DOCSLIB.ORG

Subduction Initiation Along Transform Faults: the Proto-Franciscan Subduction Zone

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Subduction Initiation Along Transform Faults: the Proto-Franciscan Subduction Zone Subduction initiation along transform faults: The proto-Franciscan subduction zone John W. Shervais1* and Sung Hi Choi2 1DEPARTMENT OF GEOLOGY, UTAH STATE UNIVERSITY, 4505 OLD MAIN HALL, LOGAN, UTAH 84322-4505, USA 2DEPARTMENT OF GEOLOGY AND EARTH ENVIRONMENTAL SCIENCES, CHUNGNAM NATIONAL UNIVERSITY, DAEJEON 305-764, SOUTH KOREA ABSTRACT The initiation of subduction is a process that cannot be observed directly but must be inferred from the rock record after subduction is well established. There are many approaches possible to infer the origin of subduction zones that are still active, but paleosubduction zones pres- ent special challenges, since their geodynamic setting can no longer be directly observed. In this study, we examine evidence for subduc- tion initiation of the proto-Franciscan subduction zone along a transform fault, based on a subduction initiation origin for the Coast Range ophiolite, and on the Tehama-Colusa serpentinite mélange, which underlies the ophiolite and separates it from high-pressure/temperature metamorphic rocks of the Franciscan complex. The Coast Range ophiolite consists of volcanic, plutonic, and mantle components, each of which contains elements that refl ect subduction initiation or hydrous melting within a subduction-zone setting. The volcanic assemblage includes forearc basalts and boninites, as well as more evolved calc-alkaline rocks; the plutonic complex contains intrusive suites that refl ect this same range of parent magmas. Peridotites of the mantle section include both abyssal-like and refractory peridotites formed by hydrous decompression melting. The Tehama-Colusa serpentinite mélange consists of blocks of basalt, chert, sedimentary rocks, and peridotite (harzburgite and lherzolite) in a sheared serpentinite matrix. The mélange matrix represents hydrated refractory peridotites with forearc affi nities, and blocks within the mélange consist largely of upper-plate lithologies (harzburgite, arc volcanics, and arc-derived sediments). Lower-plate blocks within the mélange include oceanic basalts and chert with rare blueschist and amphibolite. The abyssal peridotites have low pyroxene equilibration temperatures that are consistent with formation in a fracture-zone setting. However, the current mélange refl ects largely upper-plate litholo- gies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault or fracture zone that evolved into a subduction-zone mélange complex. The nucleation of subduction zones along former transform boundaries has long been proposed for both modern arc systems and for the Franciscan–Coast Range ophiolite system. Our data support this interpretation and document more fully how this mechanism is expressed by mixing within the evolving serpentinite mélange. LITHOSPHERE; v. 4; no. 6; p. 484–496 | Published online 14 December 2011 doi: 10.1130/L153.1 INTRODUCTION stable oceanic lithosphere, or adjacent to buoyant continental crust (e.g., Casey and Dewey, 1984; Leitch, 1984; Stern and Bloomer, 1992; Stern, The process of subduction initiation cannot be observed directly but 2004). In either case, the ultimate result is a subduction zone that underlies must be inferred from the rock record after a subduction zone is estab- a volcanic arc, where the arc rests on older oceanic crust or continental lished. “Induced” subduction initiation (Hall et al., 2003; Stern, 2004) crust (Tatsumi and Eggins, 1995). Investigations of early arc volcanism results from far-fi eld plate stresses that force convergence across a zone over the past two decades have shown that primitive early arc volcanics of weakness within a plate during plate-boundary reorganizations (e.g., comprise the same rock assemblages as most ophiolites, and they are com- the Maquarie-Puysegur-Fiordland system near New Zealand; Ruff et al., monly preserved in highly extended forearc regions, which are interpreted 1989) or in response to a collisional event between an existing subduction to represent the products of spontaneous subduction initiation (Bloomer et zone and continental or unusually thick oceanic crust (e.g., Ontong Java al., 1995; Stern and Bloomer, 1992; Hawkins, 2003; Reagan et al., 2010). Plateau collision with the Solomon Islands Trench; Cooper and Taylor, Most extensive ophiolite terranes are believed, on the basis of their 1985). In contrast, “spontaneous” subduction initiation occurs most com- lavas and mantle residues, to have formed in, or passed through, a supra- monly when stable ocean lithosphere, which forms by anhydrous decom- sub duction-zone environment (e.g., Miyashiro, 1973; Alabaster et