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 ⁎ 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 : 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 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 [29–33]. The tectonism with relict Grenvillian metamorphism [15,16]. contrast between the shallower, warmer lithosphere of Locations are shown in Fig. 2. At the southwest end of West Antarctica and the deeper, colder lithosphere of significant rock exposure along this coast, the Reinbolt East Antarctica is evident in all these studies. The Hills are made up of Late Proterozoic with two challenge of determining surface wave structure at a phases of later deformation and metamorphism [17]. resolution that shows detail within East Antarctica is Moving eastwards, coarse-grained granite is exposed at beginning to be addressed through deployments of Landing Bluff [11] and upper amphibolite to granulate temporary broadband instruments. paragneiss at the Larsemann Hills [18]. The Rauer Group contains both Archaean and Proterozoic components with 1.3. Aims of the SSCUA deployment and scope of this a Pan-African metamorphic overprint [19].Atthe study eastward limit of exposed rock along the Princess Elizabeth Land coast lie the Archaean Vestfold Hills The SSCUA deployment aims to extend the coverage [11]. The Grove Mountains are an isolated group of of seismic structure into the interior of Mac Robertson nunataks in the Princess Elizabeth Land interior. They Land (West of the Lambert Glacier) and Princess consist mainly of high-grade felsic gneisses intruded by Elizabeth Land (East of the Lambert Glacier) towards thick granite sheets. In common with the Prydz Bay a better understanding of this key region in Gondwana coastal exposures, metamorphism and emplacement of (Fig. 2). Use of remote broadband seismic recording and granite are likely to have Pan-African age associations receiver function analysis presented in this study allows [20]. a regional-scale investigation of the seismic structure without the need for large-scale, costly refraction lines. 1.2. Previous determinations of seismic structure in The depth of the Moho and other seismic discontinuities East Antarctica may be used to compare and contrast deep crustal structure in relation to the main geological terranes (e.g. The seismic structure of the crust has been [34]). In addition, the determined crustal structures form determined along two reflection/refraction lines cross- baseline information in tracing the path of main tectonic ing the Amery Ice Shelf [21,22] and interpreted and units beneath the Antarctic ice-sheet. The broadband extrapolated in the light of reconnaissance aerogeophy- data are also suitable for the determination of wider- sical data [23] covering the wider Lambert Glacier scale structure using surface-wave methods that are the region [22]. The Moho is found to be 22–24km deep subject of further work. beneath the Amery Ice Shelf, increasing to 30–34km deep on the flanks of what is interpreted as the Lambert 2. Data and methods Graben. Determinations of seismic structure using teleseismic 2.1. Data collection data include an examination of the crust and mantle beneath the Lambert Glacier region of East Antarctica Remote stations of the SSCUA seismic deployment using early reflections of P′–P′ (PKP–PKP) phases were installed at locations shown (Fig. 2). Beaver Lake 48 A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57

