SW U. S. diabase province: A 1. 1-Ga intrusion event of middle Grenville and middle Keweenawan age
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Diabase in the southwestern US intrudes Middle Proterozoic stratified rocks as sills and Early and Middle Proterozoic crystalline rocks as subhorizontal sheets and subvertical dikes. It is discontinuous in a broad belt extending from western Texas to southeastern California. The best known intrusions are sills in Middle Proterozoic strata in Death Valley, Grand Canyon, and central Arizona. Sparse to rare dikes in some of these strata trend mostly north but range from north-northeast to west-northwest. Diabase dikes widespread in crystalline rocks in western Arizona and adjacent parts of southeastern California strike from north to west-northwest, but are predominantly northwesterly. Dikes and sheets are also present in crystalline rocks in the southern Pinaleno Mountains, southeastern Arizona, where dikes strike west-northwest. The northwest trend of the diabase province and prevalent northwesterly trend of dikes in crystalline rocks suggest that intrusion was controlled by an approximately horizontal least compressive stress field roughly parallel to the Grenville Front. Radiometric ages of Arizona and California diabase indicate emplacement at [approximately]1,100 Ma. Paleomagnetic poles from diabase sills and enclosing stratified rocks in Arizona correlate with poles reported from middle and early-late Keweenawan rocks of Lake Superior. Emplacement of the diabase coincides with: (1) the middle Keweenawanmore » eruptive and intrusive episode of the Midcontinent Rift System; (2) a major episode of (middle) Grenville thrusting and deformation documented in the Van Horn area; and (3) a time of abrupt reversal in North American apparent polar wander. These interrelated manifestations presumably arose in response to a major episode of plate interaction and collision between North American and a plate that encroached from the southeast.« lessKeywords:
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Paleomagnetic data have been obtained from Miocene intrusions, Proterozoic Paleomagnetic data have been obtained from Miocene intrusions, Proterozoic crystalline rocks and cross-cutting mafic to felsic dikes to evaluate footwall deformation during extension and unroofing of the crystalline core of the Black Mountains, Death Valley, California. Synrift intrusions contain a well-defined and, at the site level, well-grouped magnetization, interpreted to be of dual polarity, whose in situ direction is discordant in declination and inclination with an expected late Cenozoic reference direction. In situ site mean directions of this magnetization are directed towards the west and west-northwest with moderate to shallow positive and negative inclinations. The variation in magnetization direction, particularly inclination, with site locality around the turtleback structures along the western flank of the Black Mountains suggests folding of the intrusion after remanence acquisition. Two populations of in situ site means are identified: one with southwest declination and negative inclination, the other with northward declination and positive inclination. A preferred interpretation for footwall deformation involves, from oldest to youngest: (1) northeast-side up tilting of 20--40[degree] and local folding of the crystalline rocks associated with early structures (the Death Valley turtlebacks) between 11.6 and 8.7 Ma, (2) progressive east to west footwallmore » unroofing between 8.7 and [approximately]6.5 Ma, and (3) large-scale clockwise rotation (50--80[degree]) after the core detached from stable terrane to the west. The authors interpret late rotation as oroflexure related to right-lateral shear along the Death Valley fault zone.« less
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Buried basement rocks of the central United States are mainly plutonic granitic rock, mafic and felsic metamorphic rock, and diabase. Rhyolite, granitic rock, and gabbro-diabase form a discontinuous belt from eastern New Mexico to eastern Missouri. The 0.5 b.y. rocks in the Wichita Mountains, Oklahoma, are the result of the last major igneous event. The Ouachita structural belt is the southern limit of petrographic knowledge of plutonic rocks. None of these rock groups bears a simple relationship to basement topography and isotopic age. The Black Hills uplift-Cambridge arch-Central Kansas uplift is underlain dominantly by metamorphic rock with ages ranging from 1.7 b.y. in the north to 1.2 b.y. in the south. Siouxia arch contains 1.4 b.y. granitic and metamorphic rock. Nemaha uplift is underlain by 1.2-1.5 b.y. granite. Diverse rock types of 1.2-1.4 b.y. underlie Amarillo uplift and Red River-Matador arch. Several large gravity anomalies correspond to major basement structures. The Williston basin is bounded on the south and west by a series of major west- and northwest-trending gravity anomalies and on the east by a belt of south-trending gravity anomalies extending from Canada into the central Dakotas that coincides with the boundary between the 2.5 b.y. Superior and the 1.7 b.y. Churchill Provinces of the Canadian Shield. The Sioux formation (minimum age 1.2 b.y.) lies along the west-trending anomaly in southeastern and central South Dakota. Keweenawan basaltic and sedimentary rock coincides with the mid-continent gravity anomaly and extends nearly continuously from Lake Superior to northeastern Kansas. The prominent gravity feature along the Red River-Matador arch coincides with the boundary between 1.2-1.4 b.y. rocks in the central United States and the 1.0 b.y. rocks in Texas. End_of_Article - Last_Page 539------------