al., pression melting at mid-oceanic spreading centers and essentially “fl oats” 1982; Shervais, 1982; Pearce et al., 1984; Shervais and Kimbrough, 1985; on top of the underlying asthenosphere, becomes gravitationally unstable Metcalf and Shervais, 2008). While the specifi c environment (backarc, and begins to sink back into the mantle, either adjacent to gravitationally arc, and forearc) is often debated, it is clear that the volcanic rock series found in ophiolites most closely corresponds to lavas now found within the forearc region of active arc terranes (e.g., Metcalf and Shervais, 2008). Further, it has been proposed that at their time of formation, there is little *E-mail: [email protected]. or no evidence for the existence of a volcanic arc (Stern and Bloomer, Editor’s note: This article is part of a special issue titled “Initiation and Termina- tion of Subduction: Rock Record, Geodynamic Models, Modern Plate Boundaries,” 1992; Pearce, 2003; Metcalf and Shervais, 2008). edited by John Shervais and John Wakabaya shi. The full issue can be found at Shervais (2001) showed that suprasubduction-zone ophiolites display http://lithosphere.gsapubs.org/content/4/6.toc. a consistent sequence of events during their formation and evolution, sug- 484 For permission to copy, contact [email protected] | |Volume © 2012 4 Geological | Number Society6 | LITHOSPHERE of America Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/4/6/484/3038916/484.pdf by guest on 26 September 2021 Subduction initiation along transform faults: The proto-Franciscan subduction zone | RESEARCH gesting that they form in response to processes that are common to all such 2004; Metcalf and Shervais, 2008; Whattam and Stern, 2011), the specifi c ophiolites. This sequence includes: (1) birth—formation of the ophiolite setting or circumstances of this event are commonly unclear. Our second above a nascent or reconfi gured subduction zone; (2) youth—continued focus here is on the evidence provided by serpentinite mélange zones, melting of refractory asthenosphere (depleted during birth) in response to which are commonly associated with suprasubduction-zone ophiolites. fl uid fl ux from the subducting slab; (3) maturity—onset of semistable arc The petrologic and geochemical character of blocks within these mélange volcanism; (4) death—the sudden demise of active spreading and ophio- zones provides clues regarding their origin, which may be unrelated to the lite-related volcanism; and (5) resurrection—emplacement by obduction adjacent ophiolite, or may refl ect a linkage among the mélange, subduc- onto a passive margin or accretionary uplift with continued subduction. tion initiation, and ophiolite formation. This sequence of events is similar to that inferred by Reagan et al. (2010) We take as our case example the Coast Range ophiolite of Califor- from their detailed studies of the Izu-Bonin-Mariana forearc, with tholei- nia, which has been studied extensively for over three decades (Hopson itic “forearc basalts” preceding boninites, and with both forming during et al., 1981, 2008; Shervais and Kimbrough, 1985; Shervais, 1990, 2001; an episode of rapid extension in the forearc prior to establishment of the Coleman, 2000; Shervais et al., 2005a, 2005b). The Coast Range ophiolite calc-alkaline volcanic arc. is closely linked to the Franciscan assemblage—one of the world’s most Our primary focus here is on the fi rst two stages proposed by Shervais intensely studied subduction complexes and a model for convergent bound- (2001): (1) birth, characterized by forearc basalts (Reagan et al., 2010), ary plate processes (Bailey et al., 1964; Blake and Jones, 1974; Blake et represented by a variety of early arc tholeiites with strong mid-ocean- al., 1982; Wakabayashi, 1999; Ernst, 1993). In northern California, the ridge basalt (MORB)–like characteristics, and (2) youth, characterized by ophiolite is separated from younger rocks of the Franciscan assemblage by high-Mg basalts and andesites of the boninite suite, which are among the the Tehama-Colusa mélange—a serpentinite-matrix mélange containing most depleted volcanic rocks on Earth (e.g., Metcalf and Shervais, 2008). blocks of peridotite, metabasalt, chert, and high-grade metamorphic rocks These stages are thought to represent the onset and development of sub- that has been interpreted as an oceanic fracture-zone assemblage (Hopson duction initiation, followed by the transition to stable subduction (stage and Pessagno, 2005) linked to subduction initiation (Choi et al., 2008b). three: maturity and calc-alkaline volcanism). Ophiolites that represent the fi rst two stages in this progression (birth-youth) are interpreted to have GEOLOGIC SETTING formed during a subduction initiation event, regardless of their current apparent setting (Fig. 1). Further, these stages can be established
