DOCSLIB.ORG

Tectonics of the Panama Basin, Eastern Equatorial Pacific

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Tectonics of the Panama Basin, Eastern Equatorial Pacific TJEERD H. VAN ANDEL" G. ROSS HEATH BRUCE T. MALFAIT Department of Oceanography. Oregon State University. Corralhs, Oregon 97331 DONALD F. HEINRICHSj JOHN I. EWING Lamont-Doherty Geological Observatory. Columbia University. Palisades. New York 10964 Tectonics of the Panama Basin, Eastern Equatorial Pacific ABSTRACT from being fully understood. Similar enigmatic The Panama Basin includes portions of the features are found at complex boundaries be- Nazca, Cocos and South America Hthospheric tween continental and oceanic plates. plates and borders the Caribbean plate. The In this paper we describe and attempt to ex- complex interactions of these units have largely plain the morphological and structural features determined the topography, pattern of faulting, of such a complex region; the area bordered on sediment distribution, and magnetic character the east and north by South and Central of the basin. Only heat flow data fail to corre- America, and on the south and west by the late with major structural features related to Carnegie and Cocos Ridges. This region (Fig. these units. 1) contains the aseismic Cocos and Carnegie The topographic basin appears to have been Ridges, portions of the Peru and Middle created by rifting of an ancestral Carnegie America Trenches, an actively spreading east- Ridge. The occurrence of a distinctive smooth west rift zone, several major fracture zones, a acoustic basement and a characteristic overly- complex continental margin between the ex- ing evenly stratified sedimentary sequence on treme ends of the two trenches, and the large virtually all elevated blocks in the basin suggest volcanic block of the Galapagos Islands. It en- that they all once formed part of this ancestral compasses portions of the Pacific, Nazca, South ridge. The present Carnegie Ridge is the rela- America and Caribbean Hthospheric plates. tively undeformed southern half of this feature, This paper synthesizes a large volume of geo- while the Cocos Ridge is the northern half frag- physical data obtained by the Lamont-Doherty mented by left-lateral north-south transcurrent Geological Observatory and Scripps Institution faulting. As blocks of the Cocos Ridge reach of Oceanography prior to 1969, and on cruise the Middle America Trench, they appear to YALOC-69 of Oregon State University. Al- clog the subduction zone and become welded though we offer a hypothesis for the tectonic to the Nazca plate. Thus, the active transform evolution of the region, the available data are fault at the eastern edge of the Cocos plate has insufficient for a full test. The validity of this episodically shifted west as segments of the hypothesis depends in part on an analysis of the trench were deactivated. Such a shift appears to tectonic evolution and the plate movements in be occurring at the present time. adjacent areas, an aspect we hope to return to in a later paper. INTRODUCTION The plate tectonics concept as proposed by RELIEF Morgan (1968), LePichon (1968), and Isacks The first reasonably detailed bathymetric and others (1968), has linked tectonic chart of the Panama Basin was prepared by phenomena at mid-ocean ridges and trenches, Chase (1968). We have revised and updated and provided a simple tectonic model for large this chart using new data obtained in 1969 by portions of the ocean. In many cases, however, Scripps Institution and Oregon State Univer- the details remain obscure. Where three or sity, with some supplementary information more plates join, or at locations where large from the Lamont-Doherty Geological Observa- changes in direction and rate of motion have tory. The new chart is based on approximately occurred, tectonic features exist that are far twice as much data as the old one. The two are Geological Society of America Bulletin, v. 82, p. 1489-1508, 13 figs., June 1971 1489 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/6/1489/3428244/i0016-7606-82-6-1489.pdf by guest on 26 September 2021 1490 VAN ANDEL AND OTHERS—TECTONICS OF PANAMA BASINS Figure 1. The Panama Basin. Track lines represent are major fracture zones, double dashed lines are axis of seismic reflection profiles taken by Lamont-Doherty rift zone ( <///£•;• Heinrichs, in prep.). Dotted pattern: high Geological Observatory (V and C prefixes) and Oregon blocks and Yaquina graben walls; horizontal shading: State University (YALOC-69). Numbered heavy track trenches and margin troughs. lines are profiles of Figures 3 and 4. Wide dashed lines similar in gross aspect but differ significantly in Peru Trench and, as a continuous topographic detail. The line spacing compared to the com- unit, the Middle America Trench, do not ex- plexity of the relief is adequate for a broad tend into the Panama Basin beyond the land- topographic