle Mountains, Colorado RICHARD G. GIBSON; RICHARD G. GIBSON 1Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Search for other works by this author on: GSW Google Scholar CAROL SIMPSON CAROL SIMPSON 2Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD G. GIBSON 1Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 CAROL SIMPSON 2Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (12): 1957–1970. https://doi.org/10.1130/0016-7606(1988)100<1957:PPIBRO>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation RICHARD G. GIBSON, CAROL SIMPSON; Proterozoic polydeformation in basement rocks of the Needle Mountains, Colorado. GSA Bulletin 1988;; 100 (12): 1957–1970. doi: https://doi.org/10.1130/0016-7606(1988)100<1957:PPIBRO>2.3.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 SocietyGSA Bulletin Search Advanced Search Abstract Proterozoic rocks in the Needle Mountains include ∼1,755-m.y. old amphibolite-grade gneisses and ∼1,690-m.y.-old granites that comprise basement to the siliciclastic Uncompahgre Group. The mafic and felsic gneisses, which contain primary structures indicative of volcanic and plutonic protoliths, under-went two phases of isoclinal folding and foliation development prior to emplacement of the ∼1,690-m.y.-old plutons. Synkinematic garnet porphyroblast textures suggest that these fabrics developed during a progressive deformation event, DB. Subsequent DBC deformation caused folding of DBfabrics in the gneisses; development of a subvertical, east-striking foliation in the granites; and generation of a macroscopic sigmoidal foliation pattern throughout the area. DBC structures in the basement are correlated with structures in the Uncompahgre Group and formed prior to pluton emplacement at ca. 1440 Ma. Gently east-plunging mineral lineations in the granites and quartz c-axis fabrics of both the gneisses and granites indicate subhorizontal extension on steeply dipping foliation surfaces during DBC. Asymmetric c-axis fabrics and rare mesoscopic shear sense indicators in the gneisses record a component of dextral shear in domains of east-striking foliation and sinistral shear in areas of northeast-striking foliation. In contrast, symmetric c-axis fabrics in the granites suggest nearly coaxial extension. A model for DBC involving the development of conjugate strike-slip shear zones in response to north-northwest shortening is most consistent with the kinematic and fabric-orientation data. Partitioning of deformation into subparallel zones of noncoaxial and coaxial deformation was controlled by the presence of a strong pre-existing anisotropy in the gneiss complex. The shortening direction during DBC is consistent with that related to north-northwest-directed thrusting in rocks of similar age in central Arizona and northern New Mexico. Therefore, DBC structures are tentatively interpreted to have accommodated transverse shortening and orogen-parallel extension within a foreland during regional crustal shortening between ca. 1690 and 1400 Ma. 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.
Previous work in hydraulic fracture performance identifies rock quality and completion quality as key drivers of good production; however, quantifying rock quality in a systematic method along the wellbore is a difficult task. Novel methodologies are developed for calculating rock-quality measurements along the wellbore using seismic data calibrated to geologic information encompassing petrophysical and geomechanical parameters and merging these metrics with stage-level engineering observations. Far-field wellbore seismic attributes correlate with stage completion performance and are promising predictors for improved well design. Further, these integrated attributes may contribute to the fundamental understanding of the hydraulic fracturing process and to the development of more robust and powerful computation models of overall well performance.