Load more

Recommended publications
  • Podiform Chromite Deposits—Database and Grade and Tonnage Models
    Podiform Chromite Deposits—Database and Grade and Tonnage Models Scientific Investigations Report 2012–5157 U.S. Department of the Interior U.S. Geological Survey COVER View of the abandoned Chrome Concentrating Company mill, opened in 1917, near the No. 5 chromite mine in Del Puerto Canyon, Stanislaus County, California (USGS photograph by Dan Mosier, 1972). Insets show (upper right) specimen of massive chromite ore from the Pillikin mine, El Dorado County, California, and (lower left) specimen showing disseminated layers of chromite in dunite from the No. 5 mine, Stanislaus County, California (USGS photographs by Dan Mosier, 2012). Podiform Chromite Deposits—Database and Grade and Tonnage Models By Dan L. Mosier, Donald A. Singer, Barry C. Moring, and John P. Galloway Scientific Investigations Report 2012-5157 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 This report and any updates to it are available online at: http://pubs.usgs.gov/sir/2012/5157/ For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Suggested citation: Mosier, D.L., Singer, D.A., Moring, B.C., and Galloway, J.P., 2012, Podiform chromite deposits—database and grade and tonnage models: U.S.
    [Show full text]
  • Chromite Deposits of the North Elder Creek Area Tehama County, California
    UNITED STATES DEPARTMENT OF THE INTERIOR J. A. Krug, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director Bulletin 945-G CHROMITE DEPOSITS OF THE NORTH ELDER CREEK AREA TEHAMA COUNTY, CALIFORNIA By G. A. RYNEARSON Strategic Minerals Investigations, 1944 (Pages 191-210) UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1946 CONTENTS Page Abstract................................................... 191 Introduction............................................... 192 History and production..................................... 192 Geology.................................................... 194 Franciscan formation................................... 194 Knoxville formation.................................... 195 Argillite and metavolcanic rocks................... 195 Shale and sandstone................................ 195 Peridotite and serpentine.............................. 195 Saxonite........................................... 196 Dunite............................................. 196 Wehrlite........................................... 196 Serpentine......................................... 196 Dike rocks............................................. 197 Alteration............................................. 197 Structure.............................................. 198 Ore bodies................................................. 199 Mineralogy............................................. 199 Character of ore....................................... 200 Localization........................................... 202 Origin................................................
    [Show full text]
  • Geologic Gems of California's State Parks
    STATE OF CALIFORNIA – EDMUND G. BROWN JR., GOVERNOR NATURAL RESOURCES AGENCY – JOHN LAIRD, SECRETARY CALIFORNIA GEOLOGICAL SURVEY DEPARTMENT OF PARKS AND RECREATION – LISA MANGAT, DIRECTOR JOHN D. PARRISH, Ph.D., STATE GEOLOGIST DEPARTMENT OF CONSERVATION – DAVID BUNN, DIRECTOR PLATE 1 The rugged cliffs of Del Norte Coast Redwoods State Park are composed of some of California’s Bio-regions the most tortured, twisted, and mobile rocks of the North American continent. The California’s Geomorphic Provinces rocks are mostly buried beneath soils and covered by vigorous redwood forests, which thrive in a climate famous for summer fog and powerful winter storms. The rocks only reveal themselves in steep stream banks, along road and trail cut banks, along the precipitous coastal cliffs and offshore in the form of towering rock monuments or sea stacks. (Photograph by CalTrans staff.) Few of California’s State parks display impressive monoliths adorned like a Patrick’s Point State Park displays a snapshot of geologic processes that have castle with towering spires and few permit rock climbing. Castle Crags State shaped the face of western North America, and that continue today. The rocks Park is an exception. The scenic beauty is best enjoyed from a distant exposed in the seacliffs and offshore represent dynamic interplay between the vantage point where one can see the range of surrounding landforms. The The Klamath Mountains consist of several rugged ranges and deep canyons. Klamath/North Coast Bioregion San Joaquin Valley Colorado Desert subducting oceanic tectonic plate (Gorda Plate) and the continental North American monolith and its surroundings are a microcosm of the Klamath Mountains The mountains reach elevations of 6,000 to 8,000 feet.
