DOCSLIB.ORG

Internal Deformation of the Southern Gorda Plate: Fragmentation of a Weak Plate Near the Mendocino Triple Junction

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Internal Deformation of the Southern Gorda Plate: Fragmentation of a Weak Plate Near the Mendocino Triple Junction Internal deformation of the southern Gorda plate: Fragmentation of a weak plate near the Mendocino triple junction Sean P.S. Gulick* Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, Anne S. Meltzer USA Timothy J. Henstock* Department of Geology and Geophysics, Rice University, Houston, Texas 77005, USA Alan Levander ABSTRACT Gorda plate that may serve as the northern North-south compression across the Gorda-Paci®c plate boundary caused by the north- limit of in¯uence of the triple junction on the ward-migrating Mendocino triple junction appears to reactivate Gorda plate normal Juan de Fuca±Gorda plate system. Near the faults, originally formed at the spreading ridge, as left-lateral strike-slip faults. Both seis- triple junction, the plate shows evidence of mically imaged faults and magnetic anomalies fan eastward from ;N208E near the Gorda fragmenting (Fig. 1B). ridge to ;N758E near the triple junction. Near the triple junction, the Gorda plate is faulted pervasively and appears to be extending east-southeast as it subducts beneath GORDA PLATE DEFORMATION North America. Continuation of northeast-southwest±oriented deformation in the southern Oceanic crust imaged on seismic lines Gorda plate beneath the continental margin contrasts with the northwest-southeast±trend- MTJ-3, MTJ-5, and MTJ-6 is rough and per- ing structures in the overlying accretionary prism, suggesting partial Gorda±North Amer- vasively faulted (Fig. 3). The crust appears in ican plate decoupling. Southeast of the triple junction, a slabless window is generated by places to be broken into crustal blocks bound- removal of the subducting Gorda plate. Southwest of the triple junction, the Paci®c plate ed by major faults and deformed by minor acts as a rigid barrier forcing southern Gorda crust to rotate clockwise, fragment, and faults (Fig. 3, A and B; Gulick et al., 1998). ¯ow into the slabless window. Net clockwise rotation of the southern Gorda crust forms High-angle faults, imaged in the southeastern a boundary with the nonrotating northern Gorda plate, which is observed as a bend in Gorda plate, have on average eastward dips of the magnetic anomalies. This boundary, which is compressional on the western end and up to 808 and lack evidence of shortening extensional to the east, may separate the stress regime of the southern Gorda plate from within the sediments they cut (Fig. 3). South- the remainder of the Cascadia subduction zone. east-side-down basement offsets overlain by southeast-downthrown or seaward-tilted sedi- Keywords: Mendocino triple junction, Gorda plate, plate boundary, deformation, fragmentation. ments (Fig. 3, B and C) suggest a component of extension. We interpret these faults as pri- INTRODUCTION Gorda plate near the triple junction and di- marily strike-slip, consistent with earthquake The Gorda microplate subducts northwest minishes rapidly south of the Mendocino frac- data (Fig. 2A), with a secondary component of the Mendocino triple junction (Atwater, ture zone (Fig. 2A). Focal mechanism and mo- of east-southeast extension not re¯ected in the 1970); evidence of deformation within the mi- ment tensor solutions de®ne stress orientations, modern earthquake record. Pervasive faulting croplate led Wilson (1986) to term its southern which suggest that the Gorda plate is being de- suggests crustal-scale fragmentation of the part the Gorda deformation zone (Fig. 1). formed within the plate interior by northeast- brittle part of the Gorda plate near the triple West of the triple junction, the Mendocino southwest±trending, left-lateral strike-slip faults junction. fracture zone separates the 25±27 Ma Paci®c (Wilson, 1986). Paralleling the Gorda ridge in the northern plate from the 0±7 Ma Gorda plate. Southeast Direct evidence for a northeast-southwest Gorda plate, faults with an orientation of of the triple junction is a slabless window orientation of the faults (Fig. 2A) in the north- N208±308E (Silver, 1971) are truncated at their where removal of the subducting Gorda lith- western Gorda plate comes from single-chan- southern end by a series of faults trending osphere from beneath North America causes nel seismic data (Silver, 1971) and surface ex- N808E (Fig. 2A). In the southern Gorda plate, asthenospheric upwelling (Dickinson and Sny- pressions of fault-related basement ridges faults trend N208±N458E near the Gorda ridge, der, 1979). imaged on GLORIA sidescan sonar data but they rotate clockwise along the Mendoci- Gorda plate magnetic anomalies 2, 2A, and (EEZ-SCAN 84 Scienti®c Staff, 1986; Wil- no fracture zone to an orientation of N758Ein 3 are kinked between the northern and south- son, 1986). This orientation is also suggested the southeast, subparallel to the anomalies. In ern Gorda plate (Fig. 1) (Raff and Mason, in the aftershock sequence of the magnitude the southeast Gorda plate, a region of sea¯oor 1961; Silver, 1971). North of this kink, the 7.2 Eureka, California, seismic event on No- scarps observed on the GLORIA data (Fig. anomalies parallel the Gorda ridge. South of vember 8, 1980 (Wilson, 1989). 