(BVLK) station was installed in January 2002 during the 2.2. Receiver function methods initial ‘pilot’ year of field activity and retrieved during the austral summer of 2004/2005. Other stations were Receiver functions provide a means of determining deployed for shorter time intervals: Jacklyn Peak the S-wave structure of the crust and uppermost mantle (JACK) January 2003–December 2003, Fisher Massif immediately beneath the recording station—exploiting (FISH) December 2003–January 2005, Mount Cress- energy from distant earthquakes. The P-wave coda well (CRES) November 2002–December 2003, Grove includes information on the seismic discontinuities that Mountains (GROV) December 2003–January 2005, define this structure (e.g. the Moho) in the form of energy North Mawson Escarpment (NMES) December 2002– that has converted from P- to S-wave as it travels upwards January 2005 and Reinbolt Hills (REIN) January 2003– on the last part of its path from source to receiver (Fig. 3). December 2003. Site access was generally by fixed- This converted S-wave energy may be extracted from 3- wing aircraft, operating from Davis station or the component data by deconvolving the radial, horizontal PCMEGA field camp at Mount Cresswell, landing on component with the vertical component in the frequency the adjacent glacier and moving equipment to the rock domain [35,36]. In this work, receiver functions are by sledge. Helicopters were an alternative means of calculated [37] and then stacked to produce a composite accessing the more northerly sites and REIN station was receiver function with improved signal-to-noise ratio. accessed only by helicopter. Stations were also deployed Stacks are made using receiver functions from all back- at Komsomolskiy Peak (KOMS) and Wilson Bluff azimuths with the inherent assumption that the structure (WILS). It was not possible to reach these stations, and beneath the station is sufficiently close to 1-D for this hence retrieve data, owing to aircraft operational approximation to be valid. Departures from this situation problems (2004/2005 austral summer). At the time of are discussed as they arise. writing these problems are unresolved and data retrieval is not be possible in the foreseeable future. These 2.3. Inversion for structure stations are therefore not included in the analysis presented in this paper. The relationship between the receiver function Each remote station consisted of a high-fidelity waveform and the seismic structure giving rise to this broadband seismic sensor (Guralp CMG-ESP, compact waveform is very non-linear. In this work, an adaptive, type) that was buried in moraine adjacent to outcropping non-linear inversion technique that searches the param- rock. The station at Mount Cresswell was buried in ice eter space using the efficient Neighbourhood Algorithm owing to extensive crevassing which prevented access [38] is used. The 1-D model structure is found which to the nunatak from the aircraft landing area. The corresponds to the synthetic receiver function that most seismic data was recorded using Nanometrics ‘Orion’ closely fits (by a least squares measure) the observed seismographs which employ data cartridges with an receiver function. Departures of the best-fit synthetic internal heating facility. Timing was controlled using a receiver function from the observed receiver function in small GPS receiver. Three 53W photo-voltaic (‘solar’) reconnaissance-level work are generally due to 3-D panels were used to maintain power in three 76A h sealed structure which can only be investigated with a much gel-type batteries. The batteries, voltage regulator, longer station deployment and earthquakes at a range of seismometer ‘break-out’ box and seismograph were back-azimuths. Reverberant structure may also lead to housed in a large insulated case. Seismic recording some parts of the wave-forms which cannot be fitted, generally ceased in late April when the solar panels could since reverberant ray paths are not included in the 1-D no longer supply sufficient power to maintain the battery modelling. Nevertheless, a best-fit, 1-D, S-velocity voltage required by the seismograph (10.8V). Most model (as derived in this study) remains an excellent stations resumed recording in October when the longer starting point from which to investigate the deep crust. hours of day-light on the solar panels raised the battery Examination of the ensemble of possible models that voltage above the restart voltage on the seismograph closely fit the observed receiver function enables trade- (11.8V). offs between parameters and other non-unique aspects of The station at Davis operated successfully from the procedure to be addressed. In previous studies, it has December 2003 using a Guralp CMG-40T sensor and been possible to compare discontinuity depths determined Nanometrics Orion seismograph on mains power. The using receiver functions and the Neighbourhood Algo- seismic station at Mawson is installed in a 10-m deep rithm with those obtained using active source seismic vault and operated on a permanent basis by Geoscience refraction at the same location [39]. Such comparisons Australia. show that receiver functions analysis is a realistic, low- A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 49

