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Structural Investigations in Proterozoic to Lower Palaeozoic Rocks in the Read Mountains and Haskard Highlands of the Shackleton Range, Antarctica

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Structural Investigations in Proterozoic to Lower Palaeozoic Rocks in the Read Mountains and Haskard Highlands of the Shackleton Range, Antarctica Polarforschung 63 (2/3): 63-99,1993 (erschienen 1995) Structural Investigations in Proterozoic to Lower Palaeozoic Rocks in the Read Mountains and Haskard Highlands of the Shackleton Range, Antarctica By Hans-Martin Braun* Abstract: Studies on macroscopic rock deformation and microfabrics of the der Decke hin. Abschließende Heraushebung und teilweise Erosion der Decke basement rocks of the Shackleton Range, Antarctica, confirm two major in den zentralen Read Mountains ("Read-Fenster") sind vermutlich an eine groß• tectonometamorphic events. During the mid-Proterozoic, the basement was räumige, aufrechte Faltung gebunden. Während desselben Zeitraums wurden affected by prograde metamorphism reaching at least amphibolite facies Sedimente in den nördlichen Haskard Highlands unter amphibolitfaziellen Be­ conditions, and syn-metamorphic ductile deformation. In several outcrops in the dingungen deformiert und zu einem Stapel mehrerer Struktureinheiten überein• Read Mountains (southern Shackleton Range), relations between deformation, andergeschoben. Mit Hilfe der Lokalisierung größerer Bewegungshorizonte, auf metamorphism, and intrusive activity are reconstructed in detail. Proterozoic denen NW-gerichtete Überschiebungen stattfanden, und der Untersuchung tectonometamorphism terminated with uplift and erosion of the basement, charakteristischer Mikrogefüge, kann die ursprünglich für diese Region vorge­ followed by deposition of the Eocambrian Watts Needle Formation, a thin schlagene lithostratigraphische Gliederung leicht modifiziert werden. sequence of epicratonic sediments. Information about the early history of basement rocks in the Haskard Highlands (northern Shackleton Range) is not Abschließend wird die strukturelle Entwicklung der Shackleton Range, wie sie as detailed, but a major tectonometamorphic event during the mid-Proterozoic hier erarbeitet wurde, mit Erkenntnissen aus Nord-Viktorialand, dem zentralen can be demonstrated as weil. Transantarktischen Gebirge, und Neuschwabenland (westliches Königin-Maud• Land) verglichen. Nach einer Diskussion der früh-paläozoischen Decken­ During the Cambrian and Early Ordovician, basement and cover sediments in tektonik im Rahmen des Ress-Orogens im klassischen Sinne werden alternati­ the Read Mountains were overthrust by the so-called Mount Wegener Nappe ve Interpretationen aufgezeigt, die sich aus plattentektonischen Rekonstruktio­ (Mt. Wegeuer Formation). Structures in the underlying rocks indicate S 01' SE nen gemäß der neuen "SWEAT"-Hypothese ergeben. transport of the nappe. Final uplift, and partial erosion of the nappe unit in the central Read Mountains ("Mount Wegener Window") are probably related to large-scale upright folding. At the same time, sedirnentary rocks in the northern Haskard Highlands suffered amphibolite-facies metamorphism and intense 1. INTRODUCTION rotational deformation, leading to a stack of several structural units. By defining major zones ofNW directed tectonic transport (thrust planes) and characteristic The Shackleton Range is situated at about 80 "S close to the SE microstructures, the lithostratigraphic setting originally inferred for this region is slightly modified. boundary of the Filchner Ice Shelf (Fig. 1). Results of the first geological studies in the area, including some general descrip­ The structural history of the Shackleton Range, as derived from this study, is tions of tectonic structures, have been published by CLARKSON compared with that of northern Victoria Land, the central Transantarctic Moun­ (1982a, b, 1983), HOFMANN & PAECH (1983), PAECH (1985), and tains, and Neuschwabenland (western Dronning Maud Land). After discussing the early Palaeozoic tectonics of the Shackleton Range within the classical MARSH (l983a, b, 1984). The general pattern of geological units concept of the Ross orogen, alternative geotectonic interpretations are suggested in the Shackleton Range, as derived from these papers, is shown that are based on the hypothetical,"SWEAT" reconstruction of continents. in Fig. 1. Due to the reconnaissance level of exploration, however, interpretations dealing with the tectonometamorphic Zusammenfassung: Untersuchungen makroskopischer und mikroskopischer evolution of the Shackleton Range have remained rather spe­ Gefüge im Grundgebirge der Shackleton Range belegen zwei Hauptphasen tektonometamorpher Aktivität. Parallel zu mindestens amphibolit-fazieller culative. Metamorphose unterlagen die Gesteine im Mittleren Proterozoikum intensiver duktiler Deformation. Für Aufschlüsse in den Read Mountains (südliche During the "Geological Expedition into