Journal of the Geological Society, London, Vol. 153, 1996, pp. 65-82, 13 figs, 3 tables. Printed in Northern Ireland
Fission-track evidence for the thermotectonic evolution of a Mesozoic-Cenozoic fore-arc, Antarctica
B.C. STOREY', R. W. BROWN2,A. CARTER2, P. A.DOUBLEDAY'*3, A. J. HURFORD2,D. I. M. MACDONALD'*3& P. A .R.NELL'94 'British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET UK 'London Fission Track Research Group,Research School of Geological and Geophysical Sciences, University and Birkbeck Colleges London, Gower Street, London WClE 6BT, UK 'Present address: Cambridge Arctic Shelf Programme, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 9EU, UK 4Present address: Department of Geology, University of Manchester, Manchester M15 9PL, UK
Abstract: Zircon and apatite fission-track data from the LeMay Group accretionary complex and Fossil Bluff Group fore-arc basin sequence on Alexander Island (Antarctica)record a common regional Cretaceous and Cenozoic thermal and denudational history. With the exception of zircon data from samples closest to the trench, the apatite and zircon central ages are substantially less than known and, inferred stratigraphic ages. Thermal modelling of the data indicate cooling from maximum palaeotemperatures in the range 180-350 "C at c. 100 Ma. A younger period of accelerated cooling occurred between 40 and 35 Ma with final cooling to surface temperatures taking place at reduced rates through the Tertiary. The start of cooling was close in time to the end of deposition within the fore-arc basin and is consistent with structural evidence for Cretaceous deformation in a strike-slip setting. The accelerated, Early Tertiary cooling episode was broadly coeval with, and may have been caused by ridge-trench collisions and cessation of subduction off-shore Alexander Island. Zircon and apatite age data, frommagmatic rocks emplaced in thefore-arc region in Late Cretaceous and Tertiary times, are close to their age of crystallization. This indicates rapid cooling of both fission-track systems from temperatures >350"C to
Keywords: Antarctica, fore-arc basins, fission track, apatite.
Alexander Island is the largest island west of the Antarctic rising to 3000 m above sea level (Fig. 2).The Douglas Range Peninsula (Fig. 1); it represents part of afore-arc region, and its southerlycontinuation, the LeMay Range, flank active in Mesozoic times during E- or SE-directed George VI Sound,a spectacular trough which separates subduction of proto-Pacific ocean floor beneath the Alexander Island from the Antarctic Peninsula. The sound Antarctic Peninsula. Thisfore-arc region preservesa is25-30 km wide and up to900m deep, and has been well-exposed Mesozoic accretionary prism andfore-arc interpreted as a transtensional rift basin formed by dextral basin, and aTertiary igneous suite,intruded and erupted strike-slip motion during Tertiary times (Storey & Ne11 1988; during late-stage arc migration. Detailed stratigraphical and Maslanyj 1991; Nell & Storey 1991). The Douglasand structural work over the past ten years has provideda LeMay ranges may represent a rift shoulder flanking this rift framework to place the results of fission-track analysis in a system. regional tectonic context. The aim of this paper is to The boundarybetween the Mesozoic fore-arc rocks of document the thermal history, both as aconstraint on Alexander Island and the plutonic and volcanic rocks of the models of Antarctic Peninsula tectonic evolution, and as a Antarctic Peninsula (Palmer Land) in theeast is hidden test of more general models of fore-arc regions. beneath George VI Sound.During Tertiary times, the magmatic focus migrated from the Antarctic Peninsula into Regional geology and tectonic setting of Alexander the fore-arc region of Alexander Island, with consequent Island volcanic and plutonic activity (Burn 1981; Storey & Garrett 1985). Subductionalong the western seaboard of the Antarctic Peninsula ceased due to a series of collisions of Tectonic setting the Pacific-Phoenix spreading ridge with the trench (Barker Alexander Island, is about 450 km long and up to 250 km 1982): the age of these collisions decreased progressively wide. Thenorthern part of the island hasdramatic northwardsalong the peninsula aseach segment of the topography with the Rouen Mountains and Douglas Range ridge, separated from its neighbour by an oceanic fracture 65
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and volcanic complex (Burn 1984). It has been interpreted as a Mesozoic accretionary prism, which shows a progressive westward younging in stratigraphic ages (Holdsworth & Nell 1992). The oldest part is pre-Early Jurassic (?Palaeozoic) in age, andthe youngest dated parts tothe west are mid-Cretaceous. (2) The Fossil Bluff Group, which crops out on the east side of the island, represents the sedimentary fill of a coeval fore-arc basin. Itrests unconformably onand in faulted contact with the LeMay Group (Edwards 1979 Ne11 & Storey 1991) and is Mid-Jurassic to Albian in age (Doubleday et al. 1993). (3) Magmatic arc rocks consist of plutonicrocks that intrude (Care 1983), and volcanic rocks that unconformably overlie (Burn 1981) the LeMay Group. A single calc-alkaline dyke of this suiteintrudes the Fossil Bluff Group. These rocks are interpreted as the result of an Early Tertiary arc-migration event. (4)Post-subduction alkali igneous rocks consist of Miocene-age basalts (erupted <7.7 Ma, Smellie et al. 1988) unconformable on uplifted LeMay Group. Similar rocks occur throughout the Antarctic Peninsula and have been related to both a late Cenozoic extensional event (Storey & Garrett 1985), and to the development of a window in the underlying subductingslab (Hole er al. 1991). Several basaltic dykes intrude the southern part of the Fossil Bluff Group. Only one has beendated, it is older thanthe volcanic rocks and gives K-Ara whole rock age of 15 f 1 Ma (Rex 1976).
Neogene alkaline FossilalkalineNeogene Bluff Group Objectives volcanic rocks In an attemptto understand betterthe low-temperature thermal history of Alexander Island, and hence provide + + + + + Cretaceous toTertiary additionalconstraints onthe thermotectonicevolution of calc-alkalineFaults plutonic this fore-arc region, apatite and zircon fission-track analysis were completed on samples from the Fossil Bluff and LeMay groups. Specific objectives were to detect whether Fig. 1. Simplified geological map of Alexander Island showing the the exhumation history of Alexander Island could be location of analysed samples. CH, Ganymede Heights. Inset shows correlated with either the ridge-trench collision and/or the Alexander Island in relation to the Antarctic Peninsula. formation of the George VI Sound transtensional rift basin, and to investigate the thermal effect of arc migration into fore-arc regions. Additional analyses of apatite and zircon zone, collided with the trench (Barker 1982). Offshore fromplutonic rocks of Alexander Island were madeto marinedata suggest collision occurred off southern provide constraints on the age and post-formational cooling Alexander Island between 53.5 & 1 and 45 f 3 Ma, and off history of these magmatic rocks. The time-scale of Harland northern Alexander Island at 32 f 3 Ma (Larter & Barker et al. (1990) is used throughout this paper. 1991). There is no clearly defined topographic onshore expression onAlexander Island of acontinuation of the trace of the Heezen FractureZone separating the above Stratigraphy ridge segments (Fig. 2).However, one possible trace may correspondto the offset betweenDouglas and LeMay ranges and, for the most partseparates the high areas of The LeMay Group northern Alexander Island from more subdued topography in the south (Fig. 2). After ridge crest-trench collision, the The LeMay Group accretionary complex contains trench-fill spreading ridge was probably abandoned. turbidites, trench-slope sequences, and allochthonous slivers of ocean-floor and ocean-island igneousand sedimentary rocks (Burn 1984; Tranter 1987, 1991; Nell 1990; Regional geology Holdsworth & Nell 1992; Doubleday et al. 1994). The Alexander Island (Fig. 1) can bedivided into four main accretionary complex is deformed,and includes belts of geological units. mClange. The trench-fill sequences are mostly comprised of (1) The LeMay Group, forming the structural basement medium-grainedarkose and greywacke interbedded with to Alexander Island, is avariably-deformed sedimentary black shale and siltstone, whereas the trench-slope deposits
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Fig. 2. NOAA AVHRR nearvisible satellite image of Alexander Island illustrating dramatic topography flanking George VI Sound and the possible trace (dashed line) of the Heezen fracture zone.