3. Seismic structure

3.1. Observed receiver functions

Events that occurred during the deployment and are suitable for receiver function analysis are shown for one of the remote stations, NMES, and the year-round station DAVI (Fig. 4). These were located at an epicentral distance of 30–80° from the station and recorded with a good signal-to-noise ratio. Observed receiver function stacks are shown (Fig. 5) for stations of the SSCUA deployment together with the permanent station, MAW, at Mawson. The short arrows indicate the converted waveform due to the seismic discontinuity at the Moho. Where there is some debate about which discontinuity corresponds to the Moho, alternatives are shown and discussed later in the text. On some station stacks, the converted waveform due to the Moho discontinuity is clear (e.g. REIN), in others (e.g. FISH) it is of lower amplitude. In the case of a lower amplitude conversion, the arrows have been assigned after carefully viewing receiver functions from individ- ual events together with the stacks shown. The conversion due to the Moho can be generally be seen on individual receiver functions and is therefore a robust feature of the stack, even though it is a relatively low amplitude feature. The amplitude of noise preceding the main P arrival (at 0s), together with the number of events in each stack provides a measure of the reliability of the receiver function stacks. Noisiest is JACK, with only two events in the stack (due to the loss of one horizontal component a few weeks after installation). The noise at REIN and MAW is due to their coastal locations. MAW in particular is exceptionally windy. The stations in the Antarctic interior generally produced a better stack (in comparison to MAW) from a smaller number of receiver functions that were recorded over a shorter time period. The receiver function stack for MAW shows several high-amplitude conversions within the first 5s indicat- ing several strong discontinuities within the crust. The peak most likely to correspond to the seismic Moho is at 4.8s. The stack for station JACK shows a similarly Fig. 3. Sketch showing how a 1D seismic structure may be derived reverberant structure with a Moho arrival at 4.9s. BVLK from a teleseismic earthquake: (a) simplified Earth structure and shows a strong discontinuity at 3.6s and a smaller incoming earthquake energy; (b) simplified radial receiver function amplitude conversion at 5.2s. The observed receiver calculated from three-component data; (c) the corresponding derived function stack for station FISH shows a markedly seismic velocity profile (reproduced from [26] with permission from the Geological Society of London). different character, with more subdued discontinuities in structure in the upper crust and a low-amplitude Moho cost alternative and that careful use of the non-linear conversion at 4.4s. The rather different appearance of Neighbourhood Algorithm allows robust determinations the receiver function at CRES is due to the station being of structure to be made in remote locations. located on ice and shows a Moho conversion at 3.9s. 50 A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57

3.2. The effect of an ice-layer

Synthetic receiver functions demonstrating the effects of an ice-layer are shown (Fig. 6). Fig. 6a shows the synthetic receiver function for a Moho at 35km deep, Fig. 6b substitutes a layer of sediment/ regolith with a base at 5km. An ice layer of increasing depth (0.5, 1.0, 1.5 and 2.0km) takes the place of some of this sediment/regolith in Fig. 6c–f. The receiver function for the simple one-layer crust and Moho discontinuity shows a conversion at 3.9s. The corresponding phase arrives later, at 4.6s, with the substitution of the upper 5km of crust with material of lower seismic velocity. The ice layer gives rise to a broad, complex pulse following the initial P-wave arrival. Beneath CRES, the ice thickness is known to be 1.30–1.33km from ice-radar measurements [40] which matches well to a synthetic intermediate to those shown in Fig. 6d and e.

3.3. Results of inversion for structure

Best-fit structures, found using the Neighbourhood Algorithm, are shown in Figs. 7, 8 and 9. If the Moho is not modelled as a clear step in the S-velocity profiles (the broad white lines) then it is interpreted as the base of the velocity-gradient-zone in the lower crust. The receiver function stack from MAW (Fig. 7) reveals a best-fit seismic structure with several crustal disconti- nuities and a Moho at 44km (±2km) depth. The structure beneath JACK is similar, showing marked discontinuities at 12km, 20km and the Moho at 39km. Beneath BVLK, the best-fit structure shows a discon- tinuity at 14km and at 30km with a moderately broad transition at 40–44km deep. The interpretation of these discontinuities is discussed in Section 4. Beneath FISH (Fig. 8), the structure shows slight discontinuities Fig. 4. Distribution of earthquakes recorded during the SSCUA representing small changes in seismic velocity, and the deployment at: (a) NMES, an example of a remote station; (b) DAVI, the year-round station. Earthquakes are indicated by open circles, the Moho in the best-fit model is at 42km (±2km) depth. station by an open triangle and the approximate raypath between them CRES shows a dramatic change in deep structure with a by the solid lines. Moho at 33km (±2km) deep. Upper crustal structure beneath this station is overwhelmed by the part of the receiver function due to the ice-layer and is not On the east side of the Lambert Glacier, towards the significant. The Moho beneath NMES is at a similar north end of the Mawson Escarpment, NMES shows a depth, 34km (±2km) and shows relatively low Moho conversion at 3.2s, and the station in the Grove velocities in the upper crust. Some reverberations are Mountains, GROV, shows a very reverberant structure seen at 8–9s on the NMES receiver function that are not with a Moho conversion at 4.8s. On the margin of Prydz modelled as part of the 1-D structure and have not, Bay, the station at Reinbolt Hills, REIN, shows a clear therefore, been matched by the synthetic waveform. The Moho conversion at 3.9s while the record from Davis, unsatisfactory fit of the waveform around 5s is likely to DAVI, shows little discontinuity in the upper crust and a be due to the influence of 3-D structure in the crust that Moho conversion at 4.6s. is not accounted for in the parameterisation investigated A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 51