the Shackleton Range" Shackleton Range) können detaillierte Angaben über die wechselseitigen Be­ (GEISHA) in 1987/88, the author had the opportunity to visit ziehungen zwischen Deformation, Metamorphose, und der Platznahme von Intrusionen gemacht werden. Die proterozoische Tektonometamorphose endet several areas in the Read Mountains (see Fig. 2) to study the hier mit dem Aufstieg und der Erosion des Grundgebirges, und der Ablagerung medium- to high-grade metamorphic rocks of the Read Group, der eokambrischen Watts Needle Formation, einer geringmächtigen Abfolge and sediments of the Watts Needle Formation. In addition, epikratonischer Sedimente. In der nördlichen Shackleton Range (Haskard High­ metamorphic basement and supracrusta1 rocks in the northern lands) sind die Kenntnisse über frühe Ereignisse im Grundgebirge nicht so de­ tailliert wie im Süden, doch kann eine proterozoische Überprägung der Gestei­ Haskard Highlands (Fig. 1) were investigated. The aim of the ne auch hier belegt werden. present paper is to identify and describe structures and fabrics that are characteristic of certain units and tectonometamorphic Während des Kambriums und des frühen Ordoviziums wurden Grundgebirge events. The relative and absolute timing of these events in re­ und Decksedimente der Read Mountains von der sogenannten Mount-Wegener­ Decke (Mount Wegener Formation) überfahren. Gefüge in den autochthonen lation to the overall geo1ogical history of the Shack1eton Gesteinen deuten auf einen von N (-NW) nach S (-SE) gerichteten Transport Range will also be analyzed. * Hans-Martin Braun, Geologisch-Paläontologisches Institut, Johann Wolfgang Goethe­ Universität, Senckenberganlage 32-34, 0-60325 Frankfurt am Main, Germany. Manuscriptreceived 14 April 1994; accepted 30 September 1994 63 0W 0 30 SLESSOR GLACIER 20 W 80°5 80°5 ()\.()~~_:-..:==::::::~ Herbert 50 km ~~:''.';;:- Fuchs Dom~ ~~!I~~~~'\; --==~~~~~~::~~~~~~~~::t~~~3 '~~/' - ...... .s: ~~:::~::~~:: .. :~~~ --::~::~::::::::::::::::::::::::::~::- 7 <;:.,::>: ::i:it»>· ·<·l<..... ·~~ ---:-:-==3=3::::::(;:::- Otter ~ . :~.~ '.<' ':<'.' ···if····' ''1''1~ -_-.., ~ H·19 hlan d s .StephensonR ea d Mountains . Bastion REC0 VER YGLA cis R 81°5 300W 200W 81°5 SOUTHERN SHACKLETON RANGE NORTHERN SHACKLETON RANGE Stephenson Bastion Formation ,. metaclastics (part of former Turnpike Bluff Group) B1aiklock Glacier Group Mt. Wegener Formation, Wyeth Heights Formation clastic sedirnents, not metamorphic ,. metaclastics (part of former Turnpike Bluff Group) Pioneers Group Watts Needle Formation supracrustal metasediments clastic and carbonatic sediments locally possibly interleaved with basement rocks i-. Read Group Metamorphic basement ......... ·.. · CIJ....... metamorphie basement Fig, 1: The Shackleton Range, Antarctica, showing the main topographical features and geologie al units. Abb. 1: Übersicht über Topographie und Geologie der Shackleton Range, Antarktis. 2. TECTONOMETAMORPHIC EVOLUTION OF THE ner Formation represents an allochthonous unit that overrode READ MOUNTAINS both the metamorphic basement and the Watts Needle Forma­ tion sediments, truncating the latter at different levels (BRAUN 2.1 General outline et al. 1988, ROLAND et al. 1988, BUGGISCH et al. 1990, KLEIN­ SCHMIDT & BRAUN 1991, BUGGISCH et al. 1994a, b). The large Data and maps from previous studies provide a rough impres­ block of crystalline rock at the top of Watts Needle and some sion ofthe overall structure ofthe Read Mountains (e.g. CLARK­ smaller lenses at Mount Wegener are thought to have been tec­ SON 1982a). Crystalline rocks of the Read Group form a lenti­ tonically transported within the base of the nappe. The central cular body surrounded by clastic sediments and measuring about Read Mountains today form a classical window structure ("Read 70 km from E to W. The foliation in the basement rocks dips N Window", BUGGISCH et al. 1994a). in the northern part, and S in the southern part. HOFMANN & PAECH (1983) and PAECH (1985), therefore, assumed a large E­ The present study deals with following aspects of the geology W-striking anticline in the Read Mountains. In the central and of the Read Mountains: southern Read Mountains, the contact between the crystalline The structural evolution of the Read Mountains, as derived basement and the sedimentary cover is exposed at only a few from detailed investigations within the crystalline basement locations (Watts Needle area, Nicol Crags, Mount Wegener; and the Watts Needle Formation. Fig. 2). Sediments of the Watts Needle Formation are found The thrust-induced deformation structures in the units below between the Read Group basement and the metaclastic rocks of the Read Nappe. the Mount Wegener Formation. MARSH (1983b), on the basis of The kinematics of nappe transport, as deduced from the cor­ the sequence of lithological units at Watts Needle, was the first responding fabrics of the underlying units. to mention the possibility of thrusting
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