synthesis (Fig. 2), but fails to do ward terminations of the Carnegie and Cocos justice to the complexity of the relief. More Ridges. detailed surveys are in preparation for some The Cocos and Carnegie Ridges are similar portions of the basin (P. J. Grim, 1970, oral in profile. Both have relatively level, undulat- commun.). It has not been feasible to incorpo- ing crests studded with small pinnacles and rate these surveys into the regional chart. ledges. Both are bordered by steep slopes The Panama Basin can be subdivided into (Figs. 3 and 4) which descend in steps toward four physiographic units: the Cocos and Carne- the adjacent deep ocean floor. The Carnegie gie Ridges; the rugged, low-lying basin en- Ridge is simple in outline and has a marked closed by them and by a major east-west trend with a northeastward curve at its north-south-trending fracture zone at 83° W. eastern end. The Galapagos volcanic pedestal long.; and a highly fractured eastern basin con- forms its western termination. A broad saddle sisting of several marginal troughs, high blocks, in the center of the ridge separates shoaler east- and intervening rugged deep-sea floor. The ern and western areas. The Peru Trench shoals Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/6/1489/3428244/i0016-7606-82-6-1489.pdf by guest on 26 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/6/1489/3428244/i0016-7606-82-6-1489.pdf by guest on 26 September 2021 STRUCTURE 1491 abruptly northward and loses its identity in the the East Pacific Rise near 102° W., 2° N. The gap between the ridge and the continental mar- rift separates the Cocos and Carnegie Ridges gin. north of the Galapagos pedestal and terminates The Cocos Ridge has a more complex out- against the Coiba fracture zone. Within the line, being bordered by alternating north-south western basin the rift is offset northward by and east-west-trending marginal slope segments several smaller fracture zones (Molnar and which give it a distinctive staggered shape. The Sykes, 1969; Grim, 1970a; Heinrichs, in Cocos and Carnegie Ridges do not meet at the prep.). A large fracture zone also extends Galapagos block but are separated by a broad northward from the western end of the Galapa- low zone 2000 to 2600 m deep studded with gos, but is not well defined by our topographic pinnacles and small seamounts and, near the information. Galapagos pedestal, a few larger volcanoes. At The eastern part of the Panama Basin, en- its northeastern end, the Cocos Ridge joins the closed by the Coiba fracture zone, the continental margin. Bathymetric and seismic northeastern hook of the Carnegie Ridge, and reflection control show that the Middle the continental margin, has a complex relief. It America Trench ends against the western slope contains several steep-sided high blocks that are of the ridge with no evidence of the trench or similar in relief and sediment cover to the even a buried equivalent east of 84° W. long. A Carnegie and Cocos Ridges but are much deep, sediment-filled round depression occurs smaller. The two principal blocks are the Coiba between the Cocos and Coiba Ridges at the end and Malpelo Ridges. These blocks are located of the Coiba fracture zone, but it is structurally in an undulating terrain of variable and some- very different from the Middle America times considerable roughness with a regional Trench. depth ranging from 3000 to 3600 m. Our data The Carnegie and Cocos Ridges merge are not adequate to establish detailed trends gradually into the Pacific basin. Several cross- within the deeper terrain, except in the south- ings south of the Carnegie Ridge show this east where a central down warp of 500 to 2000 transition to be interrupted by broad terraces m defines the narrow and sharply bounded with steep outer slopes. Yaquina graben (Fig. 3, profiles 3 and 4). The area between the two ridges is occupied The continental margin is paralleled by a se- by a broad basin that deepens from 2200 m in ries of rather broad, shallow, elongate troughs. the west to 3400 m in the east, where it is Seismic reflection and gravity data show that bounded at 83° W. long, by a series of narrow these are the surface expressions of much more elongate north-south trending troughs and pronounced subsurface depressions. They be- ridges. From the distribution of earthquakes gin just north of the point where the Carnegie and analysis of focal mechanisms at 83° W. Ridge and the continent approach each other long., Molnar and Sykes (1969) named this most closely, but are separated from the Peru boundary the Panama fracture zone (Grim, Trench by a shallow saddle. Swinging around 1970a). In this paper it is referred to by its in a single or double arc, the trough sequence more appropriate name, the Coiba fracture terminates against the eastern edge of the Coiba zone, after the Coiba Ridge of which it forms Ridge, which merges into the continental mar- the western scarp.