Fig. 5. Stacked receiver functions (observed) from stations shown in Fig. 2. The number of events in each stack is indicated below the station code and the arrival due to conversion at the Moho is indicated with an arrow. Where there is some debate as to which discontinuity is to be interpreted as the Moho, two arrows are shown. 52 A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57

4. Discussion

4.1. Depth of Moho and seismic discontinuities within the crust

A summary of Moho depths determined in this study is shown (Fig. 10) together with the Moho depths determined from earlier reflection/refraction work [22,21] and two of the inferred discontinuities from earlier teleseismic data [24]. The locations of the reflection/refraction lines, which were shot toward the northeast, are also shown but extrapolated depths in the southern part of the Lambert Glacier region [22] are not, since the determinations of structure from receiver function analysis provide much stronger controls. The Rayner Terrane stations show modelled Moho depths of 44 (±2) km and 39km for MAW and JACK respectively that are similar to the earlier result of 42km at MAW [25]. We take the intermediate value of 42km as the characteristic Moho depth for the Rayner Terrane. The slight disparity between the receiver function determination of Moho depth at JACK and the depth determined in the earlier reflection/refraction work [21] is probably related to problems associated with depth determination over dipping interfaces in unreversed seismic lines. The receiver function from Beaver Lake station, BVLK, shows a sharp interface at 30km deep that is interpreted as the Moho [22] and is also the most likely interpretation in this study. It is possible that broad transition over the depth interval of 40–44km is a structure related to the base of the crust on the western Fig. 6. Synthetic receiver functions showing the effects of an ice layer. side of the major fault bounding the Lambert transten- Layer thicknesses/depths are given and seismic S-wave velocities are as follows: ice, 1.5–2.0km s− 1; sediment, 2.5–3.0km s− 1; crustal sional basin (also referred to in various studies as the basement, 3.0=3.8km s−1; mantle, 4.4km s−1. Lambert rift or graben). Beaver Lake is close to this fault, possibly located over the fault at depth and the 1D using the Neighbourhood Algorithm. The major features approximation of structure may be an oversimplification of the observed waveform are accounted for and the for this station. The 40–44km broad transition could determination of Moho depth is sufficiently robust for also be a sub-horizontal structure in the upper mantle this reconnaissance-scale survey. The receiver structure possibly related to underplating or collision tectonic beneath GROV (Fig. 9) is highly reverberant with processes. Station FISH, in the Fisher Terrane appears several discontinuities in a relatively low-velocity crust to contrast significantly with the stations in the Rayner and a Moho at 38km (±2km) depth. The negative Terrane (MAW and JACK) in the simplicity of its crustal amplitudes of the waveform at 2.5s and after 6.5s are structure even though the Moho depth is similar. again due to reverberations, not low-velocity layers, and Between FISH and CRES (33km), the difference in have not been matched by the synthetic. Again, the Moho depth is striking. It is likely that the southern Moho depth determination is sufficiently robust to boundary of the Fisher Terrane represents a significant justify the conclusions made in this work. Beneath tectonic boundary. The similarity of Moho depth at REIN, the Moho is very sharp at 30km (±2km) depth, CRES and NMES (34km), across the Lambert Glacier in resulting in a good match between the observed and the north Mawson Escarpment, is consistent with the synthetic Moho pulse at this station. Beneath DAVI, a Lambert Terrane underlying both stations. In this best-fitting crustal structure with a broad high-velocity- proposed interpretation, the Lambert Terrane extends gradient zone above a Moho at 36km depth is found. westward, across the axis of the present-day Lambert A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 53

Fig. 7. Best-fit receiver functions and corresponding S-wave velocity models for stations of Mac Robertson Land. Upper plots: solid line=observed receiver function, broken line=synthetic. Lower plots: heavy white line=best-fit model, fine grey lines=other models searched, fine black lines=limits of models searched.