contain coarse conglomerate-sandstone bodies interbedded the age of the enclosing ocean-floor lavas. Grikurov et al. with thinly bedded turbidite deposits. (1967) reported K-Ar whole-rock ages of 110-105 Ma from polymict sandstones atCorelli Horn (NW LeMay Range), Age of rhe LeMay Group. The age of the LeMay Group is and 165 Ma from arkosic sandstones in the LeMay Range. poorly constrained.Age-indicative fossils recovered from Theyinterpreted all of theseas metamorphic (and hence the accretionary complex include: an Early Jurassic ammon- probablyaccretion) ages. The LeMay Group is unconfor- itefrom accreted tuffs of the Lully FoothillsFormation mably overlain or in faultedcontact with the Fossil Bluff (Thomson & Tranter 1986); Late Jurassicto Early Creta- Group on the eastern side of the island, placing a minimum ceous radiolaria from accreted oceanic cherts in the western age of pre-Mid-Jurassic on theparts of the accretionary Sullivan Glacier area of northern Alexander Island (Hold- complex underlying the Fossil Bluff Group. sworth & Ne11 1992);and mid-Cretaceous radiolaria from similarly accreted cherts in the Havre Mountains and Deb- ussy Heightsareas of northernAlexander Island (Hold- The Fossil Bluff Group sworth & Nell 1992). As all of these are from allochthonous The Fossil Bluff Group (Fig. 3) is upto 7 km thick and units, they can only place maxima on the ages of accretion in consists dominantly of silty mudstone, with significant those parts of the prism, but it is noticeable that they show a amounts of sandstone and conglomerate. Palaeocurrent and pattern of younging towards the margin, consistent with the provenance analyses indicate that the bulk of the unit was accretionary prism model. Holdsworth & Ne11 (1992) calcu- derived from the volcanic arcto the east (Butterworth lated their age of accretion from plate tectonic models as 1991), although there is a minor component of accretionary 95 Ma for the Jurassic-Cretaceous samples, and 86 Ma for complex-derived material,mainly in the western parts of the the later samples assuming that the radiolarian age was also group (Nell & Storey 1991; Doubleday er al. 1993). The
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Stage Unit formations (Fig. 3) were provided by Butterworth et al. 8; (1988), andthe PlutoGlacier and Neptune Glacier 100 formations by Moncrieff & Kelly (1993), while Doubleday et al. (1993) provided details of the lowest two, the Selene Nunatak and AtollNunataks formations. Biostratigraphic Albian 105 control is based mainly on invertebrate macrofauna (Crame & Howlett 1988; Howlett 1989) and some radiolarian faunas (Holdsworth & Ne11 1992). ‘Howlett (1989) published zonal 110 schemes using ammonites and belemnites, which together Pluto Glacier Formation provide an average resolution of about three zones for every 115 1400 m two stages, with the best resolution in the Tithonian and Berriasian stages. Aptian Calliato Cliffa The stratigraphyrecords two major stages in the 120 development of the basin, punctuated by short-duration, tectonically induced events. The Selene Nunatak and Atoll Nunataks formations, exposed on the west side of the basin, 125 record the first stage, the transition from trench-slope to Barremian fore-arc basin sedimentation (Doubleday et al. 1993). These units werederived from the accretionary complex rather 130 Spartan Glacier Formation than the volcanic arc. All other strata in the basin are part of 1000 m Hauterivian the secondstage fore-arc basin sedimentationand form a 135 large-scale, shallowing-upward cycle of Kimmeridgian to Albian age (Butterworth & Macdonald 1991; Butterworth Valanginian 1991). The Ablation Point Formation is restricted to the NE 140 part of the outcrop, and consists of large-scale slide deposits Jupiter Glaciel of sandstone turbidites in a mudstone matrix, interpreted as Berriarian Member collapse of a submarine fan complex in latest Kimmeridgian 145 Himalia Ridge Formation times. Undisturbed strata of the same age suggest that the sliding was a localized, tectonically induced event. Tithonian 150 \ 2200 m The Himalia RidgeFormation, from which the fission-track samplescame, consists of sandstone and Kimmeridgia m conglomerate turbidites, representing a variety of inner fan 155 Oxfordian environments (Butterworth 1991). Its thickness varies from \ 2200 m at itstype area toabout 450111. All the Callovian conglomerates were arc-derived, although there is a change 160 from volcanic-rich at the base to plutonic-rich at the top of Atoll Nunatakr Formation the unit, reflecting incision of the volcanic arc (Butterworth Bathonian 165 A 1050 m 1991). The Spartan Glacier Formation is a unit of slope and outer shelf mudstones, with subsidiary thin sandstone units. Itrepresents basin shallowing (Butterworth & Macdonald 170 Bajocian 1991), and contains a number of submarine slide units up to 50m thick and several km wide thought to have been tectonically induced. The overlying Pluto Glacier Formation 175 Formation Aalenian is similar, but with thicker coarser sandstone interbeds. The ? unit represents inner- to mid-shelf deposition, and is affected Fig. 3. Stratigraphic section of the Fossil Bluff Group based on by majora slump which cuts nearly 250 m of strata Butterworth et al. (1988); Moncrieff & Kelly (1993); Doubleday et (Moncrieff & Kelly 1993). The Neptune Glacier Formation al. (1993). is almostentirely sandstone, and represents a regressive- transgressive cycle from outer shelf to terrestrialdeposits and back to shelfal (Moncrieff & Kelly 1993). Its outcrop is restricted to the southern part of the group. relationship with the volcanic arc is obscured by the Tertiary Anumber of short-duration events including sediment rift now occupied by George VI Sound. sliding and slumping, instantaneous sea-level falls, and Most stratigraphic work hasbeen carried outon the increases in clastic supply may signify tectonic events. Large eastern side of the exposed Fossil Bluff Group rocks, where slide deposits are widespread atthe stratigraphic levels stratarange in agefrom Kimmeridgian-Albian (But- indicatedabove (Macdonald et al. 1993). Theyprobably terworth et al. 1988 and references within); these strata are originated during periods of tectonic instability, although an presumed to represent the middle part of the palaeo-basin, origin due to ‘instantaneous’ lowering of base level during a lying almostequidistant between the volcanic arc and the eustatic sea-level fall cannot be excluded for some of these western edge of that basin. However, recent fieldwork in the deposits. The main sliding eventsoccurred in late western partdemonstrated that deposition of thegroup Kimmeridgian, late Tithonian, Barremian-early Aptian, and spanned Mid-Jurassic (?Bathonian)to Albian times mid-Aptian times. (Doubleday et al. 1993). Formal lithostratigraphic definitions Thereappear to be two records of instantaneous of the Ablation Point, Himalia Ridge and Spartan Glacier widespread sea-level falls. First, the Jupiter Glacier Member
