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Geologische Rundschau

, Volume 77, Issue 1, pp 191–209 | Cite as

Rapid upward transport of mid-crustal mylonitic gneisses in the footwall of a Miocene detachment fault, Whipple Mountains, southeastern California

  • Gregory A. Davis
Article

Abstract

Mylonitic gneisses of upper greenschist to middle amphibolite facies grade are exposed below the Whipple detachment fault in the Whipple Mountains metamorphic core complex. Fabric and microstructural analyses of the thick (>3.5 km) mylonitic sequence indicate that it represents an intracrustal zone of non-coaxial laminar flow with a predominant sense of northeastward shear. The top of this shear zone is the Whipple mylonitic front, the abruptly gradational (locally within several meters) upper limit of pervasive ductile strain between a distinctive sequence of non-mylonitized crystalline rocks and their lower, mylonitized equivalents. Mylonitization of Oligo-Miocene age (26±5 Ma) is estimated to have occurred at depths of 16±4 km (4.4±1.1 kb) and at temperatures between 460–535 °C. Fission track and40Ar/39Ar age determinations from the mylonitic rocks collectively document their rapid cooling from above 450 °C to below 200 °C between 20 and 18 Ma ago. Rapid cooling is attributed to post-20 Ma uplift of mylonitic gneisses in the footwall of an evolving low-angle detachment fault system of extensional origin. The NE-rooting Whipple fault system and the mylonites are kinematically coordinated (same sense and direction of shear), but the faults of the system appear to have cut across the mylonites several million years after their formation.

Lower-plate mylonites reached the earth's surface, where they were eroded, prior to 16 Ma ago. Minimum uplift rates for the mylonites and detachment fault system slip rates for the period 20−16 Ma ago are 3 and 7.2 mm/yr, respectively, assuming that the mylonites were captured at a minimum depth of 12 km by a fault system that dipped 25° through the upper crust. From available cooling data, higher rates for 20−18 Ma ago are likely. Cumulative displacement of rock units across major faults of the Whipple system appears to exceed 40–45 km.

Keywords

Footwall Slip Rate Fission Track Detachment Fault Metamorphic Core Complex 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Zusammenfassung

Im metamorphen Komplex der Whipple Mountains sind unterhalb der Whipple-Abscherung mylonitische Gneise der oberen Grünschieferfazies und mittleren Amphibolitfazies aufgeschlossen. Gefügeuntersuchungen innerhalb der mächtigen (>3,5 km) Mylonitsequenz zeigen, daß hier eine intrakrustale Zone nichtkoaxialen laminaren Fließens mit vorherrschend nordost gerichtetem Schersinn repräsentiert ist. Den obersten Teil dieser Scherzone bildet die Whipple Mylonitfront. Sie überspannt kontinuierlich innerhalb weniger Meter die Obergrenze durchdringender duktiler Deformation bis zu eindeutig nichtmylonitisierten Abfolgen kristalliner Gesteine und ihren tieferen mylonitisierten Äquivalenten. Es wird angenommen, daß die Mylonitisierung während des Oligo-Miozäns (26 ±1–5 Ma) in einer Tiefe von 16 ±4 km (4,4 ±1,1 kb) und bei Temperaturen zwischen 460–535 °C stattgefunden hat. Spaltspurenuntersuchungen und40Ar/39Ar-Datierungen aus den mylonitischen Gesteinen belegen zusammen ein rasches Abkühlen von über 450°C auf unter 200 °C im Zeitraum vor 20 bis 18 Ma. Die rasche Abkühlung wird dem vor 20 Ma beginnenden Aufstieg der mylonitischen Gneise am Fuß eines sich entwickelnden flachwinkligen, dehnungsbedingten Abscherungssystems zugeschrieben. Das nordost verwurzelte Whipple-Störungssystem und die Mylonite sind kinematisch gleichgerichtet, sie haben dieselbe Richtung und denselben Schersinn, dennoch haben die Störungen die Mylonite einige Millionen Jahre nach ihrer Bildung überschnitten. Tiefere Plattenmylonite erreichten so die Erdoberfläche und wurden vor 16 Ma erodiert. Die Minimalraten für den Aufstieg der Mylonite und die Bewegungen entlang der Abscherungssysteme für den Zeitraum vor 20 bis 16 Ma liegen jeweils zwischen 3 und 7,2 mm/yr. Dies unter der Annahme, daß die Mylonite in einer Tiefe von mindestens 12 km von einem mit 25° einfallenden Störungssystem geschnitten wurden. Aus den verfügbaren Daten müssen für den Zeitraum vor 20−18 Ma höhere Bewegungsraten angenommen werden. Der kumulative Versatz der Gesteinseinheiten entlang der Hauptstörungen des Whipplesystems scheint damit 40–45 km zu überschreiten.