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The Gogebic Iron Range extends for some 100 km from near Clam Lake, northwestern Wisconsin to Lake Gogebic, westernmost northern Michigan. It consists of an allochthonous package of Early Proterozoic (EP) strata sandwiched between Archean basement rocks on the south and unconformably overlying Middle Proterozoic (MP) Keweenawan strata on the north. Volcanic rocks occur at either end of the Gogebic range, but the central part of the range consists mainly of monoclinically north-dipping sedimentary strata. Initial phases of Penokean deformation detached Early Proterozoic strata from underlying Archean basement and created northverging, bedding-parallel shears in the monoclinal sequence. Exposures of mylonitized rocks at several places along the length of the district demonstrate the allochthonous nature of EP strata. The main zones of detachment are the shaly portions of the Sunday Quartzite and Palms Formation. Bedding and the shear zones were next deformed into northeast and northwest-plunging folds, most prominently at the east and west ends of the range respectively. Subsequent deformation involved faulting of both the Archean and EP strata. During formation and closure of the MP Midcontinent Rift, the Pre-Keweenawan strata were tilted steeply northward along listric thrusts (Cannon et al, 1993), resulting in a northwestern-oriented, strike-parallel, cross-sectional view ofmore » the Pre-Keweenan crust.« less
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Foliated and unfoliated rhomb-porphyry dikes, genetically related to the andesitic Marron Formation, cut some westernmost exposures of the Shuswap Complex in the southern Okanagan Valley of British Columbia. Emplacement of these dikes was associated with northerly trending fractures and flexural-slip folding, localized high heat flow, re-setting of country rock K–Ar systems and hydrothermal alteration. All these events are set at 42–48 m.y. B.P. based on K–Ar measurements on dikes and country rocks.Rhomb-porphyry dikes are part of an andesitic volcanic province in south-central British Columbia that coincides in time with ductile deformation within the Cascade Fold Belt to the west and latest thrust faulting in the Rocky Mountains to the east. The volcanic province is associated in space with thinning of the crust, whereas thickening of the crust is associated with ductile shortening to the west. It is likely that the volcanic province marks the position of the ductile–brittle transition zone in the crust during late Eocene time.
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The north end of the Pioneer Mountains is underlain by the following rocks: lower Proterozoic gneiss and amphibolite; middle proterozoic clastic rocks of the Missoula Group (Mount Shields Formation?); basal Cambrian clastic unit; Cambrian-Devonian-Carboniferous shelf sequence; Permian Phosphoria Formation; and Lower Triassic Dinwoody Formation. Jurassic rocks are missing. The Lower Cretaceous Kootenai Formation is of lagoonal to fluviatile facies, overlain by a thick (^approx 2 km, 1 mi) section of fluviatile Colorado Group. An upper member of the Colorado Group has yielded a Campanian-to-Maestrichtian pollen flora. The sedimentary rocks are cut by calc-alkalic plutonic rocks (80 to 65 m.y.B.P.), the oldest of which are synchronous with upper Colorado sedimentation. The y ungest pre-Quaternary rocks are Eocene and Oligocene calc-alkalic lavas and Oligocene pumiceous tuff. The Missoula Group is entirely in thrust sheets that postdate the Colorado, so the thrusting is no older than Campanian, but the thrusts are cut by 72 to 74 m.y.B.P. plutons. The Johnson thrust of Fraser and Waldrop, on the 1972 U.S. Geological Survey Geologic Quadrangle Map 988, is part of this thrust system. A klippe of Missoula Group on Morrison Hill is an erosional remnant, but most of the overthrust rocks are west of the Wise River valley and probably overlie Phanerozoic strata. In addition to the thrust sheets, two families of high-angle faults dominate. One family trends west-northwest and displacements along these faults can be measured in kilometers. The Johnson thrust is interpreted to have been displaced by a sinistral fault of this system along the straight valley of Big Hole River between Seymour Creek and Dewey. The eastward projection of the mountain front at Maiden Rock, just south of Divide, resulted from block displacement along two strands of this fault. This particular fault is of major significance because it marks an abrupt jump in the initial strontium ratios of intrusive rocks, from values typical of the Boulder batholith (0.706 to 0.709) to those of the Pioneer batholith (0.711 to 0.716), indicating that different crustal blocks were juxta osed. The second family of high-angle faults trends north-northeast. Wise River valley is interpreted to be a graben in this system. One important fault in the system is the Fourth of July fault, with downdrop (to west) of several kilometers; the fault may be continuous with the Comet Mountain fault. The west-northwest fault system began at least before the Eocene lava flows, but complex field relations between the two high-angle fault systems indicate their growth must have overlapped in age, possibly through the late Tertiary. End_of_Article - Last_Page 1362------------