Load more

Recommended publications
  • 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]
  • Shape of the Subducted Rivera and Cocos Plates in Southern Mexico
    JOURNALOF GEOPHYSICAL RESEARCH, VOL. 100, NO. B7, PAGES 12,357-12,373, JULY 10, 1995 Shapeof the subductedRivera and Cocosplates in southern Mexico: Seismic and tectonicimplications Mario Pardo and Germdo Sufirez Insfitutode Geoffsica,Universidad Nacional Aut6noma de M6xico Abstract.The geometry of thesubducted Rivera and Cocos plates beneath the North American platein southernMexico was determined based on the accurately located hypocenters oflocal and te!eseismicearthquakes. The hypocenters ofthe teleseisms were relocated, and the focal depths of 21 eventswere constrainedusing a bodywave inversion scheme. The suductionin southern Mexicomay be approximated asa subhorizontalslabbounded atthe edges by the steep subduction geometryof theCocos plate beneath the Caribbean plate to the east and of theRivera plate beneath NorthAmerica to thewest. The dip of theinterplate contact geometry is constantto a depthof 30 kin,and lateral changes in thedip of thesubducted plate are only observed once it isdecoupled fromthe overriding plate. On thebasis of theseismicity, the focal mechanisms, and the geometry ofthe downgoing slab, southern Mexico may be segmented into four regions ß(1) theJalisco regionto thewest, where the Rivera plate subducts at a steepangle that resembles the geometry of theCocos plate beneath the Caribbean plate in CentralAmerica; (2) theMichoacan region, where thedip angleof theCocos plate decreases gradually toward the southeast, (3) theGuerrero-Oaxac.a region,bounded approximately by theonshore projection of theOrozco and O'Gorman
    [Show full text]
  • Interplate Coupling Along the Nazca Subduction Zone on the Pacific Coast of Colombia Deduced from Geored GPS Observation Data
    Volume 4 Quaternary Chapter 15 Neogene https://doi.org/10.32685/pub.esp.38.2019.15 Interplate Coupling along the Nazca Subduction Published online 28 May 2020 Zone on the Pacific Coast of Colombia Deduced from GeoRED GPS Observation Data Paleogene Takeshi SAGIYA1 and Héctor MORA–PÁEZ2* 1 [email protected] Nagoya University Disaster Mitigation Research Center Nagoya, Japan Abstract The Nazca Plate subducts beneath Colombia and Ecuador along the Pacific Cretaceous coast where large megathrust events repeatedly occur. Distribution of interplate 2 [email protected] Servicio Geológico Colombiano coupling on the subducting plate interface based on precise geodetic data is im- Dirección de Geoamenazas portant to evaluate future seismic potential of the megathrust. We analyze recent Space Geodesy Research Group Diagonal 53 n.° 34–53 continuous GPS data in Colombia and Ecuador to estimate interplate coupling in the Bogotá, Colombia Jurassic Nazca subduction zone. To calculate the interplate coupling ratio, in addition to the * Corresponding author MORVEL plate velocities, three different Euler poles for the North Andean Block are tested but just two of them yielded similar results and are considered appropriate for discussing the Pacific coastal area. The estimated coupling distribution shows four main locked patches. The middle two locked patches correspond to recent large Triassic earthquakes in this area in 1942, 1958, and 2016. The southernmost locked patch may be related to slow slip events. The northern locked patch has a smaller coupling ra- tio of less than 0.5, which may be related to the large earthquake in 1979. However, because of the sparsity of the GPS network, detailed interpretation is not possible.