2A) is coincident with the region of shallow the bend, the anomalies trend obliquely to the We present multichannel seismic re¯ection crust and fault scarps imaged on MTJ-5 (Fig. ridge, progressively changing from N358E data from the Mendocino triple junction seis- 3A). This map view indicates constraints for (anomaly 2) near the spreading ridge to N658E mic experiment (TreÂhu et al., 1995) that image some of the faults on MTJ-5 to ;N708E. (anomaly 4) near the triple junction. Silver the southeastern Gorda plate in advance of the Where MTJ-5, MTJ-6, and MTJ-3 are in prox- (1971) noted that the Gorda plate magnetic migrating Mendocino triple junction (Fig. imity, prominent mappable faults have general anomalies are tens of kilometers shorter than 1A). When integrated with single-channel the corresponding Paci®c plate anomalies. east-northeast orientations (Fig. 2B), consis- seismic, GLORIA, seismicity, and magnetic Seismicity is concentrated in the southern tent with the progressively more eastward- anomaly data, these pro®les provide an im- oriented pattern of basement ridges and faults. proved understanding of kinematics of the This apparent relationship suggests that these *Present addresses: GulickÐInstitute for Geo- Gorda plate. Speci®cally, the internally de- faults may have formed at the spreading ridge physics, University of Texas, Austin, Texas 78759, forming southern Gorda plate is rotating but were later reactivated as strike-slip faults USA; HenstockÐSchool of Ocean and Earth Sci- ence, University of Southampton, Southampton clockwise toward the triple junction. This ro- (Wilson, 1989). SO14 3HZ, UK. tation creates a boundary with the northern Depth-to-basement structure contours (Fig. q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; August 2001; v. 29; no. 8; p. 691±694; 3 ®gures. 691 the deformation front. Uplift and erosion of the continental margin in proximity to the tri- ple junction (Gulick, 1999) provide a sedi- ment source with transport down the Eel Can- yon and deposition of the Gorda fan (Fig. 1A), which is coincident with the deepest oceanic basement. DISCUSSION Eastward-fanning geometry of southern Gorda plate faults and magnetic anomalies suggests clockwise rotation of the southern part of the Gorda plate (Riddihough, 1980), resulting in a boundary with the nonrotating northern Gorda plate (Fig. 1B). Clockwise ro- tation of the southern plate is consistent with compression along the western end of the bend of the magnetic anomalies (Fig. 1B), where we observe faults oriented N808E and a shallow basement ridge (Fig. 2A) (Silver, 1971). Clockwise rotation also predicts exten- sion along this boundary somewhere to the east. Seismic stratigraphic analysis (Gulick, 1999) concluded that part of the forearc over- lying the subducting eastern end of the bend in the anomalies collapsed within the past 500 k.y., consistent with the subduction of either topography or a crustal-scale graben related to this boundary (Fig. 1B). The boundary be- tween the rotating and nonrotating parts of the Gorda plate may be the northern limit of the in¯uence of the triple junction on the Juan de Fuca±Gorda plate system. Fox and Dziak (1999) reported a band of mi- croseismicity that trends north-northeast across the bend in the magnetic anomalies and oblique to the trends of the southern Gorda plate faults. Given the discrepancy with mapped faults and basement ridges, we interpret these events to represent stress relief from plate bending forces generated by the plate subduction. Seismic transects MTJ-5 and MTJ-3 (Fig. 3) and available seismicity data (Fig. 2) show no evidence for underthrusting of the Gorda plate beneath the Paci®c plate or obduction of the Gorda plate onto the Paci®c plate; these sug- gestions were made by Silver (1971) and Stod- Figure 1. A: Gorda plate study area showing plate boundaries with Paci®c dard (1987), respectively, to explain the short- and North American plates. Gray areas are magnetic anomalies (Atwater and ened Gorda plate magnetic anomalies. We Severinghaus, 1988); dashed line indicates Gorda deformation zone (Wil- son, 1986); area with dotted lines is Gorda fan. Anomaly ages (Ma) are suggest an alternative: a combination of shear- shown to right. Dark gray lines are parts of industry and academic multi- ing of the anomalies along the fracture zone channel data; black lines are single channel seismic data of Silver (1971). suggested by changes in anomaly shape, short- Bold arrows show Gorda and Paci®c plate motions relative to stable North ening of the crust near the spreading ridge due America. Heavy-dash box indicates area of B. B: Tectonic model for Gorda plate, where boundary between rotating southern Gorda crust and more sta- to convergence between the northern and ble northern Gorda crust is shown along bend in anomalies.