    [Show full text]
  • Multi-Stage Origin of the Coast Range Ophiolite, California: Implications for the Life Cycle of Supra-Subduction Zone Ophiolites
    International Geology Review, Vol. 46, 2004, p. 289–315. Copyright © 2004 by V. H. Winston & Son, Inc. All rights reserved. Multi-Stage Origin of the Coast Range Ophiolite, California: Implications for the Life Cycle of Supra-Subduction Zone Ophiolites JOHN W. S HERVAIS,1 Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505 DAVID L. KIMBROUGH, Department of Geological Sciences, San Diego State University, San Diego California 92182-1020 PAUL RENNE, Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709 and Department of Earth and Planetary Science, University of California, Berkeley, California 94720 BARRY B. HANAN, Department of Geological Sciences, San Diego State University, San Diego California 92182-1020 BENITA MURCHEY, United States Geological Survey, 345 Middlefield Road, Menlo Park California 94025 CAMERON A. SNOW, Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322-4505 and Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305 MARCHELL M. ZOGLMAN SCHUMAN,2 AND JOE BEAMAN3 Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208 Abstract The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Trans- verse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid- Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen.
    [Show full text]
  • High-Temperature Ultramafic Complexes in the North Norwegian Caledonides: I - Regional Setting and Field Relationships
    High-temperature ultramafic complexes in the North Norwegian Caledonides: I - Regional setting and field relationships M.C. BENNETT, S,R. EMBLIN, B, ROBINS & W.J.A. YEO Bennett, M.C., Emblin, S.R., Robins, B. & Yeo, W.J.A. 1986: High+emperature ultramafic complexes in the North Norwegian Caledonides: I - Regional setting and field relationships. Nor. geol. unders. Bull.405, r40. Four major (25-100 km¡) ultramafìc complexes were developed in the Sørøy Nappe during the second phase of Finnmarkian deformation in the North Norwegian Caledonides. They were emplaced into layered mafic intrusions and show broadly similar emplacement histories and petrographic variations suggesting a related petrogenesis. The Nordre Bumannsford, Melkvann and Kvalford group of ulramafic complexes developed in part by emplacement ofboth replacive and dilational ne-normative ultramafic sheets and dykes which progressively fragmented the layered olivine gabbro envelope. The earliest dykes and sheets are from a few cm to l00m wide and show highly inegular, nondilational contacls. They are dominantly coarse-grained, xenolithic olivine clinopyroxenite varying locally to dunite, poikilitic wehrlite, feldspat- hic olivine clinopyroxenite and olivine melagabbro. Iater dykes are from a few cm to 20 m wide and also lack chilled margins but show regular, dilational contact relationships. They include poikilitic wehrlite (commonly spatially associated with veins and patches of secondary wehrlite and dunite), olivine clinopyroxenite, olivinehornblende clinopyroxenite, hornblende peridotite and homblende melagabbro. Some members of the dyke suite exhibit mineral lamination, modal layering and cyclic units, and others contain mosaic-porphyroclastic spinel lher¿olite nodules. Variably foliated and metamorphosed olivine gabbro, ankaramite and picrite dykes with chilled maryins were emplaced during the latest stages in the evolution ofthe complexes.