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of the Himalia Ridge Formation is a shallow marine unit volcanic rocks are considered to cogeneticbe and deposited within wave-base; it sits on submarine fan contemporaneous with the plutonic rocks. deposits,and is overlain by deep-water mudstones. Although Butterworth (1991) interpreted this as the result of a eustatic sea-level fall, he did not rule out the possibility Fission-track analysis: methodology and results of it being caused by tectonic uplift of the underlying Fission-track analyses were carried outon apatite and zircon accretionary prism. Second, the base of the Neptune Glacier grains separated from 15 samples from the Fossil Bluff Group, 11 Formation appears to be a synchronous event, reflecting the samples form the LeMay Group,and 12 samplesfrom igneous start of a sea-level fall, althoughwhether this is a global intrusions. The Fossil Bluff Group samples were all taken from the eustatic or local tectonic effect is uncertain. These two Himalia Ridge Formation (Kimmeridgian-Berriasian), the only unit instantaneous sea-level falls occur in latest Berriasian times of basin-wide extent which hasthe correct lithologies forthe and early in the late Albian respectively (Butterworth 1991; extraction of zircon and apatite. Conventional magnetic and heavy Kelly & Moncreiff 1992; Moncreiff & Kelly 1993). liquid mineralseparation techniques, polishing and etching procedures as summarized by Hurford et al. (1991) were used. All sampleswere analysed by theexternal detector method (e.g. Magmatic rocks Gleadow 1981) which allows us to determinethe uranium Plutonic rocks, including the large composite Rouen concentration and age for individual crystals. Mountains Batholith (Care 1983) andnumerous small Sampleswere irradiated using thethermal facilities in the stocks, intrude the LeMay Group throughoutAlexander HERALD reactor, Aldermaston, UK, the HIFAR reactor at Lucas Island. Although some have a marginal foliation, they are Heights, NSW, Australia, and the RiseReactor at theNational Research Centre, Roskilde,Denmark. Neutron fluences were post-tectonic with respect to the deformation of the LeMay monitored using uranium glass dosimeters (Coming CN-1 or CN-2 Group(Care 1983). The exposed magmatic rocks do not for zircon irradiations, and CN-5 or NBS glass SRM 612 for apatite have amarked aeromagnetic signature, although a large irradiations). anomaly exists to the west of Alexander Island close to the Ages were calculated using theIUGS-recommended zeta formertrench site (Maslanyj et al. 1991), possibly calibrationapproach, whereby a proportionality constant zeta is representing unusual near-trench magmatism. The plutonic evaluated by multiple analysis of mineral standards of known age rocks vary in composition from gabbro to granite and have a (e.g. Hurford & Green 1983). Zeta values used in this study have broadly calc-alkaline chemistry (Care 1983). The intrusions been determined from repeated measurements of standard apatites are generally considered to represent the Tertiary migration and zircons fromthe Fish Canyon Tuff, Colorado;the Durango of subduction-related igneous activity intothe fore-arc deposit, Cerro de Mercado, Mexico; the Mt. Dromedary banatite, region prior to cessation of subduction (Burn 1981; Storey & New South Wales, Australia; the Buluk Member tuff, Kenya; and Garrett 1985). Alate adamellite phase of theRouen the Tardree Rhyolite, Co. Antrim, N. Ireland (Hurford 1990). The Mountains batholith from NW Rouen Mountains gave a six analyses reported in Tables 1, 2 and 3 were conducted over several point Rb-Sr isochron age of 46 f 3 Mawith a verylow years by different analysts utilizing different irradiation facilities. initial "Sr/"Sr ratio of 0.7030 (Pankhurst 1982). A more Consequently, the zetacalibration values appropriate foreach recent seven point Rb-Sr isochron (high MSWD 10.8) gives analysis vary and these are listed in the data tables. an age of 56 f 4 Ma with an initial X7Sr/X'Srratio of 0.7045 Spontaneous and induced fission-track densitieswere deter- (Kamenov & Lilov 1989). Although validity of the Kamenov mined for individual crystals using Zeiss Axioplan microscopes with & Lilov isochron is questionable since it comprised analyses X100 objectives (dry for apatite and oil immersion for zircon) and a of samples from widely spaced localities and included late total magnification of X1250. Computer controlled stages were used to effect the rapid translationbetween crystal and mica detector dykes andaplites, new U-Pb zircon data, based on four with a precision of c. 1 pm. Only crystals with prismatic sections zircon fractions, plot on a discordia with concordiaa parallel tothe c-crystallographic axis were acceptedfor analysis, intercept of 55.9 f 2.5 Ma (I. L. Millar & J. McCarron, pers. these crystals having a high etching efficiency; for such crystals a com. 1994) close to the Rb-Sr age. geometry factor of 0.5 for the relationship between spontaneous and Numerous dykes, commonly hornblende-bearing induced track densities has been shown to be appropriate. porphyries up to 10-m wide, with sharp, chilled contacts Horizontal confined fission-track lengths (e.g. Laslett et al. 1987; intrude the LeMay Group. Dykes also intrudethe Rouen Gleadow et al. 1986) were measured in the apatite crystals using a Mountains batholith and one known calc-alkaline dyke, with drawing tubeand digitizing tabletcalibrated against stagea a K-Ar whole rock age of 41 f 1.5 Ma (sample KG.3656.4, micrometer. Individual track length measurements were determined I. L. Millar pers. corn.), intrudes the Fossil Bluff Group. The with a precision of c. 0.3 pm. However, reliable estimates of the Fossil Bluff Group is also intruded in the SE part of its mean track length andthe distribution of lengths could only be outcrop by several alkali camptonite dykes part of the made on a few of the samples (where 2100 individual track lengths post-subduction alkali igneous suite. These dykes trend at a were measured), because of thegenerally very low spontaneous high angle to George VI Sound; no similar dykes have been track densities within the apatite grains (c. 0.05 to 0.5 X 10' cm-'). found in the LeMay Group, suggesting that the dykes may be related to the transtensional rift formation of George VI Sound. A single K-Ar whole-rock analysis gave an age of Data presentation 15 f 1 Ma on one of these dykes (Rex 1976). Tables 1, 2 andpresent3 the analytical dataand Subaerial volcanic rocks, considered to be cogenetic and fission-track ages and mean track lengths measured for the contemporaneous with the plutonic rocks, restunconfor- Alexander Island samples. The ages presented in Tables 1,2 mably on uplifted LeMay Group. K-Ar whole-rock analyses and 3 are termed central ages, following Galbraith (1992). suggest eruption ages of 60-40Ma (Burn 1981), and The conventional age calculation (e.g. Naeser 1967; Hurford 65-40Ma (Kamenov & Pimpirev 1992), with a further & Green 1982) makes the implicit assumption that all K-Ar biotite age of 69Ma (Grikurov et al. 1967). The crystals possess a single, heterogeneous age population, and
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Table 1. Fission track-dara measured for the Fossil Bluff Group samples, Alexander Island, Antarctica
Sample Locality Mineral Strat. P, P, Pd Dosimeter Px2 RE FT Length SD an d age andno. age and (4 (4) (nd) glass & (70) central (pm) lithology (Ma) no. of Zeta (ilu) age *lu crystals (no. (Ma) tracks)