Fig. 8. Best-fit receiver functions and corresponding S-wave velocity models for stations in the central Lamber Glacier region. 54 A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57

Fig. 9. Best-fit receiver functions and corresponding S-wave velocity models for stations in Princess Elizabeth Land. glacier, at least as far as CRES seismic station (on ice, resolution over the Grove Mountains and hence approximately 15km northwest of Mt. Cresswell nuna- provides no further insight while the (Bouger) gravity tak). Regional scale magnetic and gravity anomaly maps anomaly map mostly reflects changes in ice-loading [23] show notable changes across the approximate region [23]. Beneath DAVI, the Moho depth determined in this of the boundary between the Fisher and Lambert Terranes study is 36km. The complexity of outcrop geology and generally support the interpretation of contrast in between REIN and DAVI, together with the regional Moho depth as indicating a major tectonic boundary. scale magnetic anomaly map [23], suggests that the The receiver function determination of Moho depth Prydz Coast is an amalgamation of smaller blocks such beneath station REIN (30km) is in agreement with the that there is no single significant tectonic or terrane reflection/refraction results. At this point the active boundary between the two stations. source line runs almost parallel to the strike of the gentle Crustal features and Moho depths may be preserved Moho dip and there is no problem with accepting this throughout very long periods of geological time [34], depth estimate from the unreversed line. The difference but later influences on crustal structure must also be in character of the lower crust (low seismic velocity considered. It is arguable that the shallower crustal beneath REIN, high seismic velocity beneath NMES) depths seen at BVLK, CRES, NMES and REIN are due suggests that the deep crust beneath REIN is not related to crustal thinning associated with the failed transten- to that which underlies NMES, i.e. the northern extent of sional Lambert basin. However, the reflection/refraction the Lambert Terrane lies south of REIN. The Moho results [21,22] suggest that the major faults bounding depth from the receiver function at GROV beneath the the ‘rift’ are within the current bounds of the main Grove Mountains (38km) shows a distinct contrast to Lambert Glacier. The only station that might show some NMES. The discontinuity depths inferred from the influence from a linear ‘rift’ is therefore BVLK. It is teleseismic observations close to NMES (25km) and possible that the axis of the transtensional basin curves GROV (49km) [24] provide an independent confirma- westward at the latitude of the Mawson Escarpment. tion that such a contrast exists. The contrast may This scenario provides an alternative explanation for the indicate a significant tectonic boundary, or the Grove shallow crustal depth observed at CRES, although REIN Mountains block may be showing structure attributable and NMES would not be affected [22] and the receiver to more extensive intrusive activity and underplating. function structure characterising the Lambert Terrane The available magnetic anomaly map changes to lower remains appropriate. A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 55

Fig. 10. Summary of Moho depths determined in this study and comparison with depths determined in earlier work [22,24]. The approximate location of the single-channel seismic lines [21,22] which constrain the depth contours near the northern Amery Ice Shelf are shown by A–A′ and B–B′.

Erosion and uplift/subsidence processes may also be boundary. In this case complementary geological and significant in producing apparent structural boundaries, geophysical techniques might show a terrane to be especially if uplift/subsidence occurs in a heterogenous continuous across such an apparent boundary. manner across the terrane under analysis. At the present time, uplift is occurring at a negligible rate across the 4.2. Tectonic implications Lambert Glacier region [41]. Two phases of denudation, Permo-Triassic and Early Cretaceous, have been This study has revealed a major tectonic boundary quantified in the Northern Prince Charles Mountains between FISH and CRES and similarities in structure [3]. The total denudation is of the order of 2–3km away between CRES and NMES. These observations are from the Lambert Glacier, but may be close to 10km consistent with the location of the proposed suture of near Beaver Lake. Should heterogenous uplift or crustal Indo-Antarctica and the Antarctic craton. The Lambert thinning have been significant earlier in the tectonic Terrane could therefore represent a major feature history of the region then contrasts in crustal structure associated with this suture—it follows the location of and Moho depth alone may falsely imply a tectonic the broad grey-shaded band to the north of the Southern 56 A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57

