Zusammenfassung
Beim Übergang schmaler Ozeanbecken oder Pullapart-Becken vom Riftstadium mit Dehnung der kontinentalen Kruste zur Akkretion neuer ozeanischer Kruste kann es in den Spreading-Trögen zur Bildung von Lagergängen (Sills) innerhalb der Sedimente kommen. Injektionen nachfolgender starker magmatischer Pulse werden über und neben der Kontaktzone der zuvor gebildeten vertikalen Basaltgänge (Dikes) und Sills eingeschoben. Auf diese Weise wird ein charakteristischer Sill-Sediment-Komplex aufgebaut.
Die Mächtigkeit dieser Übergangszone zwischen dem Sheeted Dike-Komplex (seismische »Lage« 2) und dem jüngeren Sediment (seismische »Lage« 1) wird in erster Linie von der Sedimentationsrate gesteuert. Extrem hohe Sedimentationsraten, welche die Spreadingrate übertreffen, bewirken eine Auffüllung der Becken und behindern vermutlich stark den Magmenaufstieg. Bei sehr geringen Sedimentationsraten wie z. B. im Bereich mittelozeanischer Rücken breiter, reifer Ozeanbecken, extrudiert das aufsteigende Magma praktisch ohne Sedimenteinfluß am Meeresboden. Mittlere, aber immer noch relativ hohe Sedimentationsraten können Spreading-Tröge nicht auffüllen, sondern ermöglichen die Bildung von Sill-Sediment-Komplexen mit Mächtigkeiten bis zu einigen hundert Metern.
Dieses Phänomen wurde im Guaymas Becken des Golfs von Kalifornien beobachtet. Ähnliche Sill-Sediment-Komplexe können auch an den heutigen passiven Kontmentalrändern der großen Ozeane vorkommen und tragen vielleicht zum Auftreten einer »gestörten seismischen« und »magnetisch ruhigen« Zone bei. Auch aus fossilen Riftzonen und Orogen-Gürteln sind schon Beispiele für ähnliche Sill-Sediment-Komplexe bekannt.
Abstract
During the transitional stage from stretching of continental crust to accretion of new oceanic crust, the upwelhng basaltic magma in spreading troughs of narrow ocean basins and pull-apart basins may form sills in soft sediment. The injections of subsequent great magmatic pulses are empla ced next to or on top of the contact zone of the older ones. In this manner a distinctive sill-sediment complex is built up.
The thickness of this transitional zone between the sheeted dike complex (seismic layer 2) and overlying sediments (seismic layer 1) is controlled chiefly by the rate of sedimentation. Extremely high sedimentation rates surpassing the spreading rate cause filling up of the basins and hamper the rise of magma to the land surface. Very low rates, e.g. along mid-oceanic ridges of wide mature ocean basins, hardly influence the magma extruding at the sea floor. Intermediate (but comparatively still high) sedimentation rates cannot fill up the spreading troughs, but they generate sill-sediment complexes of a thickness up to a few hundred meters.
The processes leading to this phenomenon were observed in the Guaymas Basin of the Gulf of California. Sill-sediment complexes similar to those described here may occur at several sections along the present passive continental margins and may contribute to the existence of a »disturbed seismic« and »magnetic quiet« zone. Examples of such complexes were also found in ancient rift zones and orogenic belts.
Résumé
Au cours de la phase de transition entre l'extension de la croûte continentale et la formation de nouvelle croûte océanique, la montée de magma basaltique dans des bassins océaniques étroits et des bassins »pull-appart« peut conduire à la formation de sills dans les sédiments. Les puissantes injections magmatiques qui suivent se mettent en place au dessus et à côté de la zône de contact des venues plus anciennes. Ainsi peut s'édifier un complexe caractéristique de sills et de sédiments.
L'épaisseur de cette zône de transition entre le complexe de dykes (couche sismique 2) et le sédiment susjacent (couche sismique 1) dépend principalement du taux de sédimentation.
Des taux de sédimentation élevés, dépassant le taux d'ouverture, produisent le remplissage des bassins et s'opposent à la montée du magma.
Dans le cas de taux de sédimentation très bas comme par exemple dans les dorsales des bassins océaniques larges et matures, le magma s'épanche sans obstacle sur le fond de la mer. Des taux moyens, mais toutefois relativement élevés, ne peuvent pas remplir les fossés d'ouvertures océaniques; par contre, ils rendent possible la formation de complexes sills-sédiments dont l'épaisseur peut atteindre plusieurs centaines de mètres. Ce phénomène a été observés dans le bassin de Guaymas dans le Golfe de Californie.
Des complexes analogues peuvent se recontrer le long des marges continentales des grands océans et contribuer à l'existence d'une zône sismiquement perturbée et magnétiquement calme. On connaît de tels complexes dans les anciennes zônes de rift et les ceintures orogéniques.
Краткое содержание
При преобразовании у зких океанических ба ссейнов, или бассейнов Pullapart на риф товой стадии расшире ния материковой коры, при наростании новой океанической коры мо гут образовывать в не плотных седиментах трогов ра сширения жилы силлы. Последующее внедрен ие магм смещает образованные до этог о вертикальные базал ьтовые жилы — дайки — и силлы. Т о. получается характе рный силло-седиментный ко мплекс. Мощность таких перех одных зон между компл ексом Sheeted Dike (сейсмическое “поло жение” 2) и поздними седиментам и (сейсмическое “поло жение” 1) зависит в первую очер едь от скорости осадконакопления. Большая скорость нак опления осадков, прев ышающая скорость расширения, приводит к заполнени ю бассейна и, повидимом у, сильно тормозит поднятие магмы. При оч ень незначительной с корости осадконакопления, на пр.: в регионе среднеокеаническог о хребта широкого, уже с формировавшегося, океанического бассе йна, восходящая магма изливается по дну океана фактиче ски без влияния на это т процесс осадконакоп ления. Осадконакопле ние, происходящее со сред ней скоростью, при кот орой седиментых не успева ют заполнить трог рас ширения, способствует образо ванию описанных выше силло-седиментнх ком плексов с мощностью д о несколько сот метров. Этот феномен наблюда ют в бассейне Гвиама, Калифорнийского залива. Сходный компл екс находят на сегодня пассивном материковом крае бол ьших океанов; они, повидимо му, ответственны за по явление “нарушенных сейсмич еских” и “магнитно-сп окойных” зон. Подобные силло-се диментные комплексы известны также в фосс ильных рифтовых зона х и в поясах орогенов.
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Einsele, G. Interaction between sediments and basalt injections in young Gulf of California-type spreading centers. Geol Rundsch 75, 197–208 (1986). https://doi.org/10.1007/BF01770188
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DOI: https://doi.org/10.1007/BF01770188