Analysis of metamorphic textures and mineral chemistry in Wedowee Group metapelites near the Blakes Ferry pluton reveals a regional metamorphic P-T path dominated by pressure fluctuations attributable to thrusting. Country rocks contain the post-S/sub 2/ equilibrium assemblage Mu+Bt+Chl+Gar+Ep+Pc+Qz+/-l 1+/-Ti (M/sub 2/); St inclusions in Gar and graphite inclusions in Bt oblique to S/sub 2/ record the presence of an earlier Bt+St+Gar assemblage (M/sub 1/). Published experimental data, Mu-Bt-Gar-Pc thermobarometry, and aluminosilicate reaction textures found elsewhere in the Wedowee Gp constrain P-T conditions to 610+/-40/sup 0/C, 4+/-1.5kb for M/sub 1/ and 580+/-40/sup 0/C, 9.5+/-1 kb for M/sub 2/. Post-S/sub 2//M/sub 2/ emplacement of the Blakes Ferry pluton caused garnet resorption and growth of radiating Chl clots within 50m of the pluton. Ca- and Fe/Mg-enriched Gar rims and more calcic Pc in the aureole than country rocks indicate a reversal of reaction (2) during contact metamorphism. Thermobarometry suggests conditions of 560+/-40/sup 0/C at 5.5+/-1kb. Shear bands and weak crenulations (S/sub 3/) post-date the contact metamorphism. These data define a P-T path initially characterized by isothermal compression over a range of 5kb which is interpreted to record emplacement of a approx. 15km thick Piedmont Allochthon onto the Wedowee Gp. Subsequently decreasing P reflectsmore » tectonic uplift and partial unroofing prior to pluton emplacement. This scenario implies progressive SE to NW nappe stacking during Paleozoic crustal thickening and provides independent verification for large-scale thrust tectonics recently suggested on the basis of structural analysis in the southernmost Appalachians.« less
Magma mixing in silicic volcanic rocks of the Inyo chain is manifest by bimodal mineral chemistry and distinctive banding, defined by either variations in (1) glass color and chemistry or (2) microlite abundance within colorless glass. Bands composed of brown or microlite‐rich, colorless glass are characterized by more Mg‐rich mafic silicates and more calcic plagioclase than microlite‐poor, colorless glass domains. A rhyolitic and a dacitic mixing end‐member can be defined on the basis of this mineral distribution. Thermobarometry using the mineral assemblages unique to each end‐member indicates that prior to mixing, the dacite was more oxidized and water‐rich than the rhyolitic magma. Determined mineral crystallization conditions compare favorably with experimentally derived phase equilibria for melts of broadly similar composition. The spatial association of contrasting mineral assemblages with the flow banding in these rocks implies that the banding is a direct product of magma mixing. It formed as either (1) volumes of the comingled magmas were immediately vented to the surface and quenched, preserving domains of two distinct glass compositions, or (2) bands of dacitic magma precipitated microlites when comingled with the rhyolitic magma in an attempt for the two magmas to approach equilibrium.
In the Columbus Basin, offshore Trinidad, evaluating the controls on fault seal is a prerequisite for understanding how the petroleum fields were charged. In this paper, we present a case study from Mahogany field, where interbedded Pliocene–Pleistocene shales and reservoir sands occur in a broad four-way-closed anticline cut by numerous normal faults. Fault seals in this stratigraphic sequence can be successfully evaluated using shale gouge ratio (SGR), with a transition between sealing and nonsealing faults occurring in the SGR = 0.15–0.25 range. Because of the high net-to-gross ratio of individual sands, low SGR values typically correspond to areas of reservoir self-juxtaposition, whereas good seals (SGR 0.2) exist where different sands are juxtaposed against one another. The larger structural geometry, which changes significantly from the shallow reservoirs to the deeper ones, closely controls the distribution of stacked, fault-sealed petroleum accumulations in this field. Petroleum column heights in individual fault blocks within the structure are limited either by (1) a cross-fault spill point at a low-SGR window on the west side of a fault block or (2) a synclinal spill point within a fault block from which petroleum leaves the overall four-way closure. The pattern of hydrocarbon-water contacts in the field suggests that petroleum filled and spilled its way from northeast to southwest across the structure with individual sands acting as a separate flow systems. Despite juxtaposition against each other, communication between stratigraphically different sands is minimal. Vertical migration of petroleum along faults is not required to explain the distribution of charged sands, and this is consistent with both petrophysical data and the known sealing character of the faults. This petroleum migration model serves as a tool for evaluating charge risk and column heights in untested fault blocks in the area.
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