Prince Charles Mountains (Fig. 1). Trans-tensional References extension along the axis of what is now the Lambert Glacier appears to have initiated at the eastern margin of [1] I.C.W. Fitzsimons, Grenville-age basement provinces in East the Rayner Terrane and extended southward, cutting Antarctica: evidence for three separate collisional orogens, Geology 28 (2000) 879–882. through the Lambert Terrane/suture where it turns to [2] S.D. Boger, C.J.L. Wilson, Brittle faulting in the Prince Charles trend east–west and terminating close to the contact Mountains, East Antarctica: cretaceous transtensional tectonics with the Ruker Terrane. related to the break-up of Gondwana, Tectonophys 367 (2003) 173–186. 4.3. Seismic structure and terrane characterisation [3] F. Lisker, R. Brown, D. Fabel, Denudation and thermal history along a transect across the Lambert Graben, northern Prince Charles Mountains, Antarctica, derived from apatite fission track The following seismic structures form a basis for thermochronology, Tectonics 22 (2003) 1055, doi:10.1029/ tracing the extent of the terranes further beneath the 2002TC001477. Antarctic ice sheet in future studies. The Rayner Terrane [4] H.M.J. Stagg, J.B. Colwell, N.G. Direen, P.E. O'Brien, B.J. Brown, is characterised by a Moho depth of approximately G. Bernardel, I. Borissova, L. Carson, D.B. Close, Geological framework of the continental margin in the region of the Australian 42km and significant seismic discontinuities in the Antarctic Territory, Geoscience Australia Record 2004/25. upper and mid-crust (e.g. 10km and 26km at JACK). [5] T. Rivers, Lithotectonic elements of the Grenville Province: The Fisher Terrane also shows a crustal depth of 42km review and tectonic implications, Precambrian Res. 86 (1997) and a more uniform increase of seismic velocity 117–154. throughout the crust. The Lambert Terrane is charac- [6] J.J. Veevers, Pan-Africa is Pan-Gondwanaland: oblique conver- gence drives rotation during 650–500Ma assembly, Geology 31 terised by a shallow Moho depth of approximately (2003) 501–504. 34km with a single discontinuity (at approximately [7] I.C.W. Fitzsimons, A review of tectonic events in the East 12km) in the upper crust. Antarctic Shield and their implications of Gondwana and earlier supercontinents, J. Afr. Earth Sci. (2000) 3–23. 5. Summary [8] I.C.W. Fitzsimons, Proterozoic basement provinces of south- western Australia, and their correlation with Antarctica, in: Y. Yoshida, B.F. Windley, S. Dasgupta (Eds.), Proterozoic of East The depth of the crust to the west of the Lambert Gondwana: Supercontinent Assembly and Breakup, Spec. Publ.