KG.3231.1 Ganymede Heights 141 Apatite 1.267 CN-5 60 0.1 33f4 C. 2150111 Sandstone 18 (5266) 339 * 5 Zircon 0.757 CN-l 20 8 90 +S 14 (2005) 115+3 KG.3215.11 Ganymede Heights 143 Zircon 11.455 CN-2 90 n 97+7 Sandstone In (4788) 135 f 7 KG.3076.41 Ganymede Heights 145 Apatite 2.291 SRM 612 95 n s2+9 C. 1850 m Sandstone 12 11094 354 * 5 KG.3465.1 Ganymede Heights 147 Zircon 0.899 0-2 60 n 129*11 Sandstone 4 (3020) 132 f 5 KG.3465.2 Ganymede Heights 146 Apatite 2.154 SRM-612 80 0.2 62* m Too low C. 1750 m Sandstone 14 (9895) 354 f 5 density Zircon 0.899 CN-2 60 0 115*l4 5 (3020) 132 + 5 KG.3069.2 Ganymede Heights 147 Apatite 2.291 SRM 612 70 n 36*4 15.23 f 0.431.71 c. 1550 m Sandstone 13 l1094 354 * 5 (17) Zircon 0.756 CN-l Track densities (p) are as measured and are (lOh tracks/cm*); numbers of tracks counted (n)shown in brackets. All analyses by external detector method using 0.5 for the 4x/2n geometry correction factor. Pxz is the probability of obtaining X* value for U degrees of freedom where U =no. crystals - I. RE is relative error on central age-see text for discussion. thus spontaneous andinduced counts from all crystals are conventional age becomes inappropriate.Use of a X* test pooled. In many cases this basic assumption is invalid, in (Galbraith 1981) provides ameans of assessing whethera particular where crystals frommultiple provenances in a data set contains error additional to that allowed for under single sediment are present, or the differential annealing of the conventional Poissonian statistics. The central age used apatite crystals with differing chemistry produces a spread of here is a modal age, weighted to account for the differing resetages (e.g. Green et al. 1989a). For such cases, the precision of the individual crystal ages. Twouncertainties Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 ANTARCTICFORE-ARCEVOLUTION, FT EVIDENCE 71 Table 2. Fission track-data measured for the LeMay Group samples, Alexander Island, Antarctica ~ Sample MineralLocality p, P, P* Dosimeter Px2 RE FT Length SD no. and and (n,) (4 (nd) glass & (%) central (pm) lithology no. of Zeta age flu crystals (Ma) (no. tracks) KG.2200.1 Charcot Island Apatite 1.113 4.491 1.264 CN-5 1s 8 54 f 2 13.28 f 0.17 1.66 Sandstone 20 (834) (3365) (10508) 345 f 10 (100) Zircon 8.1 11 1.862 0.366 CN-2 80 0.1 106 i6 20 (2221) (510) (251 1) 134 * 5 KG.2204.1 Charcot Island Apatite 0.798 3.805 1.254 CN-5 8 11 46f2 13.58 f0.19 1.53 Sandstone 17 (519) (2475’1 ( 10508) 345 f 10 (66) Zircon 8.935 2.054 0.357 CN-2 7 16 102 *8 10 (857) (197) (2511) 134f 5 KG.3368.1 NE LeMay Range Zircon 9.349 1.684 0.455 CN-2 90 0 168 f 12 Sandstone 10 (1421) (256) (4785) 135f5 KG.3391.13 NW LeMay Range Apatite 3.492 10.30 1.646 CN-2 Zircon 16.19 6.655 0.358 CN-2 ~ 0 58f11 2 (90) (37) (2511) 134f5 KG.3716.6 N Debussy Heights Apatite 0.395 2.075 1.302 CN-5 10 28 42f9 Too few granite clast from 5 (57) (300) (7218) 339 f 5 tracks conglomerate Zircon 7.259 1.588 0.364 CN-2 7 17 107 f 10 10 (1 120) (245) (2511) 134f5 KG.3785.1 W Douglas Range Apatite 0.704 4.559 1.246 CN-5 40 6 33 *2 14.41 f 0.16 1.31 Sandstone 20 (407) (2636) (10508) 345 f 10 (72) Zircon 15.90 2.134 0.362 CN-2 30 8 l78 f 13 10 (1945) (261) (2511) 134f5 KG.4275.16 Mt Umbriel Apatite 0.552 2.328 1.162 CN-5 S0 2 4812 13.38 f 0.24 2.39 (1360 m) 20 (501) (2113) (8105) 349 f 5 (100) Sandstone Zircon 13.16 3.301 0.536 CN-2 Notes as for Table 1. attach tothe central age: the f indicates the analytical distributions (presented as histograms) for a representative precision, whilst an estimate of the age dispersion (or the selection of the data from Alexander Island are presented in spread of the individual crystal data), is given by the relative Figs 4 and 5. standard error of the central age (RE in data tables). Where this dispersion is low (<10%) a single age population is indicated, and the central and conventional ages converge. Fossil Bluff and LeMay Groups All uncertainties arequoted at 1u level unless otherwise stated. Zircon fission-track results The count data are more readily digested by use of the radial plot of Galbraith (1990), a graphical device for Zircon fission-track central ages for the sedimentary rocks comparing crystals of differing ages and differing precisions. rangefrom 78 + 11 to 129 f 11 Ma for the Fossil Bluff The position on the X scale recordsthe uncertainty of Group (Table 1) and 102 f 8 to 178 * 13 Ma for the LeMay individual grain age estimates (dependent upon the number Group (Table 2). These ages are all older than the apatite of tracks counted), whilst each point has the same standard fission-track ages. With the exception of the two samples error in the y direction (indicated by the vertical +2 to -2 from Charcot Island (KG.2200.1 and KG.2204.1), in the bar). The plot also includes a circular age scale such that the extreme west of thestudy area,and two of the LeMay age of any crystal may be determined by extrapolating a line Group samples from the highest elevations (KG.3785.1 and from the origin throughthe crystal’s x,y coordinates to KG.3368.1), the zircon FT ages are all significantly younger intercept the age scale. Radial plots and apatite track length than the depositional age of the host rocks. There is also a Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 72 B. C. STOREY ET AL. Table 3. Fission track-data measured for igneous intrusions, Alexander Island, Antarctica Sample Locality Mineral PS P, Pd Dosirneter P,yz RE FT Length SD no. and and (n,) (4 (4 glass & (%) central (pm) lithology no. of Zeta age *la crystals (no. (Ma) tracks) KG.3776.1 Rouen Mts Apatite 0.447 3.838 1.264 CN-5 85 0 25*2 13.71 zt0.39 1.40 Granite 20 (240) (2059) (10508) 34s * 10 (11) Zircon 11.65 5.597 0.363 CN-2 regional correlation between sample elevation and zircon FT of some 150 km from Ganymede Heights (the type area of age, with theolder zircon FT ages occurring at higher the formation) in the north, through Planet Heights, to the elevations (Fig. 6). Quadrangle and Offset Ridge in the south (Fig. 1). Detailed The moderate to low relative errors (RE Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 ANTARCTICEVIDENCEFORE-ARCEVOLUTION, FT 73 KG.37841 Aptb. nmm KG.4270.54 Apttl. M. zircon Mean 14 79 f 0.17 pm Central Age: 57 f 4 Ma Centd Age: 57 f 3 Ma y. Mean. 13.95 f 0.1 8 pm Central Age: 48? 5 Ma Central Age: 110 f 5 Ma Sld Dev.: 1.03pm P(@) 14 % RE 18% P(x2) 6 % RE 11 % sm. N.: 1.78 pm ~(~2) 14 U 11 U Track length (pm) % relativeermr % relative error KG.4275.16 Zhon Central Age: 138t 10 Ma KG.2317.3 Aptb zlrcon sm. mw 2.39 @m P(~J47 % RE 3 % P(x2) 2 % RE 16 % Mean: 14.50 f 0.30 pm Central Age: 59 f 7 Ma centra Age: 9 f 3 Ma M. 40 r L of tracks: 100 X Xtals: 20 L6 sm. mv.: 1SI pm P(XZ) % RE 39 % P(x2) 69% RE 1 % X d tracks:25 X Xtals: 20 Track lengthTrack (pm) % reiahveerror % relahveerror Track lengthTrack (pm) % relahw error %relative em M. KG.3463.2 Apatite M11 Zlrcon Central ~ge.92 f 5 Ma Sld. Dev.: 1.83 pm P(x2) - - - - z 20 . .. . 10 pm,, , l? I0 15 20 ..- Track length (pm) % relativeerror % relative erml Track length (pm) % relalive ermr % relatirr,(Km KG.2200.1 Aptit. zlrcon Mean 13.28 f 0.17 pm Centrai Age: 54 f 2 Ma Central Age 106 f 6 Ma Fig. 5. Histograms of apatite track length data and radial plots of Std. De": 1.65pmP(@) 13% RES% R 'O r X of tracks: ?W X Xtals: 20 zircon ages for representative samples of magmatic rocks. track lengths was obtained. Mean track lengths for 8 Fossil Bluff Group samples vary between 13.87 f c. 0.30 pm Tr& lengmTr& (v) % relahve error % ralatwe ermr (n = 105) and 15.23 *0.43 pm (n = 17) with standard Fig. 4. Histograms of apatite track-length data and radial plots of deviations of0.92 to 1.95 pm, and for 5 LeMay Group apatite and zircon ages for representative samples KG.4270.54 and KG.3463.2 (Fossil Bluff Group), and KG.4275.16 and KG.2200.1 (LeMay Group). Shaded segment on zircon plot indicates stratigraphic age of sample. See text for interpretation of radial plots. 1400 1200 -E 200k74Ma) rangebetween 12+8 and94*65Ma (IF). v g 1000 Despite this large range the majority of individual crystal .- ages are between 30 and 60 Ma. Moreover, all single-crystal c apatite ages have large analytical uncertainties, reflecting the -2 800 low track numbers counted in each crystal. W 600 The generally low relative errors and P(x') values 25% - Q indicate that the range of grain ages measured within the E 400 majority of apatite samples is consistent with the range (21 v) expected from a population of grains with a single common 200 age. Only four apatite analyses yielded Pk') values 55%0 and relative errors ?lO%,and within all of these the 0. . statistics are dominated by only oneor two grains. For 0 20 40 60 80 l00 l20 140 l60 180 200 example, if the age of sample KG.4270.54is recalculated Fission Track Age (Ma) excluding the single grain with an age of c. 20 Ma (see radial plot shown in Fig. 4) a P(x') value of 25% and a relative 0 Apatite FBG, 0 Apatite LMY, W Zircon FBG, 0 Zircon LMY error of 9% is obtained. Consequently, we do not regard the Fig. 6. Plot of approximate sample elevation and fission track ages few crystal ages (less than 5% of the data) which do appear for LeMay Group (LMY) and Fossil Bluff Group (FBG) samples. to be younger (or older) than the central apatite FT age of The shaded areas represent the possible range of stratigraphic ages the host sample to be significant. for the LeMay Group and the sampled part of the Fossil Bluff Duetothe low spontaneous track densities and Group. The diagonal lines represent the time of deformation of the consequent paucity of horizontal confined tracks only a Fossil Bluff Group (c. 100 Ma) and the times of ridge-trench limited amount of information about thedistribution of collisions off northern (N) and southern (S) Alexander Island. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 B. C. STOREY ET AL 2500 1- E- 2000 91 8 C 2 1500 E P 0B 2 1000 8 B L 500 P 8 Fig. 7. (a) Stratigraphic section of Himalia Ridge Formation with zircon and apatite fission-track ages. (b) Sketch cross-section of Himalia Ridge (low ground) and Leda Ridge (high ground) at Ganymede Heights showing four prominent conglomerate units and the location of fission track samples. Cross- section drawn from unpublished field data collected by P.J. Butterworth and P.J. Howlett. samples from 13.28 f 0.17 pm (n = 100) to 14.41 f 0.16 pm for stability of tracks in apatite and zircon, is not readily (n = 72) with standard deviations of 1.31 to 2.39 pm. Track explicable. lengthhistograms for thosesamples where 2100 track The range of single grain apatite ages and confined track lengths were measured are shown in Fig. 4. For these four lengths for representative samples are shown in the radial samples the distribution of track lengths is dominated by a plots and histograms inFig. S. The distribution of apatite mode of c. 14 pm. However, all four samplescontain a track lengths is narrowand unimodal with meanlengths varying but distinct proportion of highly annealed tracks between 15.18 f 0.12 pm (n = 100) to 13.25 f 0.39 pm (c. S-l0 pm). (n = lS), and standard deviations of 0.93 to 1.54 pm (Table 3). Zircon analyses show a smaller spread of central ages, Igneous intrusions with sample ages ranging from 46 f 6 to 80 f 8 Ma (20). Most zircon analyses have moderate relative errors (Table Zircon and apatite fission-track results 3). Apart from sample KG.2705.13 and KG.3776'1 coexisting zircon andapatite from the remaining igneous Apatite and zircon were analysed (where present) from 12 rocks yielded concordant fission-track ages (within 1v). samples of plutonic and porphyrydyke rocks from throughout Alexander Island. Nine apatite analyses yielded Interpretation of fission-track-results central ages ranging between 25 f 4 Ma and 81 f 42 Ma (20) (Table 3). In addition, apatite from sample KG.2705.13 gave Fossil Bluff and LeMay groups a seemingly anomalousage of 114 f 26 Ma (20), sig- nificantly older than the sample zircon age. Thisresult, Zircon fission-track results. Vitrinite reflectance measure- inverted with respect tothe normally acceptedthresholds mentsmade on both the LeMay and Fossil Bluff groups Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 ANTARCTICFORE-ARCEVOLUTION, FT EVIDENCE 75 sedimentary rocks (Doubleday 1994) provideestimates of calibration of Sweeney & Burnham (1990) the latter value maximum palaeotemperatures relevent to interpreting the indicates maximum temperatures of 185-200 "C for heating zircon FT results. The most significant observation is that times of 10-50 Ma, sufficient only for partial annealing of there is no correlation between stratigraphic level and R, zircon. This interpretation is supported by the very low values. Furthermore, the iso-reflectance lines are near hori- Pk') value of 2% and moderate relative error (RE of zontal (dipping gently to the east) and consequently cross- 16%) which indicates that the range in single crystal zircon cut the regional structural and stratigraphic units (Double- FT ages from this sample is not consistent with a single age day et al. 1993). This clearly indicates that maximum palaeo- population. temperatures were attained after maximum burial and major The samples with mid-Cretaceous zircon FT central ages deformation. The second important observation is that occur in three geographical areas. From east to west these measured values of R. range from 1.75% to greater than are; Douglas Range, Debussy Heights areaand Charcot 7.5% (Debussy Heights area), with the lower values gene- Island (Fig. 2). The samplefrom the DouglasRange rally occurring at higher elevations. These values of R. (KG.3519.14) came from the hanging wall of majora indicate maximum palaeotemperatures of between c. 170 "C west-directed out-of-sequence thrust. The thrust has several and significantly greater than 350 f 50 "C, which is the upper kilometres of displacement,disrupts earlier-formed accre- limit of fission-track stability in zircon for heating times of tionary structures, and places greenschist facies rocks over the order of 10' years (Yamada et al. 1995). This implies that prehnite-pumpellyite facies rocks. The metamorphicgrade presently outcropping rocks across Alexander Island should of the sample precludes a provenance age and most likely exhibit arange of zircon FT ages representing various represents cooling associated with this out-of-sequence degrees of post-depositional thermal annealing: from partial thrust. through to total annealing. The zircon FT ages of samples from Debussy Heights The samples with oldest (Middle Jurassic) zircon central area most likely also represent cooling after accretion. The ages are from the LeMay Group at the highest elevations mid-Cretaceous ages (107 f 10 Ma) are less than the age of within the LeMay and Douglas ranges, onthe exposed nearbyallochthonous Kimmeridgian to early Valangian eastern margin of the complex. This is in general agreement (c. 153-137 Ma) andAlbian-Cenomanian (112-90 Ma) with the observation thatthe lower Ro values are also cherts, and are similar to the time that Holdsworth & Ne11 recorded from the highest elevations (Doubleday 1994). The (1992) predicted that these portions of oceanic crust would Fossil Bluff Group unconformably overlays the LeMay have beenaccreted to Alexander Island (95 f 7 Ma and Groupand provides a minimum EarlyJurassic age of 88 f 9 Ma respectively). The extremely high minimum accretion for these rocks. Modal analysis of the LeMay vitrinite reflectances of 7.5% from a sample in this region Group sandstones by Tranter (1991) indicates that they were indicate maximum palaeotemperatures well in excess of derivedfrom erosion of adisected magmatic arc and the 350 "C for a heating period of 10-50 Ma (using the model of zircon ages could conceivably represent a provenance age. Sweeney & Burnham 1990) which would have been However, the analysed samples range in metamorphic grade sufficient to completely anneal all pre-existing zircon from prehnite-pumpellyite to greenschist facies, and vitrinite fission-tracks. reflectance data, fromsamples close to KG.3368, suggest This interpretation is in marked contrast to that of the complex patterns with minimum mean reflectances of about samplesfrom Charcot Island. These samples come from 4.5%, but with other higher-gradeinherited components. substantially closer to the trench than the above samples and For a heating period of 10-50Ma, this indicates maximum should represent material accreted after 88 f 9 Ma ago. The palaeotemperatures of c. 290 f 10 "C using the calibration of fission-track ages (106 f 6,102 + 8 Ma) are older than this, Sweeney & Burnham (1990). suggesting that the zircons have not been reset since The results of laboratoryannealing experiments deposition and that the zircon