Résumé

Une série épaisse (>3,5 km) de gneiss mylonitiques allant du facies supérieur des schistes verts au facies moyen des amphibolites affleure sous la faille de décollement de Whipple dans le complexe métamorphique des Whipple Mountains. L'analyse des fabriques et des microstructures de cette série mylonitique montre qu'elle représente une zone intracrustale de flux laminaire non coaxial, avec un glissement prédominant vers le nord-est. Le sommet de cette shear-zone est le front mylonitique de Whipple, qui marque l'apparition brusque (localement en quelques mètres) de la déformation ductile pénétrative, entre une série supérieure cristalline non mylonitique et ses équivalents mylonitiques inférieurs. La mylonitisation, d'âge oligocène-miocène (26±5 Ma) a dû s'effectuer à une profondeur de 16±4 km (4,4±1,1 Kb) et à des températures comprises entre 460° et 535 °C Les traces de fission et des datations40Ar/39Ar montrent que les mylonites ont subi un refroidissement rapide de plus de 450 °C à moins de 200 °C entre 20 et 18 Ma. Ce refroidissement rapide est attribué à la montée, à partir de 20 Ma, des gneiss mylonitiques lors du développement du système de failles de décollement extensionnelles, dont ils formaient le mur. L'ensemble des failles, à pied NE, et les mylonites sont cinématiquement coordonnés; le glissement s'y est effectué dans la même direction et le même sens; cependant, les failles ont coupé les mylonites plusieurs Ma après la formation de celles-ci. Les mylonites de la plaque inférieure ont atteint la surface du sol et y ont été soumise à l'érosion avant 16 Ma. Si on admet une surface de décollement inclinée à 25° coupant les mylonites à une profondeur d'au moins 12 km, les vitesses minimales de la montée des mylonites et du mouvement de long du décollement entre 20 et 16 Ma, ont dû être respectivement de 3 mm/an et 7,2 mm/an. D'après les données fournies par le refroidissement, les vitesses devaient être plus élevées pendant la période de 18 à 20 Ma. Le déplacement des masses rocheuses de long des failles majeures du système de Whipple semble exéder 40 à 45 km.

Краткое содержание

В метаморфном компле ксе гор Whipple Mountains милонитизированные гнейсы верхней фации зеленого сланца и средней фаци и амфиболитов обнаже ны ниже срыва Whipple. Данные ис следования текстуры и мощность (3,5 км) милонитной свиты указывают на то, что зд есь представлены внутрикристалличес кая зона некоаксиально-л аминарного течения при господст ве направления срыва на северо-восток.

Верхняя часть этой зо ны срыва образует фронт милонита Whipple. Она о хватывает непрерывн ой мощностью в нескольк о метров верхнюю гран ицу пластичной деформац ии до явно немилонити зированной свиты кристаллиновы х пород и их милонитиз ированных эквивалентов, распол оженных ниже. Считают, что проц есс милонитизации пр оисходил в олигомиоценовом пе риоде (26 + / -1–5 миллионов лет тому назад) на глуб ине 16 + / - 4 км (4,4 + /-1,1 кб) при температурах между 460–535 °С. Исследова ние следов трещин и датир овка с помощью аргонового метода (40А r/39Аr) милонитизированн ых пород говорит о быстр ом охлаждении за пери од от 20 до 18 миллионов лет с более чем 450 °С до менее 200 °С. Такое быстр ое охлаждение припис ывают поднятию милонитизи рованных гнейсов, нач авшееся еще ранее 20 миллионов л ет тому назад у подошвы систем срыва, простирающихся поло го и вызванных растяжени ем. Система разломов Whippie, связанная с северо-во стоком, и милониты име ют одно и тоже направлен ие и ту же плоскость ск ола, но нарушения разорвали милониты несколько м иллионов лет позже, после их обр азования. Более глубо ко залегающие милониты платформы появились т. о. на поверхности и подв ерглись эрозии в пери од до 16ти миллионов лет тому на зад. Минимальная скор ость подъема милонитов и с двигов вдоль системы сколов в период от 20 до 16 миллионов лет тому на зад составляет от 3 до 7,2 мм/г од. При этом считают, что милониты залегал и, no-крайней мере, на глу бине 12 км и их разрезала сист ема разрывов, имеющая угол падения в 25∘. На ос новании полученных д анных считают, что за период от 20 до 18 миллионов лет тому назад подъем ная скорость была бол ьше. Смещение слоев пород вдоль главного разры ва системы Whipple, кажется, т. о. превосхо дящим 40–45 км.

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Copyright information

© Ferdinand Enke Verlag Stuttgart 1988

Authors and Affiliations

  • Gregory A. Davis
    • 1
  1. 1.Department of Geological SciencesUniversity of Southern CaliforniaLos AngelesUSA

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