Geologische Rundschau

, Volume 77, Issue 1, pp 275–284 | Cite as

Behavior of the decollement at the toe of the Middle America Trench

  • G. F. Moore
  • T. H. Shipley
Article

Abstract

Seismic reflection data from three areas of the Middle America Trench provide insights into the behavior of the decollement that separates subducted and offscraped sediment. The range of responses observed along this single subduction system provides clues as to how the decollement forms and is influenced by local conditions. The location of the decollement and whether or not the subducting basement topography influences the decollement are apparently controlled by the type and thickness of sediment in the trench. Where subducting basement topography and oceanic plate hemipelagic sediments are buried by sandy terrigenous turbidites, such as in the trench axis off Mexico, the decollement is localized near the base of the turbidite section. Subducting normal fault blocks in the oceanic crust control thrust ramps, and hanging wall anticlines form above the ramps.

In regions of the trench where coarse terrigenous sediment is thin or absent, the decollement is localized within the incoming sediment section. Where muddy trench turbidites bury subducting topography (e.g. off Guatemala), the decollement is approximately 100 m deep and is little affected by the underlying subducting topography. The lower 200–300 meters of trench sediment and all of the pelagic sediment are subducted. Where there is no trench sediment overlying carbonate-rich oceanic plate sediments (e.g. off Costa Rica), the decollement is located within the subducting plate sediment section. The decollement is localized at a single stratigraphic level and rides up and over subducting horst blocks.

Keywords

Subduction Trench Turbidite Hanging Wall Oceanic Plate 

Zusammenfassung

Reflektionsseismische Daten aus drei Bereichen des Mittel-Amerika-Grabens geben Einblick in das Verhalten entlang einer Abscherung in der subduziertes und »abgeschabtes« Sediment voneinander getrennt werden. Die Daten, die entlang dieses Subduktionssystems erhalten wurden, liefern Anhaltspunkte darüber, wie sich die Abscherung bildet und wie sie von lokalen Bedingungen beeinflußt wird. Der Entstehungsort der Abscherung wird, unabhängig davon, ob die abtauchende Basementtopographie die Abscherung beeinflußt oder nicht, anscheinend von Typ und Dicke der Grabensedimente kontrolliert. Dort, wo das abtauchende Basement und die hemipelagischen Sedimente der ozeanischen Platte von sandigen, terrigenen Turbiditen überlagert werden, wie im Grabenabschnitt vor Mexico, liegt die Abscherung nahe der Basis des Turbiditprofiles. Abtauchende, normal gestörte Schollenblöcke der ozeanischen Kruste kontrollieren die Bildung von Überschiebungsrampen unter Bildung von Antiklinalen oberhalb dieser Rampen.

Wo schlammige Grabenturbidite die subduzierende Topographie überdecken (z.B. vor Guatemala), liegt die Abscherung etwa 100 m unter der Sedimentoberfläche und wird kaum von der unterlagernden, abtauchenden Topographie beeinflußt. Die unteren 200–300 m der Grabensedimente sowie die gesamten pelagischen Ablagerungen werden subduziert. Wo karbonatreiche Sedimente der ozeanischen Platte nicht von Grabensedimenten überlagert werden (z.B. vor Costa Rica) liegt die Abscherung innerhalb der Sedimentsäule der abtauchenden Platte. Die Abscherung bewegt sich in einem einzigen stratigraphischen Niveau und gleitet dabei auf und über abtauchende Horstschollen.

Résumé

Des prospections par sismique-réflexion, effectuées dans trois régions de la fosse d'Amérique Centrale, éclairent le comportement du décollement qui sépare les sédiments subductés des sédiments «raclés». Les donnés recueillies le long de ce système de subduction simple fournissent des indications sur la manière dont le décollement prend naissance et est influencé par les conditions locales. L'emplacement du décollement et le fait qu'il est, ou non, influencé par la topographie du substrat en subduction dépendent, semble-t-il, de la nature et de l'épaisseur des sédiments de la fosse. Lorsque, dans la plaque océanique en subduction, la topographie du socle et les sédiments hémipélagiques surincombants sont enfouis sous des turbidites terrigènes arénacées, comme c'est le cas au large du Mexique, le décollement se produit près de la base de la série turbiditique. La subduction de blocs découpés par des failles normales dans la croûte océanique détermine des rampes de charriage au-dessus desquelles prennent naissance des structures anticlinales.

Dans les parties de la fosse qui sont pauvres en sédiments terrigènes grossiers ou qui en sont dépourvues, le décollement est situé à l'intérieur de la série sédimentaire. Lorsque le relief de la plaque en subduction est enfoui sous des turbidites boueuses (p. ex. au large du Guatemala), le décollement se situe à 100 m sous la surface supérieure des sédiments et est peu affecté par la topographie du substrat subducté. La subduction affecte alors la partie inférieure (200 à 300 m) des sédiments de la fosse, ainsi que l'entièreté des sédiments pélagiques. Lorsqu'il n'y a pas de sédiments terrigènes recouvrant les dépôts carbonates de la plaque océanique (p. ex. au large de Costa Rica), le décollement se situe au sein de ces derniers; il correspond à un niveau stratigraphique et contourne par en-haut les blocs en horst de la plaque descendante.

Краткое содержание

С помощью сейсмическ их методов исследова ния в трех регионах средне й Америки удается под робнее исследовать породы в доль плоскости срыва, которая отделяет засасываем ые и «перемещенные» с едименты. По этим данным, получе нным вдоль названной системы засасывания, можно представить се бе механизм образовани я срывов и влияние мес тных факторов на условия и х образования. Незави симо от того, повлияла-ли, или н ет топография поверх ности ландшафта погружающ егося основания на ср ыв, место возникновения последнего зависит о т типа и мощи седиментов граб ена. Там, где погружающ ийся фундамент и полупела гические седименты о кеанической плиты перегружены пе счаниками, и терриген ными турбидитами, как напр.: отрезок грабена в Мексике, срыв проходи т вблизи основания ту рбедитного профиля. Погружающие ся нарушенные глыбы океанической к оры контролируют обр азование рампы надвигов, приче м выше рамп появляютс я антиклинали. Там, где п листые турбидиты гра бенов покрывают засасывае мый топографический ландшафт (напр.: Гватемала), скол проходит примерно 100 м под поверхностью се диментов и подлежащи й погружающийся топог рафический ландшафт (Гватемала) не оказывает влияния на линию отслаивания.

Засасываются нижние 200–300 м седимента грабен а и все пелагические от ложения. Там, где богат ые карбонатами седимен ты океанической плит ы не перекрыты седимента ми грабена, напр.: Коста-Рика, отслаивание проходи т в осадочном столбе погружающейся плиты. Отслаивание см ещается в одном и том же стратиграфич еском горизонте и ско льзит при этом по и над погру жающейся глыбе горст а.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1988

Authors and Affiliations

  • G. F. Moore
    • 1
  • T. H. Shipley
    • 2
  1. 1.Department of GeosciencesUniversity of TulsaTulsaUSA
  2. 2.Institute for GeophysicsUniversity of TexasAustinUSA

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