FT ages reflect provenance (Yamada et al. 1995) indicatethat fission-tracks in zircon ages. It is probable that in this region, near the outer edge of will experience significant thermal annealing at temperatures the prism, material was accretedat shallow levels and 2200 f 30 "C but will only be completely annealed at consequently not heated sufficiently to reset the zircons. If temperatures of 350 * 50 "C (for heating times of the order the zircon ages do reflect provenance ages, they could either of10' years). This suggests that, even in the lower grade be the age of arc-derived materialreaching the trench or rocks, inherited zircon fission-tracks are likely to have alternatively they could reflect the age of recycled material experiencedsevere and possibly completeannealing. The from the older inner parts of the accretionary prism. This zircon FT ages therefore reflect different degrees of thermal latter alternative is perhaps the most likely inview of the annealingrangeata of elevatedpalaeotemperatures, similar mid-Cretaceous zircon FT ages from western following accretionary-related tectonic burial andheating, Alexander Island reflecting denudation of the prism at this rather than representing a provenance age. time. The ages from Charcot Island are within error of this The highest and structurally shallowest sample event. (KG.4275.16) from the summit of Mount Umbriel (1349m), Together, the zircon FT results provide a regional record southern Alexander Island hasacentral zircon age of of the cooling history of Alexander Island. The range of 138 f 10 Ma. The sample was collected close to the faulted zircon FT ages is interpreted here to represent differing contact with the Fossil Bluff Group fore-arc basin, and degrees of thermalannealing, with the youngest ages of comesfrom a little-deformed sequence at high structural c. 100 Ma indicating complete annealing and older samples levels in the LeMay Group. The accretionary model predicts only partial annealing. The youngest zircon FT ages of that the LeMay Group here should be some of the oldest c. 100 Ma therefore provide a maximum estimate of the time accretedmaterial. However, a mudstone sample from the of cooling below 350 f 50 "C, the maximum temperature at same locality has a vitrinite reflectance mean of 2.4%, with a which zircon fission-tracks are preservedfor geological first peak value of approximately 2.2%. Using the heating times (Yamada et al. 1995). The two samples Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 76 B. C. STOREY ET AL. fromCharcot Islandprobably represent material eroded new fission-tracks would have beenpreserved within the fromthe elevated eastern flank of Alexander Island at apatites until the rocks finally cooled below 110 f 10 "C. A c. 100 f 10 Ma, and subsequentlydeposited along the maximum estimate of c. 100 Ma for this time is provided by outboard trench to the west. the youngest zircon FT ages, anda minimum estimate of c. 30 Ma is provided by the youngest apatite FT ages. Apatitefission-track results. The vitrinite reflectance data The presence of highly annealed tracks (5-10 pm) within and zircon FT results provide an important framework for the tracklength distributions (Fig. 4) implies thatthe interpreting the apatite FT results. These data indicate that measured apatite FT ages do not directly date the time of all the analysed samples experienced maximum palaeotem- cooling below 110 f 10 "C. However, the apatite track length peratures >c. 170°Cup until c. 100 Ma. Fission tracks in information can be used to obtain a more precise estimate of apatiteare completely annealedat temperatures greater the time of cooling below 110 f 10 "C in addition to a than 110 f 10 "C, so any inherited fission-tracks within the reconstruction of the cooiing history below this temperature. detrital apatites would have been erased. Furthermore, no Twosamples were selectedfor detailed thermal history analysis; sample KG.4275.16 andKG. 3463.2. Theywere chosen because both samples yielded reliable track length information (2100 tracks) and they also represent the upper and lower limits of the range of apatite ages measured. Thermal histories which best predict the observed apatite fission-track parameters were calculated forboth samples, using theapatite track-annealing model of Laslett et al. (1987), Duddy et al. (1988), and Green et al. (1989b), and an optimisationprocedure developed and described by Gallagher et al. (1991). This procedure involves searching a specified T-tspace (these limits are shown as the dashed boxes onthe thermal history diagram in Fig. 8)for an optimal thermal history. The modelled thermal histories and acomparison of the predicted and observed fission-track parameters for both samples are shown in Fig. 8. The modelledthermal history for sample KG.4275.16 indicates that final cooling below c. 110°Coccurred at ApatiteAge Mean Length Std. Dev. c. 90 Ma, thus supporting the evidence for the major cooling Measured:48 2 2 Ma 13.38 f 0.24 pm 2.39 pm episode at c. 100 Ma indicated by the zircon data. However, Predicted:47 Ma the most distinctive aspect of the modelled thermal histories 13.702.36pmpm forboth samples is the markedincrease in therate of cooling between 40 and 35 Ma. Moreover, the similarity in 30 the form of the two thermal histories demonstrates that the full range of observed apatite ages and track lengths can be 20 explained by the same simple cooling history. The younger KG ,4275.16 ages simply reflect slightly higher palaeotemperatures (by 10 10 to 15 "C)existing immediately prior to accelerated cooling at c. 40 Ma. - To test this interpretation of the thermal history further 5 10 0 5 15 20 apatite fission-track parameters were calculated for six Track length (pm) hypothetical samples representing a vertical crustal profile of c. 1 km. The model thermal histories for the six samples are shown inFig. 9. Analysis of the vitrinite reflectance data ApatiteAge Mean Length Std. Dev. indicatesa palaeogeothermal gradient of c. 45"C km-' Measured:34 f 7 Ma13.87 ? 0.30 pm 1.83 pm (Doubleday 1994), and this value was assumed in the Predicted: 36 13.98Ma pm 1.93pm construction of the modelled thermal histories. The two historiesrepresentative of samples KG.4275.16 and v) 30- KG.3463.2 are indicated by the bold curves and the model Y results are presented in Fig. 10. The good correlation between the predicted and observed apatite fission-track parameters provides strong evidence for asubstantial increase in therate of cooling between 40 and 35 Ma. For the estimated thermal gradient of c. 45 "C km-' the apatite data indicate that between 2.5 and 3.5 km of crust has been denuded since c. 40 Ma, with at 0 5 10 15 20 least 1 km being removed between 40 and 35 Ma (mean rate Track length (pm) of c. 200 m Ma-l). Although this interpretation was derived Fig. 8. Apatite track-length histograms and thermal history diagram from detailed analysis of only two of the analysed samples, based on modelling observed apatite fission track parameters on the uniformity of the complete dataset suggests that the sample KG.4275.16 (LeMay Group) and sample KG.3463.2 (Fossil model is regionally applicable tothe whole of Alexander Bluff Group). See text for modelling details. Island. However, there will clearly be local differences in the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 ANTARCTICEVIDENCEFORE-ARCEVOLUTION, FT 77 0 peratures, experienced by the samples immediately prior to accelerated cooling at c. 40 Ma (c. 100 "C to ~110"C),is 40 precisely that which would beexpected to causesevere 80 differential annealing between apatite grains having different Cl/Cl + F ratios (e.g. Green et al. 1989; O'Sullivan & Parrish 120 1995). Differential annealingbetween grains and thus also Accelerated cooling between samples would thus further hinder resolution of any between 40-35Ma : 160 age/sample elevation trend. Finally, there is some evidence across Alexander Island for late Tertiarynormal faulting \ l/ t (Nell er al. 1989) which may have caused minor vertical \l displacements between samples and hence disrupting any Partial (e total?) annealing 240 pre-existing relationshipbetween sample elevation and of zirmn fission tracks I 280 (coolingat -100 Ma) 1 280 fission-track parameters. 