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This study provides geological background information necessary for an evaluation of the earthquake hazard in an area of induced seismic activity at Monticello Reservoir, South Carolina. This region contains a thick stratified sequence of Proterozoic Z and Cambrian metasedimentary and metavolcanic rocks. In the early to middle Paleozoic, this sequence was recrystallized and deformed under metamorphic conditions that ranged from greenschist to amphibolite facies and experienced at least two episodes of folding. The region has been intruded by late kinematic to postkinematic granitoid plutons of Silurian and Carboniferous ages and by numerous northwest trending diabase dikes of Late Triassic and Early Jurassic age. The region south of Monticello Reservoir in the Carolina slate belt experienced two episodes of faulting in the late Paleozoic and/or early to middle Mesozoic. The older group of faults trends approximately east, has only small displacements, and is characterized by extensive silicification of the fault zones. The younger group of faults trends approximately north, has experienced dip slip displacements up to 1700 m, and is characterized by carbonate mineralization in the fault zones. Both sets of faults are cut by an undeformed diabase dike of Late Triassic or Early Jurassic age. The induced seismic activity around Monticello Reservoir is occurring in a heterogeneous quartz monzonite pluton of Carboniferous age. Although laterally extensive faults have not been found in the vicinity of the reservoir, the pluton contains large enclaves of country rock and is cut by numerous, diversely oriented small faults and joints. These local inhomogeneities in the pluton, together with an irregular stress field, are interpreted to control the diffuse seismic activity around the reservoir. In view of the apparent absence of lengthy faults, it is unlikely that a large‐magnitude earthquake will occur in response to the stress and pore pressure changes related to the impoundment of Monticello Reservoir.
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Research Article| June 01, 1970 Pre-Cenozoic Tectonic History of the Salinian Block, Western California ROBERT A WIEBE ROBERT A WIEBE Department of Geology, Franklin and Marshall College, Lancaster, Pennsylvania 17604 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1970) 81 (6): 1837–1842. https://doi.org/10.1130/0016-7606(1970)81[1837:PTHOTS]2.0.CO;2 Article history received: 21 Jan 1970 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT A WIEBE; Pre-Cenozoic Tectonic History of the Salinian Block, Western California. GSA Bulletin 1970;; 81 (6): 1837–1842. doi: https://doi.org/10.1130/0016-7606(1970)81[1837:PTHOTS]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 SocietyGSA Bulletin Search Advanced Search Abstract The Salinian block is the elongate segment of the California Coast Ranges that lies between the Sur-Nacimiento and San Andreas fault zones. Known basement rocks within this block consist of Cretaceous and older granitic and high-grade metamorphic rocks that contrast sharply with Franciscan Group rocks of overlapping age east of the San Andreas and west of the Sur-Nacimiento fault zones.The metamorphic rocks of the Salinian block (often termed Sur series) are probably in part Paleozoic in age and appear to have been derived from a relatively shallow-water shelf-type sequence of psammitic rocks with subordinate calcareous and pelitic rocks. A graphite- and pyrite-bearing unit, mapped for more than 20 mi in the northern Santa Lucia Range, defines two sets of folds. Mineral lineations and mesoscopic folds indicate that major folding about northeast-trending axes occurred prior to prominant folding about north-northwest-trending axes.Assuming approximately 300 mi of post-Cretaceous right slip along the San Andreas fault, the metamorphic rocks of the Salinian block are tentatively correlated with similar Paleozoic and, perhaps in part, Precambrian rocks of the Mojave Desert and southwestern Tehachapi Mountains. The age of the early northeast-trending folds is unknown, but they may be a southwestward extension of Paleozoic tectonic and stratigraphic trends that head from Nevada into southern California. The north-northwest-trending folds are in part Cretaceous as indicated by the syntectonic emplacement of Cretaceous granitic rocks. 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.
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