- Glacier is approximately 42km with shallower crust at Geol. Soc. Lond., vol. 206, 2003, pp. 93–130. approximately 34km occurring south of the Fisher [9] S.D. Boger, C.J. Carson, C.M. Fanning, J.M. Hergt, C.J.L. Terrane. East of the Lambert Glacier crustal depths are Wilson, J.D. Woodhead, Pan-African intraplate deformation in the northern Prince Charles Mountains, East Antarctica, Earth between 34 and 30km. Beneath Davis, the crust is a Planet. Sci. Lett. 195 (2002) 195–210. little deeper, at 36km and further inland, beneath the [10] S.D. Boger, J.McL. Miller, Terminal suturing of Gondwana and Grove Mountains, 38km. It is likely that a major the onset of the Ross-Delamerian Orogeny: the cause and effect tectonic boundary exists between the Northern and of an Early Cambrian reconfiguration of plate motions, Earth – Southern Prince Charles Mountains that extends across Planet. Sci. Lett. 219 (2004) 35 48. [11] R.J. Tingey, The Geology of Antarctica, Oxford Monographs on the axis of the Lambert basin. Seismic structures Geology and Geophysics, Oxford University Press, UK, 1991. characteristic of the Rayner, Fisher and Lambert Terrane [12] E.V. Mikhalsky, J.W. Sheraton, A.A. Laiba, R.J. Tingey, D.E. have been determined which may be used to trace Thost, E.N. Kamenev, L.V. Fedorov, Geology of the Prince tectonic provinces beneath the ice. Charles Mountains, Antarctica, AGSO—Geoscience Australia, Canberra, Bulletin, vol. 247, 2001. [13] S.D. Boger, C.J. Carson, C.J.L. Wilson, C.M. Fanning, Acknowledgements Neoproterozoic deformation in the Radok Lake region of the northern Prince Charles Mountains, East Antarctica; evidence for Brian Kennett, Seismology Group Leader, and a single protracted orogenic event, Precambrian Res. 104 (2000) technical staff at RSES/ANSIR (Australian National 1–24. Seismic Imaging Resource) are warmly thanked for their [14] S.D. Boger, C.J.L. Wilson, C.M. Fanning, Early tectonism within the East Antarctic craton: the final suture support of this work. Logistics provided by Australian between east and west Gondwana? Geology 29 (2001) 463–466. Antarctic Division are greatly appreciated. Particular [15] P.H.G.M. Dirks, C.J.L. Wilson, Crustal Evolution of the East thanks to Mike Woolridge and many others at AAD Antarctic mobile belt in Prydz Bay: continental collision at Kingston and Davis Station for support over several 500Ma? Precambrian Res. 75 (1995) 189–207. field seasons. Recording equipment was made available [16] I.C.W. Fitzsimons, The Brattstrand Paragneiss and the Sostrene Orthogneiss: a review of Pan-African Metamorphism and through a grant from the ANU Major Equipment Grenvillian Relics in Southern Prydz Bay, in: C.A. Ricci (Ed.), Committee and support provided through an AAS The Antarctic Region: Geological Evolution and Processes, Terra (Australian Antarctic Science) grant (Project #2303). Antartica Special Publication, 1997. A.M. Reading / Earth and Planetary Science Letters 244 (2006) 44–57 57