    [Show full text]
  • Back-Arc Opening and the Mode of Subduction
    VOL.84, NO. B3 JOURNALOF GEOPHYSICALRESEARCH MARCH10, 1979 Back-ArcOpening and the Mode of Subduction SEIYA UYEDA EarthquakeResearch Institute, University of Tokyo,Tokyo, Japan HIROO K ANAMORI SeismologicalLaboratory, California Institute of Technology,Pasadena, California 91125 Trench-arcsystems (subduction zones) can be classified into two types depending onwhether or not activelyopening back-arc basins are associated with them. This suggests that subduction of an oceanic plateis not a sufficientcondition for back-arc opening, though it may be necessary one. Mechanisms that causethe distinction between the two types have been investigated. Earthquake studies suggest that there isa significantdifference inthe mode of plate motion at interplate boundaries between the two types of trench-arcsystems. Extreme cases are Chile, where plate motion is seismic, and the M arianas arc, where it is aseismic.This difference seems to indicatethat the stress state in theback-arc area differs between the twotypes: compression in the Chilean type and tension in theMarianas type. This difference in thestress stateis alsomanifested in other tectonic features, such as topography, gravity, volcanic activity, and crustalmovement. Two possible mechanisms forthe difference between the two types are suggested: (1) Thenature of thecontact zone between upper and lower plates chahges from tight coupling (Chile) to decoupling(the Marianas) through the evolutionary process ofsubduction. The decoupling results inan oceanwardretreat of the trench and back-arc opening. (2) The downgoing slab is anchored tothe mantle, sothat the position of a trenchis also fixed with respect to themantle. Since the motion in themantle is slowcompared tothat of surface plates, it is the motion of the landward plate which controls the opening and nonopeningof back-arcs.
    [Show full text]
  • Large Intermediate-Depth Earthquakes and the Subduction Process
    80 Physics ofthe Earth and Planetary Interiors, 53 (1988) 80—166 Elsevier Science Publishers By., Amsterdam — Printed in The Netherlands Large intermediate-depth earthquakes and the subduction process Luciana Astiz ~, Thorne Lay 2 and Hiroo Kanamori ~ ‘Seismological Laboratory, California Institute of Technology, Pasadena, CA (U.S.A.) 2 Department of Geological Sciences, University ofMichigan, Ann Arbor, MI (USA.) (Received September 22, 1987; accepted October 21, 1987) Astiz, L., Lay, T. and Kanamori, H., 1988. Large intermediate-depth earthquakes and the subduction process. Phys. Earth Planet. Inter., 53: 80—166. This study provides an overview of intermediate-depth earthquake phenomena, placing emphasis on the larger, tectonically significant events, and exploring the relation of intermediate-depth earthquakes to shallower seismicity. Especially, we examine whether intermediate-depth events reflect the state of interplate coupling at subduction zones. and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes. Historic record of large intraplate earthquakes (m B 7.0) in this century shows that the New Hebrides and Tonga subduction zones have the largest number of large intraplate events. Regions associated with bends in the subducted lithosphere also have many large events (e.g. Altiplano and New Ireland). We compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with M> 6 and depth between 40 and 200 km. The final catalog includes 335 events with 47 new focal mechanisms, and is probably complete for earthquakes with mB 6.5. For events with M 6.5, nearly 48% of the events had no aftershocks and only 15% of the events had more than five aftershocks within one week of the mainshock.
