Geology of the South Pass area, Fremont County, Wyoming

Bayley, Richard W.; Proctor, Paul Dean; Condie, Kent C.

doi:10.3133/pp793

Cited by 19 publications

(18 citation statements)

References 11 publications

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“…Laboratory data have been presented on the physical properties of mylonites analogous to the rocks at depth in the Pacific Creek or Wind River fault zones. The Precambrian basement of the Wind River range is very similar in composition [Bayley et al, 1973] as well as velocity and density [Smithson and Ebens, 1971] to the samples from the Columbia River fault zone, particularly the granodiorites.…”

Section: Modeling Of Fault Zone Reflectionsmentioning

confidence: 95%

The nature of seismic reflections from deep crustal fault zones

Jones¹,

Nur²

1984

J. Geophys. Res.

175

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Deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement. Perhaps the best example is the Consortium for Continental Reflection Profiling (COCORP) Wind River line in Wyoming, which shows the Wind River thrust fault to be a strong reflector from the surface to depths of about 30 km. To identify the physical properties responsible for the seismic reflections from fault zones, we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones. These rocks are characterized by extensive ductile deformation of plastic minerals, brittle deformation of the more rigid minerals, grain size reduction, and development of a strong fabric. Average velocity and density were not found to change significantly and systematically between the mylonites and the ddjacent undeformed rock. Seismic anisotropy of 7% or greater is present in several mylonites dependent upon their composition and fabric. Using our data on mylonite properties and models for the crustal structure near the Wind River line, we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the COCORP Wind River reflection data. When anisotropy and the finely laminated structure of the fault zone are considered, our modeling indicates fairly strong reflections from mylonite zones in most cases. We also show that elevated pore pressure in shear zones may produce strong reflections. However, a permeability of the order of 10 -16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive. COLUMBIA ",. •. RIVER I• N, CLACHNACUDAINN SALIENT INTRUSION CR5 CR7 CR3 ß ß CRt SITE MYLONITIC , t km • METASEDIMENT ,•, APPROXIMATE DUCTILE SHEAR ZONE BOUNDARY --APPROXIMATE CONTACT I I 118 ø 15' 118 ø 10' Geology of the Columbia River fault zone, southeastern British Columbia. The large dots represent sample locations. quartz, biotite, and chlorite have been reduced to a very fine grained matrix which appears to flow around large, broken fragments of feldspars. Biotite and chlorite form a strong foliation, and elongate quartz aggregates form a consistent lineation within the foliation plane. Deformation has been predominately ductile for the quartz and micas and brittle for the feldspars. The mylonitic metasediment (Figure 4a) is shown under plane polarized light which emphasizes the texture due to the pleochroic layered silicates. These rocks lack the large feldspar porphyroclasts present in the mylonitic granodiorite, but fine-grained feldspars are present. The strong fabric has been produced by ductile deformation of the quartz and phyllosilicates which have been reduced to very fine grained layers. The feldspars deformed primarily by fracturing and rigid body rotation. The cataclasite ( Figure 4b) also has high cohesion and low porosity. However, in thin section the deformation is seen to be predominately brittle, and there has been no developm...

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Section: Modeling Of Fault Zone Reflectionsmentioning

confidence: 95%

The nature of seismic reflections from deep crustal fault zones

Jones¹,

Nur²

1984

J. Geophys. Res.

175

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“…The sequence may be correlative with or compositionally analogous to the greenstone-graywacke terrane exposed in the southeastern Wind River Mountains. Here, Bayley, Proctor, and Condie (1973) have described a thick section of graywacke, greenstone, and iron-formation.…”

Section: Discussionmentioning

confidence: 99%

“…In spite of these uncertainties, the temperature and depth Gilettiand Cast (1961), Goldichand others (1966), Hills and others (1968), Naylor, Steiger, and Wasserburg (1970), Bayley, Proctor, and Condie (1973), Hills and Armstrong (1974), and Divis (1976), and M. A. Lanphere (written commun., 1977).…”

Section: Mineral Agesmentioning

confidence: 99%

Reconnaissance geology and geochronology of the Precambrian of the Granite Mountains, Wyoming

Peterman¹,

Hildreth²

1978

Professional Paper

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The Precambrian of the western part of the Granite Mountains, Wyoming, contains a metamorphic complex of gneisses, schists, and amphibolites that were derived through amphibolite-grade metamorphism from a sedimentary-volcanic sequence perhaps similar to that exposed in the southeastern Wind River Mountains. Whole-rock Rb-Sr dating places the time of metamorphism at 2,860±80 million years. A high initial s7Sr/ s6Sr ratio of 0.7048 suggests that either the protoliths or the source terrane of the sedimentary component is several hundred million years older than the time of metamorphism. Following an interval of 300±100 million years for which the geologic record is lacking or still undeciphered, the metamorphic complex was intruded by a batholith and satellite bodies of medium-to coarse-grained, generally massive biotite granite and related pegmatite and aplite. The main body of granite is dated at 2,55Q±60 million years by the Rb-Sr method. Limited data suggest that diabase dikes were emplaced and nephrite veins were formed only shortly after intrusion of the granite. Emplacement of the granite at about 2,550 million years ago appears to be related to a major period of regional granitic plutonism in the Precambrian of southern and western Wyoming. Granites, in the strict sense, that are dated between 2,450 and 2,600 million years occur in the Teton Range, the Sierra Madre, the Medicine Bow Mountains and the Laramie Range. This episode of granitic plutonism occured some 50 to 100 million years later than the major tonalitic to granitic plutonism in the Superior province of northern Minnesota and adjacent Ontario the nearest exposed Precambrian W terrane that is analogous to the Wyoming province. Initial 87Sr / 86Sr ratios of some of the Wyoming granites are higher than expected if the rocks had been derived from juvenile magmas and it is likely that older crustal rocks were involved to some degree in the generation of these granites. Slightly to highly disturbed Rb-Sr and K-Ar mineral ages are obtained on rocks of the metamorphic complex and on the granite. These ages range from about 2,400 to 1,420 million years and are part of a regional pattern of lowered mineral ages of Precambrian W rocks of southern Wyoming. A major discontinuity in these mineral ages occurs along a line extending from the northern Laramie Range, through the northern part of the Granite Mountains, to the southeastern Wind River Mountains. North of this line, Rb-Sr and K-Ar biotite ages are 2,300 million years or greater, whereas to the south, the biotite ages decrease drastically over a short distance, to a common range of 1,600-1,400 million years. We suggest that these lowered ages represent regional cooling below the 300° C isotherm as a consequence of uplift and erosion of the large crustal block occurring south of the age discontinuity. In this interpretation, the westerly-trending age discontinuity would be a zone of major crustal dislocation that resulted from vertical tectonics in late Precambrian X or early Precambrian Y time.

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“…The characteristic and unique lithologies of the Sweetwater mining district to the north (figs. 2, 4) (Bayley, 1973) dominate the clast composition of this lobe, and therefore that area is presumed to be the source.…”

Section: Placer Goldmentioning

confidence: 99%