160 140 120 100 80 60 40 20 0 Time (Ma) Igneous intrusions Fig. 9. Model thermal histories based on apatite fission track parameters calculated for samples KG.4275.16 and KG.3463.2 Apatite and zircon fission-track results. Seven samples have (bold) and hypothetical samples representing a vertical crustal concordantapatite and zircon central fission-track ages profile of c. 1 km. within la experimental uncertainties. (Notethat in three samples the apatite age is actually older than the zircon age, but that in each case the uncertainty on the apatite age is exact amount of denudation at the different localities across greaterthan thaton the zircon asa consequence of the the island. relative paucity of spontaneoustracks counted). Rapid, Animportant result of the modelling is that it volcanic-type cooling (Gleadow et al. 1986) is supported by demonstrates that the range of observed apatite fission-track the narrow, unimodal apatite confined track length distribu- parameters can be explained by the same simple cooling tions with mean lengths of 14-15 pm (Fig. 5). Such distribu- history, and that the full range occurs over a vertical crustal tions demonstratethat the majority of tracks have been section of only c. 750 m. Furthermore, the thermal history formed at near-surface temperatures and have endured neg- derived from the apatite data is consistent with the thermal ligible annealing. Such aninterpretation requires thatthe history independently derived from the zircon data. intrusion cooled rapidly, which would require high crustal The lack of any clearrelationship between apatite FT levels of emplacement (<2 km depth). Consequently, where age and sample elevation, as the model predicts, probably the zircon and apatite FT ages of a sample are concordant, results from a number of factors. Firstly, the particularly low the measured age provides a good estimate of the time of precision of the apatite FI ages (due to low track counts) intrusion. hinders the resolution of any age elevation trends. Secondly, There is somespatial variation in the distribution of the effects of variable apatite chemistry on the rate of fission-track ages of igneous rocks(and by inference the fission-track annealing is most pronouncedfor severe time of intrusion) throughout Alexander Island; the oldest degrees of annealing. The range of predictedpalaeotem- ages are recorded in the south with the younger ages to the 30 U- 2 2 1 KG 4 2.5 2.5 30 r I Fig. 10. Plots of fission-track age and mean track lengths against amount of denudation from thermal modelling. Note the good correlation between the Track length (pm) observed and predicted (solid line) Fission Track Age (Ma) Mean Track Length (pm) track-length data. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 78 B. C. STOREY ET AL. north. Furthermore, when the ages are projected onto a line betweenthese possibilities, we need to examine other which representsan approximation of the continuation of evidence of burial history. The Fossil Bluff Group totals the Heezen fracture zone and the likely subduction direction about 7 kmin stratigraphic thickness, although thetrue duringTertiary times (Barker 1982), the ages, with some maximum burial depth probably varies from place to place. exceptions, show a general decrease towards the trench (Fig. The youngest sediments in the basin are Albian (c. 97 Ma 11). This is consistent with migration of the locus of old) shelf sandstones. The coincidence within analytical magmatism towards the trenchprobably as a result of a error of the younger zircon ages of c. 100 Ma argues that slowing of subduction ratesand a steepening of the deposition of the youngest exposed rocks of the Fossil Bluff subducting slab. Groupequated with the time of deepestburial and immediateexhumation of the Himalia Ridge samples Discussion analysed. Thegroup hasbeen deformed into a broad synclinal monocline, with steep easterly dips against the LeMay Range Fault, which uplifts the LeMay Group to the Burial history and deformation west. Within the group, there has also been deformation by Post-depositional annealing in the accretionary complex and aseries of ENE-directed thrusts, with related anticlines; fore-arc basin sequences could be due either to deep burial sandstonedykes (Taylor 1982) were injected in the or to greatlyenhanced regional heat flow. To decide extensiondirection. The deformation can all be related to dextraltranspression (Nell & Storey 1991). The timing of deformation is uncertain, but the involvement of unlithified sediment in sandstone dykes, and various systematic palaeocurrent changes (Doubleday et al. 1993), suggest that deformation took place shortly after the endof deposition in thecentral part of the basin, and may even have been co-eval with the later stages of sedimentation. This would be consistent with deformation in a strike-slip setting. The folding andthrusting of the Fossil Bluff Group was accomplished by bedding-parallel flexural slip along overpressured horizons. These are now marked by fibrous, bedding-parallel calcite (andbarite) veins which cut some sandstone dykes. Whilst they were not the first tectonic structures to form, they indicate that the folding was early, and occurred while the rocks were dewatering and before lithostatic fluid pressures were lost. Theseobservations agree with the fission-track evidencepresented here, that initial exhumation followed closely on the end of deposition. v In addition, the ages are broadly similar throughout the 0 20 4080 60 120 100 entire Himalia Ridge Formation, irrespective of dip, Apatite FTAge (Ma) suggesting that deformation preceded exhumation, at least at the temperatures recorded by the fission-track systems. This conclusion is supported by vitrinite reflectance results from the western margin of the basin, which suggest that the thermal maximum was achieved after deformation (Double- day et al. 1993). An independent study of the diagenesis and metamorph- ism of the Himalia Ridge Formation at Ganymede Heights (work in progress by S. Miller & D.I.M. Macdonald), indicates that the metamorphic grade varies from prehnite- pumpellyite at the base of the Himalia Ridge Formation to laumontiteat the top.This is supported by systematic variation in the percentage of laumontite (10-40%), chlorite crystallinity (ChC[001] 0.45-0.65; ChC[002] 0.35-OS), and vitrinite reflectance (Ro= 3.71-2.46%). Diagenetictem- peratures of 250°C at the base and 150°C at the top have been estimated from these data, values consistent with those required to producethe observed levels of fission-track annealing in zircon. The maximum palaeogeothermal gradient deduced (45 "C km-') is significantly higher than might be expected from a fore-arc basin, and suggests that the Fossil Bluff Group has undergonea marked heating v 0 20 40 60 80 100 120 event. If this thermalgradient is projected to zero, it Zircon FTAge (Ma) suggests that the true burial depth of the top of the Himalia Ridge Formation at Ganymede Heights was 3-3.5 km. This Fig. 11. Distribution of apatite and zircon ages projected along the estimate is in good agreement with the estimate of between trace of Heezen fracture zone. 2.5 and 3.5 km derived from the apatite FT data. However, Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 ANTARCTICFORE-ARC EVOLUTION, FT EVIDENCE 79 this is 1-1.5 km less than the known stratigraphic thickness levels in the prism and quickly cooled below their apatite of sedimentaryrocks younger than the Himalia Ridge blocking temperatures.Both the zircon and apatitedata Formation. record a migration in the locus of arc magmatism towards Preliminary oxygen and carbon isotope studies of Fossil the trench which, asa result of the Phoenix plate being Bluff Group cements indicates that late-stage, fracture-filling surrounded on three sides by the Antarctic plate and being calcite was precipitated at temperatures in excess of 15OoC, isolated from global tectonic influences (Larter & Barker as a single pulse (P. Ditchfield pers. comm. 1993). 