Load more

Recommended publications
  • Washington Division of Geology and Earth Resources Open File Report
    l 122 EARTHQUAKES AND SEISMOLOGY - LEGAL ASPECTS OPEN FILE REPORT 92-2 EARTHQUAKES AND Ludwin, R. S.; Malone, S. D.; Crosson, R. EARTHQUAKES AND SEISMOLOGY - LEGAL S.; Qamar, A. I., 1991, Washington SEISMOLOGY - 1946 EVENT ASPECTS eanhquak:es, 1985. Clague, J. J., 1989, Research on eanh- Ludwin, R. S.; Qamar, A. I., 1991, Reeval­ Perkins, J. B.; Moy, Kenneth, 1989, Llabil­ quak:e-induced ground failures in south­ uation of the 19th century Washington ity of local government for earthquake western British Columbia [abstract). and Oregon eanhquake catalog using hazards and losses-A guide to the law Evans, S. G., 1989, The 1946 Mount Colo­ original accounts-The moderate sized and its impacts in the States of Califor­ nel Foster rock avalanches and auoci­ earthquake of May l, 1882 [abstract). nia, Alaska, Utah, and Washington; ated displacement wave, Vancouver Is­ Final repon. Maley, Richard, 1986, Strong motion accel­ land, British Columbia. erograph stations in Oregon and Wash­ Hasegawa, H. S.; Rogers, G. C., 1978, EARTHQUAKES AND ington (April 1986). Appendix C Quantification of the magnitude 7.3, SEISMOLOGY - NETWORKS Malone, S. D., 1991, The HAWK seismic British Columbia earthquake of June 23, AND CATALOGS data acquisition and analysis system 1946. [abstract). Berg, J. W., Jr.; Baker, C. D., 1963, Oregon Hodgson, E. A., 1946, British Columbia eanhquak:es, 1841 through 1958 [ab­ Milne, W. G., 1953, Seismological investi­ earthquake, June 23, 1946. gations in British Columbia (abstract). stract). Hodgson, J. H.; Milne, W. G., 1951, Direc­ Chan, W.W., 1988, Network and array anal­ Munro, P. S.; Halliday, R. J.; Shannon, W.