    [Show full text]
  • Measurements of Upper Mantle Shear Wave Anisotropy from a Permanent Network in Southern Mexico
    GEOFÍSICA INTERNACIONAL (2013) 52-4: 385-402 ORIGINAL PAPER Measurements of upper mantle shear wave anisotropy from a permanent network in southern Mexico Steven A. C. van Benthem, Raúl W. Valenzuela* and Gustavo J. Ponce Received: November 13, 2012; accepted: December 14, 2012; published on line: September 30, 2013 Resumen Abstract Se midió la anisotropía para las ondas de cortante 8SSHU PDQWOH VKHDU ZDYH DQLVRWURS\ XQGHU en el manto superior por debajo de estaciones VWDWLRQVLQVRXWKHUQ0H[LFRZDVPHDVXUHGXVLQJ en el sur de México usando fases SKS. Las records of SKS phases. Fast polarization directions direcciones de polarización rápida donde la placa ZKHUHWKH&RFRVSODWHVXEGXFWVVXEKRUL]RQWDOO\ de Cocos se subduce subhorizontalmente están are oriented in the direction of the relative orientadas aproximadamente paralelas con el PRWLRQEHWZHHQWKH&RFRVDQG1RUWK$PHULFDQ movimiento relativo entre las placas de Cocos y plates, and are trench-perpendicular. This América del Norte y además son perpendiculares SDWWHUQLVLQWHUSUHWHGDVVXEVODEHQWUDLQHGÀRZ DODWULQFKHUD3RUORWDQWRVHLQ¿HUHTXHODSODFD and is similar to that observed at the Cascadia VXEGXFLGD DUUDVWUD HO PDQWR TXH VH HQFXHQWUD subduction zone. Earlier studies have pointed SRUGHEDMR\ORKDFHÀXLU HQWUDLQHGÀRZ 8QD out that both regions have in common the young situación similar existe en la zona de subducción age of the subducting lithosphere. Changes in the GH&DVFDGLD(VWXGLRVSUHYLRVKDQVHxDODGRTXH RULHQWDWLRQRIWKHIDVWD[HVDUHREVHUYHGZKHUH estas dos regiones tienen en común la subducción the subducting
    [Show full text]
  • Structure of the Collision Zone Between the Nazca Ridge and the Peruvian Convergent Margin
    RESEARCH ARTICLE Structure of the Collision Zone Between the Nazca Ridge 10.1029/2019TC005637 and the Peruvian Convergent Margin: Geodynamic Key Points: • The Nazca Ridge hosts an and Seismotectonic Implications overthickened lower crust E. Contreras‐Reyes1 , P. Muñoz‐Linford2, V. Cortés‐Rivas1, J. P. Bello‐González3, J. A. Ruiz1, (10–14 km) formed in an on‐ridge 4 setting (hot spot plume near a and A. Krabbenhoeft spreading center) 1 2 • The Nazca Ridge correlates with a Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile, Centro prominent continental slope scarp I‐MAR, Universidad de los Lagos, Puerto Montt, Chile, 3Grupo Minero Las Cenizas, Taltal, Chile, 4GEOMAR‐Helmholtz bounded by a narrow and uplifted Centre for Ocean Research, Kiel, Germany continental shelf • The Nazca Ridge has behaved as a seismic asperity for moderate earthquakes (e.g., 1996 Mw 7.7 and Abstract We study the structure and tectonics of the collision zone between the Nazca Ridge (NR) and 2011 Mw 6.9) nucleating at depths the Peruvian margin constrained by seismic, gravimetric, bathymetric, and natural seismological data. >20 km The NR was formed in an on‐ridge setting, and it is characterized by a smooth and broad shallow seafloor (swell) with an estimated buoyancy flux of ~7 Mg/s. The seismic results show that the NR hosts an Supporting Information: – – • Supporting Information S1 oceanic lower crust 10 14 km thick with velocities of 7.2 7.5 km/s suggesting intrusion of magmatic material from the hot spot plume to the oceanic plate. Our results show evidence for subduction erosion in the frontal part of the margin likely enhanced by the collision of the NR.