    [Show full text]
  • Major Chemical Characteristics of Mesozoic Coast Range Ophiolite in California
    JOURNAL OF OF THE U.S. GEOLOGICAL SURVEY NOVEMBER-DECEMBER 1974 VOLUME 2, NUMBER 6 Scientific notes and summaries of investigations in geology, hydrology, and related fields +r U«/ fc/ U.S. DEPARTMENT OF THE INTERIOR UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC The Journal of Research is pub­ Correspondence and inquiries concerning the Jour­ 20402. Order by SD Catalog No. lished every 2 months by the U.S. nal (other than subscription inquiries and address JRGS. Annual subscription rate Geological Survey. It contains changes) should be directed to the Journal of Re­ $15.50 (plus $3.75 for foreign mail­ papers by members of the Geologi­ search, Publications Division, U.S. Geological Survey, ing). Single copy $2.75. Make cal Survey and their professional National Center 321, Reston, VA 22092. checks or money orders payable to colleagues on geologic, hydrologic, the Superintendent of Documents. topographic, and other scientific Papers for the Journal should be submitted through Send all subscription inquiries and technical subjects. regular Division publication channels. and address changes to the Superin­ tendent of Documents at the above address. Purchase orders should not be sent to the U.S. Geological Survey The Secretary of the Interior has determined that the publication of this periodical is library. necessary in the transaction of the public business required by law of this Department. Library of Congress Catalog-card Use of funds for printing this periodical has been approved by the Director of the Office No.
    [Show full text]
  • Occurrence of Gabbro-Wehrlite Near Lochalsh, Ontario M
    OCCURRENCE OF GABBRO-WEHRLITE NEAR LOCHALSH, ONTARIO M. H. Fnonsnnc. MacassaMi.nes. Li.mited., Toromto, Ontar'i.o, Ansrnecr Gabbro-wehrlite consisting of more than80/6 dark constituents, but in excess of 1016 plagioclase, forms the marginal facies of a mela-olivine gabbro intrusion near Lochalsh, about 150 miles north of Sault Ste. Marie, Ontario. This exceptionally fresh rock is of in- terest as a link between the gabbro and peridotite clans. In his report on the iron deposits of the Missinaibi map area, E. Thompsonl mentioned the occurrenceof peridotite at the northwest end of Dog Lake, about 1$ miles southeast of the Canadian Pacific Railway station Lochalsh. The intrusive body is roughly elliptical in outline, with its major axis striking almost due east. It is slightly over one mile long and up to 1,500 ft. wide, forming a prominent ridge rising more than 300 ft. above the level of Dog Lake. Thompson describedthis remarkably fresh basic rock as consisting essentially of pyroxene and olivine, with lesser quantities of plagioclase and biotite, and some serpentine and hornblende as secondary products. The present writer investigated the occurrencereferred to by Thomp- son, in connection with a search for war-important minerals. Micro- scopic examination of a considerablenumber of samplestaken at various points of the intrusion revealed that its plagioclase content exceedsthe amount of leucocratic constituents allowed by most petrographers in rocks classedas peridotites. Actually the mass consistsof a core bearing from 20 to 45/s plagioclase,and a more melanocratic border zone aver- aging Iessthan20/s plagioclase.In no instance was the feldspar content Iessthan t0/6 of the total constituents.
    [Show full text]
  • Igneous Petrology EOSC 321 Laboratory 1: Ultramafic Plutonic and Volcanic Rocks
    1 Igneous petrology EOSC 321 Laboratory 1: Ultramafic plutonic and volcanic rocks Material Needed: a) Microscope, b) Glossary of rock names and textures (see Pages 24- 25 and 43 of Winter); c) Lab1 Manual printed off the course website; d) tables to aid determination of mineral modes in thin sections; e) classification triangles; f) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke) Microscopes: At the start of the lab period, you will be assigned a microscope and the combination to the microscope locker by your TA. You will have access to this microscope throughout the remainder of the course. The microscope will be shared with students from the other laboratories. Please keep the microscopes in working condition and, should a problem arise, let your TA know immediately so that repairs can be made. Lab Organization: Review optical properties of common rock-forming minerals so that you’re able to identify them. There is a box with reference thin sections for igneous rock- forming minerals that can be helpful in this respect. In the next two hours of the lab period you will examine reference thin sections of ultramafic rocks. Each thin section has a brief petrographic description to assist you in the identification of minerals, textures and rock classification. Please make sure you understand the reference petrographic descriptions and can find all of the minerals mentioned. You should also be able to understand why a rock is given a particular name. For these, you should recap how to assess mineral modes under a microscope, and how to plot these on rock classification triangles.