    [Show full text]
  • Cambridge University Press 978-1-108-44568-9 — Active Faults of the World Robert Yeats Index More Information
    Cambridge University Press 978-1-108-44568-9 — Active Faults of the World Robert Yeats Index More Information Index Abancay Deflection, 201, 204–206, 223 Allmendinger, R. W., 206 Abant, Turkey, earthquake of 1957 Ms 7.0, 286 allochthonous terranes, 26 Abdrakhmatov, K. Y., 381, 383 Alpine fault, New Zealand, 482, 486, 489–490, 493 Abercrombie, R. E., 461, 464 Alps, 245, 249 Abers, G. A., 475–477 Alquist-Priolo Act, California, 75 Abidin, H. Z., 464 Altay Range, 384–387 Abiz, Iran, fault, 318 Alteriis, G., 251 Acambay graben, Mexico, 182 Altiplano Plateau, 190, 191, 200, 204, 205, 222 Acambay, Mexico, earthquake of 1912 Ms 6.7, 181 Altunel, E., 305, 322 Accra, Ghana, earthquake of 1939 M 6.4, 235 Altyn Tagh fault, 336, 355, 358, 360, 362, 364–366, accreted terrane, 3 378 Acocella, V., 234 Alvarado, P., 210, 214 active fault front, 408 Álvarez-Marrón, J. M., 219 Adamek, S., 170 Amaziahu, Dead Sea, fault, 297 Adams, J., 52, 66, 71–73, 87, 494 Ambraseys, N. N., 226, 229–231, 234, 259, 264, 275, Adria, 249, 250 277, 286, 288–290, 292, 296, 300, 301, 311, 321, Afar Triangle and triple junction, 226, 227, 231–233, 328, 334, 339, 341, 352, 353 237 Ammon, C. J., 464 Afghan (Helmand) block, 318 Amuri, New Zealand, earthquake of 1888 Mw 7–7.3, 486 Agadir, Morocco, earthquake of 1960 Ms 5.9, 243 Amurian Plate, 389, 399 Age of Enlightenment, 239 Anatolia Plate, 263, 268, 292, 293 Agua Blanca fault, Baja California, 107 Ancash, Peru, earthquake of 1946 M 6.3 to 6.9, 201 Aguilera, J., vii, 79, 138, 189 Ancón fault, Venezuela, 166 Airy, G.
    [Show full text]
  • Kinematic Reconstruction of the Caribbean Region Since the Early Jurassic
    Earth-Science Reviews 138 (2014) 102–136 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Kinematic reconstruction of the Caribbean region since the Early Jurassic Lydian M. Boschman a,⁎, Douwe J.J. van Hinsbergen a, Trond H. Torsvik b,c,d, Wim Spakman a,b, James L. Pindell e,f a Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands b Center for Earth Evolution and Dynamics (CEED), University of Oslo, Sem Sælands vei 24, NO-0316 Oslo, Norway c Center for Geodynamics, Geological Survey of Norway (NGU), Leiv Eirikssons vei 39, 7491 Trondheim, Norway d School of Geosciences, University of the Witwatersrand, WITS 2050 Johannesburg, South Africa e Tectonic Analysis Ltd., Chestnut House, Duncton, West Sussex, GU28 OLH, England, UK f School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, UK article info abstract Article history: The Caribbean oceanic crust was formed west of the North and South American continents, probably from Late Received 4 December 2013 Jurassic through Early Cretaceous time. Its subsequent evolution has resulted from a complex tectonic history Accepted 9 August 2014 governed by the interplay of the North American, South American and (Paleo-)Pacific plates. During its entire Available online 23 August 2014 tectonic evolution, the Caribbean plate was largely surrounded by subduction and transform boundaries, and the oceanic crust has been overlain by the Caribbean Large Igneous Province (CLIP) since ~90 Ma. The consequent Keywords: absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into GPlates Apparent Polar Wander Path the global circuit of plate motions.
    [Show full text]
  • Tracing the Central African Rift and Shear Systems Offshore Onto
    Tracing the West and Central African Rift and Shear Systems offshore onto oceanic crust: a ‘rolling’ triple junction William Dickson (DIGs), and James W. Granath, PhD, (Granath & Associates) Abstract Compared to the understood kinematics of its continental margins and adjacent ocean basins, the African continent is unevenly or even poorly known. Consequently, the connections from onshore fault systems into offshore spreading centers and ridges are inaccurately positioned and inadequately understood. This work considers a set of triple junctions and the related oceanic fracture systems within the Gulf of Guinea from Nigeria to Liberia. Our effort redefines the greater Benue Trough, onshore Nigeria, and reframes WCARS (West and Central African Rift and Shear Systems) as it traces beneath the onshore Niger Delta and across the Cameroon Volcanic Line (CVL), Figure 1. We thus join onshore architecture to oceanic fracture systems, forming a kinematically sound whole. This required updating basin outlines and relocating mis- positioned features, marrying illustrations from the literature to imagery suitable for basin to sub- basin mapping. The resulting application of systems structural geology explains intraplate deformation in terms of known structural styles and interplay of their elements. Across the Benue Trough and along WCARS, we infer variations in both structural setting and thermal controls that require further interpretation of their petroleum systems. Introduction Excellent work has defined Africa's onshore geology and the evolution and driving mechanisms of the adjacent (particularly the circum-Atlantic) ocean basins. However, understanding of the oceanic realm has outpaced that of the continent of Africa. This paper briefly reviews onshore work. We then discuss theoretical geometry of tectonic boundaries (including triple junctions) and our data (sources and compilation methods).