    [Show full text]
  • Seismic Imaging of the Structure of the Central Ecuador Convergent Margin : Relationship with the Inter-Seismic Coupling Variations Eddy Sanclemente Ordońez
    Seismic imaging of the structure of the central Ecuador convergent margin : relationship with the inter-seismic coupling variations Eddy Sanclemente Ordońez To cite this version: Eddy Sanclemente Ordońez. Seismic imaging of the structure of the central Ecuador convergent margin : relationship with the inter-seismic coupling variations. Earth Sciences. Université Nice Sophia Antipolis, 2014. English. NNT : 2014NICE4030. tel-01005320 HAL Id: tel-01005320 https://tel.archives-ouvertes.fr/tel-01005320 Submitted on 12 Jun 2014 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. UNIVERSITE DE NICE-SOPHIA ANTIPOLIS - UFR Sciences École Doctorale de Sciences Fondamentales et Appliquées T H E S E pour obtenir le titre de : Docteur en Sciences de l'UNIVERSITÉ de Nice-Sophia Antipolis Discipline : Sciences de la Planète et de l’Univers présentée et soutenue par Eddy SANCLEMENTE IMAGERIE SISMIQUE DE LA STRUCTURE DE LA MARGE CONVERGENTE D’EQUATEUR CENTRAL : RELATIONS AVEC LES VARIATIONS DE COUPLAGE INTERSISMIQUE SEISMIC IMAGING OF THE STRUCTURE OF THE CENTRAL ECUADOR CONVERGENT MARGIN: RELATIONSHIP WITH THE INTER-SEISMIC COUPLING VARIATIONS Thèse dirigée par : Jean-Yves COLLOT et Alessandra RIBODETTI soutenue le 28 Mai 2014 Jury : M. Bertrand Delouis, Professeur, Examinateur M.
    [Show full text]
  • Geometry and Seismic Properties of the Subducting Cocos Plate in Central Mexico Y
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B06310, doi:10.1029/2009JB006942, 2010 Click Here for Full Article Geometry and seismic properties of the subducting Cocos plate in central Mexico Y. Kim,1 R. W. Clayton,1 and J. M. Jackson1 Received 31 August 2009; revised 22 December 2009; accepted 25 January 2010; published 17 June 2010. [1] The geometry and properties of the interface of the Cocos plate beneath central Mexico are determined from the receiver functions (RFs) utilizing data from the Meso America Subduction Experiment (MASE). The RF image shows that the subducting oceanic crust is shallowly dipping to the north at 15° for 80 km from Acapulco and then horizontally underplates the continental crust for approximately 200 km to the Trans‐ Mexican Volcanic Belt (TMVB). The crustal image also shows that there is no continental root associated with the TMVB. The migrated image of the RFs shows that the slab is steeply dipping into the mantle at about 75° beneath the TMVB. Both the continental and oceanic Moho are clearly seen in both images, and modeling of the RF conversion amplitudes and timings of the underplated features reveals a thin low‐velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab. By inverting RF amplitudes of the converted phases and their time separations, we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting Cocos plate and its thickness. High Poisson’s and Vp/Vs ratios due to anomalously low S wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure.
    [Show full text]
  • Tectonic Segmentation of the North Andean Margin: Impact of the Carnegie Ridge Collision
    ELSEVIER Earth and Planetary Science Letters 168 (1999) 255±270 Tectonic segmentation of the North Andean margin: impact of the Carnegie Ridge collision M.-A. Gutscher a,Ł, J. Malavieille a, S. Lallemand a, J.-Y. Collot b a Laboratoire de GeÂophysique et Tectonique, UMR 5573, Universite Montpellier II, Place E. Bataillon, F-34095 Montpellier, Cedex 5, France b IRD, Geosciences Azur, Villefranche-sur-Mer, France Received 17 July 1998; accepted 2 March 1999 Abstract The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes Mw ½ 7:8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of ¯at slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the in¯uence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 ± from 6ëN to 2.5ëN with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 ± from 2.5ëN to 1ëS showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 ± from 1ëS to 2ëS with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 ± south of 2ëS with ¯at subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc.