    [Show full text]
  • Translithospheric Mantle Diapirism: Geological Evidence and Numerical Modelling of the Kondyor Zoned Ultramafic Complex (Russian Fa R-E a S T )
    JOURNAL OF PETROLOGY VOLUME 50 NUMBER 2 PAGES 289^321 2009 doi:10.1093/petrology/egn083 Translithospheric Mantle Diapirism: Geological Evidence and Numerical Modelling of the Kondyor Zoned Ultramafic Complex (Russian Fa r-E a s t ) J.-P. BURG1*,J.-L.BODINIER2,T.GERYA1, R.-M. BEDINI2, F. BOU DI ER 2,J.-M.DAUTRIA2,V.PRIKHODKO3,A.EFIMOV4, E. PUPIER2 AND J.-L. BALANEC2 1EARTH SCIENCES DEPARTMENT, ETH ZENTRUM AND UNIV. ZU« RICH, SONNEGGSTRASSE 5, ZU« RICH, 8092, SWITZERLAND 2GE¤ OSCIENCES MONTPELLIER, UNIVERSITE¤ DE MONTPELLIER 2 & CNRS, CC 60, PLACE EUGE' NE BATAILLON, 340 95 MONTPELLIER CEDEX 05, FRANCE 3INSTITUTE OF TECTONICS AND GEOPHYSICS, RUSSIAN ACADEMY OF SCIENCES, 680063 KHABAROVSK, RUSSIA 4INSTITUTE OF GEOLOGY AND GEOCHEMISTRY, RUSSIAN ACADEMY OF SCIENCES, 620151 EKATERINBURG, RUSSIA RECEIVED MARCH 10, 2008; ACCEPTED DECEMBER 30, 2008 We report new structural, microstructural, petrological, and major- core metasomatic zone, with a decreasing melt fraction from core to and trace-element data on ultramafic rocks from the Kondyor zoned rim, and also suggest that solid-state deformation induced grain-size ultramafic complex in Far-East Russia. The ultramafic rocks are reduction towards the cooling border of the Kondyor massif. Based on subdivided into three subconcentric lithologies, from core to rim: (1) their geochemistry, the dunites are interpreted as mantle rocks a metasomatic domain where generally phlogopite-rich dykes perva- strongly affected by reaction with melts similar to the Jurassic^ sively intrude dunite; (2) a main dunite core; (3) a pyroxenite rim. Cretaceous Aldan Shield lamproites. Rim pyroxenites were formed The ultramafic rocks have nearly vertical contacts with the sur- by a melt-consuming peritectic reaction, implying the existence of at rounding Archaean basement (gneisses, quartzites and marbles) least a small, conductive thermal gradient around the dunite body and hornfelsed Riphean sediments.The hornfelsed sediments show a while the latter was still at near-solidus temperature conditions.