[17] G.T. Nichols, R.F. Berry, A decompressional P–T path, Reinbolt [28] J.F. Lawrence, D.A. Wiens, A.A. Nyblade, S. Anandakrishnan, P. Hills, East Antarctica, J. Metamorph. Geol. 9 (1991) 257–266. J. Shore, D. Voight, Upper mantle thermal variations beneath [18] C.J. Carson, C.M. Fanning, C.J.L. Wilson, Timing of the the Transantarctic Mountains inferred from teleseismic S-wave Progress Granite, Larsemann Hills: additional evidence for attenuation, Geophys. Res. Lett. 33 (2006) L03303, doi:10.1029/ Early Palaeozoic orogenesis within the east Antarctic Shield 2005GL024516. and implications for Gondwana assembly, Aust. J. Earth Sci. 43 [29] G. Roult, D. Rouland, J.P. Montagner, Antarctica II: Upper- (1996) 539–553. mantle structure from velocities and anisotropy, Phys. Earth [19] S.L. Harley, I. Snape, L.P. Black, The evolution of a layered Planet. Inter. 84 (1994) 33–57. metaigneous complex in the Rauer Group, East Antarctica: [30] M.H. Ritzwoller, N.M. Shapiro, A.L. Levshin, G.M. Leahy, evidence for a distince Archaean terrane, Precambrian Res. 89 Crustal and upper mantle structure beneath Antarctica and (1998) 175–205. surrounding oceans, J. Geophys. Res. 106 (2001) 30645–30670. [20] X. Liu, Z. Zhao, Y. Zhao, J. Chen, X. Liu, Pyroxene exsolution in [31] S. Danesi, A. Morelli, Structure of the upper mantle under the mafic granulites from the Grove Mountains, East Antarctica: Antarctic Plate from surface wave tomography, Geophys. Res. constraints on Pan-African metamorphic conditions, Eur. J. Lett. 28 (2001) 4395–4398. Mineral. 15 (2003) 55–65. [32] A. Morelli, S. Danesi, Seismological imagine of the Antarctic [21] R.G. Kurinin, G.E. Grikurov, Crustal structure of part of East continental lithosphere: a review, Glob. Planet. Change 42 (2004) Antarctica from geophysical data, in: C. Craddock (Ed.), 155–165. Antarctic Geoscience, University of Wisconsin Press, Madison, [33] A. Sieminski, E. Debayle, J-J. Leveque, Seismic evidence for 1982, pp. 895–901. deep low-velocity anomalies in the transition zone beneath West [22] L.V. Fedorov, G.E. Grikurov, R.G. Kurinin, V.N. Masolov, Antarctica, Earth Planet. Sci. Lett. 216 (2003) 645–661. Crustal Structure of the Lambert Glacier Area from Geophysical [34] A.M. Reading, B.L.N. Kennett, B. Goleby, Seismic structure of Data, in: C. Craddock (Ed.), Antarctic Geoscience, University of the Yilgarn Craton, Western Australia, Aust. J. Earth Sci. 50 Wisconsin Press, Madison, 1982, pp. 931–936. (2003) 427–438. [23] D.E. Thost, G. Leitchenkov, P.E. O'Brien, R.J. Tingey, P. [35] C.J. Ammon, The isolation of receiver effects from teleseismic P Wellman, P. Wellman, A.V. Golynsky. Geology of the Lambert waveforms, Bull. Seismol. Soc. Am. 81 (1991) 2504–2510. Glacier-Prydz Bay Region, East Antarctica. Geoscience Aus- [36] S. Stein, M. Wysession M, An Introduction to Seismology, tralia, map (1998). Earthquakes, and Earth Structure, Blackwell Publishing Ltd, 2003. [24] R.D. Adams, Reflections from discontinuities beneath Antarc- [37] T. Shibutani, M. Sambridge, B. Kennett, Genetic algorithm tica, Bull. Seismol. Soc. Am. 61 (1971) 1441–1451. inversion for receiver functions with application to crust and [25] M. Kanao, A. Kubo, T. Shibutani, H. Negishi, Y. Tono, Crustal uppermost mantle structure beneath Eastern Australia, Geophys. structure around the Antarctic margin by teleseismic receiver Res. Lett. 23 (1996) 1829–1832. function analyses, in: J. Gamble, D.N.B. Skinner, S. Henrys [38] M.S. Sambridge, Geophysical inversion with a neighbourhood (Eds.), Antarctica at the Close of a Millennium, . Bull.-R. Soc. N. algorithm: I. Searching a parameter space, Geophys. J. Int. 138 Z., vol. 35, 2002, pp. 485–491. (1999) 479–494. [26] A.M. Reading, Investigating the deep structure of terranes and [39] A.M. Reading, B.L.N. Kennett, Lithospheric structure of the terrane boundaries: insights from earthquake seismic data, in: Pilbara Craton, Capricorn Orogen and northern Yilgarn Craton, A.P.M. Vaughan, P.T. Leat, R.J. Pankhurst (Eds.), Terrane Western Australia, from teleseismic receiver functions, Aust. J. Processes at the Margins of Gondwana, Spec. Publ.-Geol. Soc. Earth Sci. 50 (2003) 439–445. Lond., vol. 246, 2005, pp. 293–303. [40] Neal Young, Volkmar Damm, personal communication (2005). [27] M. Studinger, G.D. Karner, R.E. Bell, V. Levin, C.A. Raymond, [41] P. Tregoning, P.J. Morgan, R. Coleman, The effect of receiver A.A. Tikku, Geophysical models for the tectonic framework of firmware upgrades on GPS vertical timeseries, Cahiers du the Lake Vostok region, East Antarctica, Earth Planet. Sci. Lett. Centre Européen de Géodynamique et de Séismologie, 23 216 (2003) 663–677. (2004) 37–46.