    [Show full text]
  • The Way the Earth Works: Plate Tectonics
    CHAPTER 2 The Way the Earth Works: Plate Tectonics Marshak_ch02_034-069hr.indd 34 9/18/12 2:58 PM Chapter Objectives By the end of this chapter you should know . > Wegener's evidence for continental drift. > how study of paleomagnetism proves that continents move. > how sea-floor spreading works, and how geologists can prove that it takes place. > that the Earth’s lithosphere is divided into about 20 plates that move relative to one another. > the three kinds of plate boundaries and the basis for recognizing them. > how fast plates move, and how we can measure the rate of movement. We are like a judge confronted by a defendant who declines to answer, and we must determine the truth from the circumstantial evidence. —Alfred Wegener (German scientist, 1880–1930; on the challenge of studying the Earth) 2.1 Introduction In September 1930, fifteen explorers led by a German meteo- rologist, Alfred Wegener, set out across the endless snowfields of Greenland to resupply two weather observers stranded at a remote camp. The observers had been planning to spend the long polar night recording wind speeds and temperatures on Greenland’s polar plateau. At the time, Wegener was well known, not only to researchers studying climate but also to geologists. Some fifteen years earlier, he had published a small book, The Origin of the Con- tinents and Oceans, in which he had dared to challenge geologists’ long-held assumption that the continents had remained fixed in position through all of Earth history. Wegener thought, instead, that the continents once fit together like pieces of a giant jigsaw puzzle, to make one vast supercontinent.
    [Show full text]
  • Presentation on Pacific Plate and Associated Boundaries
    PACIFIC PLATE AND ASSOCIATED BOUNDARIES The Pacific Plate • Pacific Plate is the largest plate and an oceanic plate. • It shares its boundaries with numerous plates namely; North American Plate.(Convergent and transform fault) Philippine Plate.(Convergent) Juan de Fuca Plate.(Convergent) Indo – Australian Plate.(Convergent, Transform Fault) Cocos Plate.(Divergent) Nazca Plate.(Divergent) Antarctic Plate.(Divergent,Transform Fault) Types of Plate Boundaries • Convergent Boundary: Subduction zones where two plates converges. Eg; Aleutian Islands(Alaska) • Divergent Boundary: Spreading centres where two plates move away from each other. Eg; East Pacific Rise (MOR, Pacific Ocean). • Transform Faults: Boundary where two plates slide past each other. For Eg. ; San Andreas Fault. BOUNDARY WITH ANTARCTIC PLATE DIVERGENT BOUNDARY • Pacific – Antarctic Ridge TRANSFORM FAULT • Louisville Seamount Chain Pacific – Antarctic Ridge Pacific – Antarctic Ridge(PAR) is located on the seafloor of the South Pacific Ocean. It is driven by the interaction of a mid oceanic ridge and deep mantle plumes located in the eastern portion of East Pacific Ridge. Louisville Seamount Chain It is the longest line of seamount chain in the Pacific Ocean of about 4,300 km, formed along the transform boundary in the western side between Pacific plate and Antarctic plate. It was formed from the Pacific Plate sliding over a long – lived centre of upwelling magma called the Louisville hotspot. BOUNDARY WITH PHILIPPINE PLATE CONVERGENT BOUNDARY • Izu – Ogasawara Trench • Mariana Trench Izu – Ogasawara Trench It is an oceanic trench in the western Pacific Ocean. It stretches from Japan to northern most section of Mariana Trench. Here, the Pacific Plate is being subducted beneath the Philippine Sea Plate.
    [Show full text]