1991), is most likely due to slowing of subduction rates and Iso-reflectance horizons deduced from vitrinite studiescut steepening of the subducting slab. The dramatic decrease in across the structure, suggesting that the peak of the main subductionrates in EarlyTertiary times (Fig. 12, from heating event post-dateddeformation (Doubleday et al. Holdsworth & Nell 1992) corresponded to migration of the 1993). Subsequentgentle tilting of the iso-reflectance magmatic arc probably caused by sinking and rolling back of horizons suggest further tilting of the strata occurred late in the subducting slab. Slab roll-back may also have resulted in the history of the basin indicating a history of progressive a period of transtensional faulting on Alexander Island with deformation. thepreservation of the Cenozoic volcanic rocks in down-faultedtranstensional grabens (Nell et al. (1989) reported syn-depositional extensional and dextral strike-slip Implications for fore-arc model faulting in association with calc-alkaline volcanic rocks), formation of George VI Sound andthe translation of The range of central apatite ages for the Fossil Bluff Group Alexander Island away from Palmer Land (Fig. 13). is similar tothat of the LeMay Groupand the youngest The accelerated early Tertiary cooling recorded by the zircon ages of the Fossil Bluff Group correspond tothe apatite fission-track data from Alexander Island appears to younger zircon ages from the LeMay Group (c. 100 Ma). be broadly coeval with the history of ridge-trench collisions The similarity of these data suggest that the fission-track off-shore Alexander Island (Fig. 12) but cannotbe data are recording a regional pattern of thermal history for correlated precisely with the time of collisions on either side thefore-arc region. Cooling from maximum temperatures of the Heezen fracture zone. The broad correlation is not for the fore-arc basin sequence and at least the outboard surprising in view of the fact that metamorphism or part of the accretionary complex occurred at approximately magmatism appears to be a common consequence of other 100 Ma with a further accelerated cooling episode occurring ridge-trench collisions elsewhere in the world (Forsythe et at between 40 and 35 Ma. Subsequent cooling to the surface al. 1986; Underwood 1989). This increase in therate of occurred at reduced rates through the Tertiary. The cooling was most likely caused by increased rates of mid-Cretaceous period of cooling in the outboard partof the denudation of topography created along the inboard region prism corresponded broadly with deformation(out-of- of thefore-arc during ridge-trench collision. Dextral sequence thrusting) in the older inboard part of the prism, shearing and formation of George VI Sound along the deformation and the start of exhumation of the Fossil Bluff eastern edge of Alexander Island during the Tertiary would Group in the central part of the fore-arc basin and the age have increased any topographic relief along the eastern of the youngest known sedimentary rocks now preserved in rifted margin of the island thus further augmenting thesouthern part of the basin. This may signify that denudation rates at this time. cessation of sedimentation was coincident with deformation whichis entirely consistent with inversion of a strike-slip basin during mid-Cretaceous transcurrent motion (Storey & Conclusions Nell 1988). Based on a geothermal gradient for this type of Zircon and apatite fission-track data from the Alexander tectonicsetting (20°C km-', Dumitru 1991), uplift of the Island fore-arc region recorda regional Cretaceousand basin has resulted in as much as 14 km of erosion in the Cenozoic thermaland denudational history. For the most central part, although this figure would bereduced to part apatite and zircon ages are substantially less than c. 8 km if the thermalgradient of 45 "C km-' suggested known and inferred stratigraphic ages. The apatite ages for above is accepted.This lower estimate is similar tothe the Tithonian to Berriasian fore-arc basin sequenceand maximum estimated thickness of c. 7 km for the Fossil Bluff Mesozoic accretionary complex indicate a phase of Group. Much of this material was probably redeposited in accelerated cooling between 40 and 35 Ma. The youngest the trench and recycled into the prism. Vitrinite reflectance zircon ages record an earlierperiod of cooling from data characteristically show many high gradeinherited substantially higher palaeotemperatures at c. 100 f 10 Ma. components, and fission-track data from the youngest part Thermal modelling of the data indicates thatthe highest of the prism record mid-Cretaceous provenance ages. Such temperatures, in the range of18O-35O0C, were reached recycling of material is probablyacommon feature of shortly before cooling at c. 100Ma, close to the time of the mature accretionary complexes and has been recorded from end of sedimentation. Zircon data from the LeMay Group the Franciscan Complex in southern California (Cowan & accretionary complex also recordmid-Cretaceous ages. Page 1975). These data reflect denudational cooling following The magmatic rocks record fission-track data that are accretionary-related tectonic burial and heating. Zircon and independent of the rest of the fore-arc with no evidence apatite age data from Tertiary magmatic rocks emplaced in from the zircon and apatite data from the sedimentary rocks the fore-arc region indicaterapid cooling of both of athermal pulse in Late Cretaceous or EarlyTertiary fission-track systems synchronous with emplacement at high times, the time of magmatic activity. The fission-track ages crustal levels. The emplacement of these magmatic rocks of the igneous rocks are close to their emplacement ages and together with the cooling history of the accretionary prism may record the most precise dating of these events up to the are related to changes in subduction zone parameters during present time. The magmatic rocks were emplacedat high Cretaceous and Cenozoic times. Slowing of subduction rates Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/65/4888653/gsjgs.153.1.0065.pdf by guest on 02 October 2021 B.C. STOREY ET AL. L + E E 140 ' 120 804 nU 3 v> (1982) and Barker (1982). and roll-back of the slab prior to cessation of subduction by ridge-trench collision may have been responsible for migration of the magmatic focus into the fore-arc regions, and uplift of the prism and formation of the high mountain ranges, and the extensional grabens that separate Alexander Island from the Antarctic Peninsula. We gratefully acknowledgethe field mappingand use of samples collected by P. Butterworth and P. Howlett, I. Evans for undertaken some of theearly fission-track analysis on which this paper is based, NERC support for fission-track analysis in London under research grants GR3/7068 and 8291 and the constructive and thought provoking comments of P. O'Sullivan and an anonymous reviewer. We thank S. Millar and P. Ditchfield for permission to quote the results of papers in preparation, J. McCarron, R. Missing and G. McDonnell for assistance with diagrams and preparation of manuscript. 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