    [Show full text]
  • This Report Is Preliminary and Has Not Bee Reviewed for Conformity with US
    UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Northeast-trending subcrustal fault transects western Washington by Kenneth F. Fox, Jr.* Open-File Report 83-398 This report is preliminary and has not bee reviewed for conformity with U.S Geological Survey editorial standards and stratigraphic nomenclature. *U.S. Geological Survey 3^5 Middlefield Road Menlo Park, California 9^025 Page Table of Contents Tectonic setting......................................................... 1 Seisraicity............................................................... 4 Discussion............................................................... 4 References cited......................................................... 6 Figures Figure 1. Magnetic anomalies in the northeastern Pacific................ 8 Figure 2. Bathymetry at intersection of Columbia lineament and Blanco fracture zone................................................. 9 Figure 3. Plane vector representation of movement of Gorda plate........ 10 Figure 4. Reconstruction of Pacific-Juan de Fuca plate geometry 2 m.y. before present................................................ 11 Figure 5. Epicenters of historical earthquakes with intensity greater than V........................................................ 12 TECTONIC SETTING The north-trending magnetic anomalies of the Juan de Fuca plate are off­ set along two conspicuous northeast-trending lineaments (fig. 1), named the Columbia offset and the Destruction offset by Carlson (1981). The northeast­ ward projections of these lineaments intersect the continental area of western Washington, hence are of potential significance to the tectonics of the Pacific Northwest region. Pavoni (1966) suggested that these lineaments were left-lateral faults, and that the Columbia, 280 km in length, had 52 km of offset, and the Destruction, with a length of 370 km, had 75 km of offset. Based on Vine's (1968) correlation of the magnetic anomalies mapped in this area by Raff and Mason (1961), with the magnetic reversal time scale, Silver (1971b, p.
    [Show full text]
  • The Caribbean-North America-Cocos Triple Junction and the Dynamics of the Polochic-Motagua Fault Systems
    The Caribbean-North America-Cocos Triple Junction and the dynamics of the Polochic-Motagua fault systems: Pull-up and zipper models Christine Authemayou, Gilles Brocard, C. Teyssier, T. Simon-Labric, A. Guttierrez, E. N. Chiquin, S. Moran To cite this version: Christine Authemayou, Gilles Brocard, C. Teyssier, T. Simon-Labric, A. Guttierrez, et al.. The Caribbean-North America-Cocos Triple Junction and the dynamics of the Polochic-Motagua fault systems: Pull-up and zipper models. Tectonics, American Geophysical Union (AGU), 2011, 30, pp.TC3010. 10.1029/2010TC002814. insu-00609533 HAL Id: insu-00609533 https://hal-insu.archives-ouvertes.fr/insu-00609533 Submitted on 19 Jan 2012 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. TECTONICS, VOL. 30, TC3010, doi:10.1029/2010TC002814, 2011 The Caribbean–North America–Cocos Triple Junction and the dynamics of the Polochic–Motagua fault systems: Pull‐up and zipper models C. Authemayou,1,2 G. Brocard,1,3 C. Teyssier,1,4 T. Simon‐Labric,1,5 A. Guttiérrez,6 E. N. Chiquín,6 and S. Morán6 Received 13 October 2010; revised 4 March 2011; accepted 28 March 2011; published 25 June 2011.
