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Reading Epsl 2006.Pdf Earth and Planetary Science Letters 244 (2006) 44–57 www.elsevier.com/locate/epsl The seismic structure of Precambrian and early Palaeozoic terranes in the Lambert Glacier region, East Antarctica ⁎ A.M. Reading Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia Received 29 August 2005; received in revised form 31 December 2005; accepted 16 January 2006 Available online 13 March 2006 Editor: V. Courtillot Abstract The Lambert Glacier region of East Antarctica encompasses the proposed boundary between three of the ancient continents that formed East Gondwana: Indo-Antarctica, the central East Antarctic Craton and a proposed extension of the Pinjarra Orogen of Australia. The only area of extensive rock exposure in central East Antarctica, it uniquely allows the seismic structure to be linked to surface geology. New broadband seismic stations were established at the remote sites of the SSCUA deployment, which ran between the austral summers of 2002/2003 and 2004/2005. Recorded energy from distant earthquakes is used to calculate receiver function waveforms that are then modelled to deduce the seismic structure of the upper lithosphere. The results of this study are two-fold. Firstly, seismic structure and crustal depth are determined beneath the Lambert Glacier region providing constraints on its tectonic evolution. A significant contrast in crustal depth is found between the Northern and Southern Prince Charles Mountains that may indicate the location of a major tectonic boundary. Secondly, baseline seismic receiver structures are established for the Rayner, Fisher and Lambert terranes that may be traced beneath the Antarctic ice sheet in the future. © 2006 Elsevier B.V. All rights reserved. Keywords: Lambert; East Antarctica; seismic structure; receiver functions; terranes 1. Introduction the modern-day Antarctic coastline. These belts may be correlated with outcrops in continents that were joined The concept of East Antarctica as the ancient to East Antarctica within the supercontinent of Gond- keystone at the centre of the assembly of continents wana, moreover, the edges of the belts act as ‘piercing forming Gondwana has been dramatically revised in points’ that precisely constrain the relation of the recent years in the light of new syntheses of geochro- continents prior to break-up. Although there is reason- nological data and geological observations from Africa, able outcrop exposure around the Antarctic coastline, India, East Antarctica and Australia [1]. It is now the mountain ranges surrounding the Lambert Glacier understood that former mobile belts run perpendicular to are the only outcrop in the interior of East Antarctica and thus provide a window into understanding the assembly of Gondwana and earlier supercontinents. ⁎ Corresponding author. Tel.: +61 2 6125 3213; fax: +61 2 6257 This study presents the first determinations of crustal 2737. and upper lithospheric structure in this part of the East E-mail address: [email protected]. Antarctic interior using the techniques of broadband 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.01.031 A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 45 earthquake seismology. Additionally, the correlation of Antarctica including the Southern Prince Charles seismic structure with the terrane boundaries observed Mountains, which lie far inland, to the south of Prydz on the surface represents a significant advance in the Bay. Modern geochronological techniques have shown long-term goal of using earthquake seismology to trace that Grenville-age rocks (1190–980Ma, after the North- major tectonic provinces beneath the ice. American orogen bearing this name [5]), exposed in a wide band around the East Antarctic coast, fall into three 1.1. Tectonic framework of the Lambert Glacier Region distinct age provinces [1]. It has also been established that two, younger, Pan-African-age belts (650–500Ma The Lambert Glacier, the largest in East Antarctica, [6]) truncate the Ancient and Grenville provinces [7]. drains into southern Prydz Bay. It exploits a transten- Ancient cratons and tectonic belts of both Grenville and sional basin that formed during the late Mesozoic during Pan-African ages may be correlated between Antarctica the breakup of Gondwana [2–4]. The location of (the and neighbouring continents in Gondwana: Africa, India future) Prydz Bay is shown (Fig. 1) in the context of the and Australia (Fig. 1) [8]. East Gondwanan continents prior to break-up. The East Antarctic geology and geochronology is consis- region encompasses a rare three-way junction between tent with a tectonic history of ocean closure and Ancient, Grenville and Pan-African terranes and there is subsequent