    [Show full text]
  • Eclogite Formation and the Rheology, Buoyancy, Seismicity, and H2O
    Eclogite Formationand the Rheology,Buoyancy, Seismicity,and H20 Contentof OceanicCrust BradleyR. Hacker1 Departmentof Geologicaland EnvironmentalSciences, Stanford University, Stanford, California A broad spectrumof variably altered igneous rocks with a wide range of grain sizes are compressedand heated over a wide range of pressure-temperaturepaths in subductionzones. Although experimentalkinetic data cannotbe extrapolatedto predict the rates of blueschistand eclogite formation in nature, textural data from rocks indicate that transformationbelow temperaturesof 150øCis minimal. Completetransformation of volcanicrocks occurs by •-250øC, but incompletetransformation of gabbroicrocks heatedto 800øC has been observed.There are important consequencesto the rapid transformation of volcanic rocks and the metastable persistenceof gabbroicrocks into the blueschistand eclogite stability fields. Fast seismic velocities shouldbe evident first in the upper oceaniccrust and may be substantiallyretarded in the lower oceaniccrust. The upper oceaniccrust will be denserthan asthenospherebefore the lower oceanic crust.Early in the processof eclogiteformation, volcanic rocks will be placedin deviatorictension and the underlyingcoarser grained rocks in compression;with furtherreaction, the stateof stressin gabbroicrocks will changefrom compressiveto tensile.Earthquakes at shallowdepths should be extensional in basalt and contractionalin gabbro, changing at deeper levels to extensional throughoutthe crust. INTRODUCTION This paper summarizes the rates and
    [Show full text]
  • Characteristics and Petrogenesis of Alaskan^Type Ultranaafic-Mafic Intrusions, Southeastern Alaska
    Characteristics and Petrogenesis of Alaskan^Type«/ J: Ultranaafic-Mafic Intrusions, Southeastern Alaska U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1564 AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current- year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and Availability List" are no longer available. Reports released through the NTIS may be obtained by writing to the National Technical Information Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry. Order U.S. Geological Survey publications by mail or over the counter from the offices given below. BY MAIL OVER THE COUNTER Books Books and Maps Professional Papers, Bulletins, Water-Supply Papers, Tech­ Books and maps of the U.S. Geological Survey are available niques of Water-Resources Investigations, Circulars, publications over the counter at the following U.S. Geological Survey offices, of general interest (such as leaflets, pamphlets, booklets), single all of which are authorized agents of the Superintendent of Docu­ copies of Earthquakes & Volcanoes, Preliminary Determination of ments. Epicenters, and some miscellaneous reports, including some of the foregoing series that have gone out of print at the Superin­ • ANCHORAGE, Alaska-4230 University Dr., Rm.
    [Show full text]
  • Petrological, Geochemical and Isotopic Characteristics of the Collo Ultramafic Rocks (NE Algeria)
    Journal of African Earth Sciences 125 (2017) 59e72 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci Petrological, geochemical and isotopic characteristics of the Collo ultramafic rocks (NE Algeria) * Rabah Laouar a, c, , Adel Satouh b, Sihem Salmi-Laouar a, Nachida Abdallah c, Jean-Yves Cottin d, Olivier Bruguier e, Delphine Bosch e, Aziouz Ouabadi c, Adrian J. Boyce f, Anthony E. Fallick f a Departement de geologie, FST, Universite Badji Mokhtar Annaba, B.P. 12, 23000, Annaba, Algeria b Departement de Geologie, Universite Kasdi Merbah Ouargla, Algeria c Laboratoire de Geodynamique, Geologie de l’Ingenieur et Planetologie, F.S.T.G.A.T., USTHB, BP. 32, Bab Ezzouar, 16111, Algiers, Algeria d Univ-Lyon, UJM Saint Etienne, UMR 6524 “Magmas et Volcans”, 23 rue P. Michelin, 42023, Saint Etienne Cedex, France e Geosciences Montpellier, Universite de Montpellier, CNRS-UMR 5243, Place E. Bataillon, 34095, Montpellier Cedex 5, France f Isotope Geosciences Unit, SUERC, East Kilbride, Glasgow, G75 0QU, Scotland, United Kingdom article info abstract Article history: The ultramafic rocks of the Collo region in northeastern Algeria crop out as “stratified” masses that cut Received 27 June 2016 across older metamorphic formations of the Petite Kabylie basement. Based on petrological compositions Received in revised form and mineralogical observations, these rocks are mainly peridotites and serpentinites. The peridotites are 27 September 2016 identified as lherzolites, but dunites may occur rarely. The lherzolites are composed of olivine, ortho- Accepted 25 October 2016 pyroxene, clinopyroxene and chromian spinel. Their chemical composition shows high MgO (34.4 Available online 26 October 2016 e37.5 wt%), Cr (0.14e0.27 wt%), Ni (0.14e0.26 wt%) and Co (34e133 ppm) contents and low CaO and Al2O3 concentrations (0.02e2.2 wt% and 0.5 to 2.8 wt%, respectively).
    [Show full text]