    [Show full text]
  • Subduction Controls the Distribution and Fragmentation of Earth’S Tectonic Plates Claire Mallard, Nicolas Coltice, Maria Seton, R.D
    Subduction controls the distribution and fragmentation of Earth’s tectonic plates Claire Mallard, Nicolas Coltice, Maria Seton, R.D. Müller, Paul J. Tackley To cite this version: Claire Mallard, Nicolas Coltice, Maria Seton, R.D. Müller, Paul J. Tackley. Subduction controls the distribution and fragmentation of Earth’s tectonic plates. Nature, Nature Publishing Group, 2016, 535 (7610), pp.140-143. 10.1038/nature17992. hal-01355818 HAL Id: hal-01355818 https://hal.archives-ouvertes.fr/hal-01355818 Submitted on 24 Aug 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Subduction controls the distribution and fragmentation of Earth’s tectonic plates Claire Mallard1, Nicolas Coltice1,2, Maria Seton3, R. Dietmar Müller3, Paul J. Tackley4 1. Laboratoire de géologie de Lyon, École Normale Supérieure, Université de Lyon 1, 69622 Villeurbanne, France. 2. Institut Universitaire de France, 103, Bd Saint Michel, 75005 Paris, France 3. EarthByte Group, School of Geosciences, Madsen Building F09, University of Sydney, NSW, 2006, Australia 4. Institute of Geophysics, Department of Earth Sciences, ETH Zürich, Sonneggstrasse 5, 8092 Zurich, Switzerland The theory of plate tectonics describes how the surface of the Earth is split into an organized jigsaw of seven large plates1 of similar sizes and a population of smaller plates, whose areas follow a fractal distribution2,3.
    [Show full text]
  • California North Coast Offshore Wind Studies
    California North Coast Offshore Wind Studies Overview of Geological Hazards This report was prepared by Mark A. Hemphill-Haley, Eileen Hemphill-Haley, and Wyeth Wunderlich of the Humboldt State University Department of Geology. It is part of the California North Coast Offshore Wind Studies collection, edited by Mark Severy, Zachary Alva, Gregory Chapman, Maia Cheli, Tanya Garcia, Christina Ortega, Nicole Salas, Amin Younes, James Zoellick, & Arne Jacobson, and published by the Schatz Energy Research Center in September 2020. The series is available online at schatzcenter.org/wind/ Schatz Energy Research Center Humboldt State University Arcata, CA 95521 | (707) 826-4345 California North Coast Offshore Wind Studies Disclaimer This study was prepared under contract with Humboldt State University Sponsored Programs Foundation with financial support from the Department of Defense, Office of Economic Adjustment. The content reflects the views of the Humboldt State University Sponsored Programs Foundation and does not necessarily reflect the views of the Department of Defense, Office of Economic Adjustment. This report was created under Grant Agreement Number: OPR19100 About the Schatz Energy Research Center The Schatz Energy Research Center at Humboldt State University advances clean and renewable energy. Our projects aim to reduce climate change and pollution while increasing energy access and resilience. Our work is collaborative and multidisciplinary, and we are grateful to the many partners who together make our efforts possible. Learn more about our work at schatzcenter.org Rights and Permissions The material in this work is subject to copyright. Please cite as follows: Hemphill-Haley, M.A., Hemphill-Haley, E. and Wunderlich, W. (2020).
    [Show full text]
  • Detailed Structures of the Subducted Philippine Sea Plate Beneath
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B1, PAGES 1015-1033,JANUARY 10, 1999 Detailed structuresof the subductedPhilippine Sea plate beneath northeast Taiwan' A new type of double seismiczone Honn Kao and Ruey-JuinRau Instituteof Earth Sciences,Academia Sinica, Taipei, Taiwan Abstract. We studiedthe detailedstructure of the subductedPhilippine Sea plate beneath northeastTaiwan whereoblique subduction, regional collision, and back arc openingare all activelyoccurring. Simultaneousinversion for velocitystructure and earthquakehypocenters areperformed using the vast,high-quality data recorded by the TaiwanSeismic Network. We furthersupplement the inversionresults with earthquakesource parameters determined from inversionof teleseismicP and $H waveforms,a criticalstep to definethe positionof plateinterface and the stateof strainwithin the subductedslab. The mostinteresting feature is that relocatedhypocenters tend to occuralong a two-layeredstructure. The upperlayer is locatedimmediately below the plate interfaceand extendsdown to 70-80 krn at a dip of 40ø- 50ø. Below approximately100 km, the dip increasesdramatically to 700-80ø. The lower layer commencesat 45-50 km andstays approximately parallel to the upperlayer with a separationof 15+5 km in betweendown to 70-80 km. Below thatthe separationdecreases andthe two layersseem to graduallymerge into one Wadati-BenioffZone. We proposeto term the classicdouble seismic zones observed beneath Japan and Kuril as "type I" andthat we observedas "typeII," respectively.A globalsurvey indicates that type II doubleseismic zonesare alsoobserved in New Zealandnear the southernmostNorth Island,Cascadia, just northof the Mendocinotriple junction, and the Cook Inlet areaof Alaska. All of them are locatednear the terrniniof subductedslabs in a tectonicsetting of obliquesubduction. We interpretthe seismogenesisof type II doubleseismic zones as reflecting the lateralcompres- sivestress between the subductedplate and the adjacentlithosphere (originming from oblique subduction)and the downdipextension (from slabpulling force).