plate-reorganisation with the following pro- considerable ongoing debate concerning its likely posed account [9] relating directly to the region under tectonic evolution. study. (1) Before 600Ma three continental blocks exist; Ancient (>1600Ma, Archaean and Palaeoprotero- one consisting of South America, Africa and Dronning zoic) rocks are found in several locations across Maud Land, another consisting of Madagasgar, India and Fig. 1. The location of Prydz Bay and the Southern Prince Charles Mountains related to their tectonic setting in a reconstruction of East Gondwana [8] and the proposed sutures (grey shaded bands) including that between Indo-Antarctica and East Antarctica [9]. The rectangle represents the area shown in Fig. 2. DML=Dronning Maud Land, M=Madagascar, EL=Enderby Land, MRL=Mac Robertson Land, SPCM=Southern Prince Charles Mountains, PB=Prydz Bay, PEL=Princess Elizabeth Land, DG=Denman Glacier, BH=Bunger Hills, WI=Wilkes Land, MR=Miller Ranges, TAM=Transantarctic Mountains, GC=Gawler Craton. 46 A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 Enderby Land/Mac Robertson Land, and the third, the Terrane boundaries in the Lambert Glacier region are remaining parts of East Antarctica and Australia. (2) At shown (Fig. 2) with province names denoted by upper- 590–560Ma, the African and Indian groups collide along case italics. The Rayner Terrane corresponds to the the Mozambique Suture. (3) From 560Ma, subduction Grenville-aged region of Indo-Antarctica that became along the African/Indian margin leads to convergence part of the Antarctic plate following collision along the with Antarctica/Australia. (4) At 535–520Ma, the proposed Kuunga Suture [10]. Within the Rayner, along African/Indian continental group collides with the the Mac Robertson coast near Mawson station, high- Antarctic/Australian group along the Kuunga Suture. grade granulite facies gneiss, with large orthopyroxene This proposed suture runs west–east, to the north of granite (charnockite) intrusions, is exposed [11]. The the present-day Southern Prince Charles Mountains, Northern Prince Charles Mountains, including Jacklyn and then turns north through Princess Elizabeth Land Peak and Beaver Lake, are similar in lithology but also [10]. include upper amphibolite facies exposures and Permo- Fig. 2. Station locations of the SSCUA deployment and rock exposure localities in the Lambert Glacier and Prydz Bay regions. Where localities have the same name as the station, they are not stated separately. MAW=Mawson, JACK=Jacklyn Peak, BVLK=Beaver Lake, FISH=Fisher Massif, CRES=Mt Cresswell, KOMS=Komsomolskiy Peak, WILS=Wilson Bluff, NMES=North Mawson Escarpment, GROV=Grove Mountains, REIN=Reinbolt Hills, DAVI=Davis. Terrane boundaries are shown by dashed lines where they are constrained by published geological literature (see text). Terrane names are given in upper case italics. Approximate location in relation to the reconstructed Gondwana continents is shown in Fig. 1. Basemap from Australian Antarctic Division. A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 47 Triassic sediments [12]. The Fisher Terrane lies at the from nuclear test sources in Novaya Zemlya recorded at southern boundary of the Rayner and consists of more North American seismic stations [24].Noregular mafic volcanic rocks of much lower metamorphic grade reflecting horizons were found in the upper lithosphere [13,12]. On the east side of the Lambert Glacier, the but this early, novel study provides extends the coverage Mawson Escarpment preserves the boundary between of the early active source work and provides some the Lambert and Ruker Terranes [14]. The Lambert independent determinations of the depth of seismic Terrane is characterised by Pan-African deformation discontinuities observed in this study. Across central with upper amphibolite facies exposures near the North East Antarctica, the following previous estimates of Mawson Escarpment station. The extent of the Lambert crustal structure and Moho depth using receiver function Terrane to the north and (possibly) the west is not analysis have been made: 36km at Syowa, Enderby constrained by surface observations. The Ruker Terrane Land [25]; 42km at Mawson, Mac Robertson Land comprises Archaean granitic basement rocks and [25,26]; 30km at Vostok in the Antarctic interior [27]; Archaean and Palaeoproterozoic metasediments [12]. 34km in the region between Vostok and the Transan- The outcrops of the Prydz Bay coast consist of high- tarctic Mountains [28 and references therein]. grade metamorphic rocks, intruded by granite plutons On a continental scale, deeper lithospheric structure [11]. The outcrops record dominantly Pan-African has been determined using surface waves
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