    [Show full text]
  • On the Dynamics of the Juan De Fuca Plate
    Earth and Planetary Science Letters 189 (2001) 115^131 www.elsevier.com/locate/epsl On the dynamics of the Juan de Fuca plate Rob Govers *, Paul Th. Meijer Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, The Netherlands Received 11 October 2000; received in revised form 17 April 2001; accepted 27 April 2001 Abstract The Juan de Fuca plate is currently fragmenting along the Nootka fault zone in the north, while the Gorda region in the south shows no evidence of fragmentation. This difference is surprising, as both the northern and southern regions are young relative to the central Juan de Fuca plate. We develop stress models for the Juan de Fuca plate to understand this pattern of breakup. Another objective of our study follows from our hypothesis that small plates are partially driven by larger neighbor plates. The transform push force has been proposed to be such a dynamic interaction between the small Juan de Fuca plate and the Pacific plate. We aim to establish the relative importance of transform push for Juan de Fuca dynamics. Balancing torques from plate tectonic forces like slab pull, ridge push and various resistive forces, we first derive two groups of force models: models which require transform push across the Mendocino Transform fault and models which do not. Intraplate stress orientations computed on the basis of the force models are all close to identical. Orientations of predicted stresses are in agreement with observations. Stress magnitudes are sensitive to the force model we use, but as we have no stress magnitude observations we have no means of discriminating between force models.
    [Show full text]
  • Mid-Cretaceous Tectonic Evolution of the Tongareva Triple Junction in the Southwestern Pacific Basin
    Mid-Cretaceous tectonic evolution of the Tongareva triple junction in the southwestern Paci®c Basin Roger L. Larson Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, Robert A. Pockalny USA Richard F. Viso Elisabetta Erba Dipartimento di Scienze della Terra, UniversitaÁ di Milano, 20133 Milano, Italy Lewis J. Abrams Center for Marine Science, University of North Carolina, Wilmington, North Carolina 28409, USA Bruce P. Luyendyk Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA Joann M. Stock Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California Robert W. Clayton 91125, USA ABSTRACT The trace of the ridge-ridge-ridge triple junction that con- nected the Paci®c, Farallon, and Phoenix plates during mid-Creta- ceous time originates at the northeast corner of the Manihiki Pla- teau near the Tongareva atoll, for which the structure is named. The triple junction trace extends .3250 km south-southeast, to and beyond a magnetic anomaly 34 bight. It is identi®ed by the inter- section of nearly orthogonal abyssal hill fabrics, which mark the former intersections of the Paci®c-Phoenix and Paci®c-Farallon Ridges. A distinct trough is commonly present at the intersection. A volcanic episode from 125 to 120 Ma created the Manihiki Pla- teau with at least twice its present volume, and displaced the triple junction southeast from the Nova-Canton Trough to the newly formed Manihiki Plateau. Almost simultaneously, the plateau was rifted by the new triple junction system, and large fragments of the plateau were rafted away to the south and east.
    [Show full text]
  • Rift-Valley-1.Pdf
    R E S O U R C E L I B R A R Y E N C Y C L O P E D I C E N T RY Rift Valley A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift. G R A D E S 6 - 12+ S U B J E C T S Earth Science, Geology, Geography, Physical Geography C O N T E N T S 9 Images For the complete encyclopedic entry with media resources, visit: http://www.nationalgeographic.org/encyclopedia/rift-valley/ A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift. Rift valleys are found both on land and at the bottom of the ocean, where they are created by the process of seafloor spreading. Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion. Tectonic plates are huge, rocky slabs of Earth's lithosphere—its crust and upper mantle. Tectonic plates are constantly in motion—shifting against each other in fault zones, falling beneath one another in a process called subduction, crashing against one another at convergent plate boundaries, and tearing apart from each other at divergent plate boundaries. Many rift valleys are part of “triple junctions,” a type of divergent boundary where three tectonic plates meet at about 120° angles. Two arms of the triple junction can split to form an entire ocean. The third, “failed rift” or aulacogen, may become a rift valley.
    [Show full text]