Earth and Planetary Suen(e Letten, 81 (1986/87) 67-78 67 ElseVIer SCIence Pubhshers B V , Amsterdam - Pnnted m The Netherland,
[3]
Asymmetric extension associated with uplift and subsidence in the Transantarctic Mountains and Ross Embayment
Paul G. Fitzgerald 1,2, Michael Sandiford 1,*, Peter J Barrett 2 and Andrew J W Gleadow 1
I Department of Geology, Unll'eniTI of Melhourne, Parkl'dle, VI( 3052 (A ustralw) ] Antar((l( Research Centre, Vl(tona UnlVersllL Welhngton (Ne'W Zealand)
ReceIVed June 19, 1986, reVIsed ver,lon accepted September 9, 1986
Apatite fl"lOn track data combmed WIth regIOnal geologIcal observatlom mdlcate that the uplift of the Tran,· antarctIc Mountams has been coeval wllh thmnmg and sub'ldence of the crust beneath the Ross Embayment In the Dry Valleys regIOn of south Vlctona Land, the moun tams have been uplifted about 5 km smce the early CenozoIC at an average rate of about 100 m/Ma Dunng upllft, the crust remamed at constant thIckness or was slightly thIckened by magmatic underplatmg In contrast, the crust beneath the Ross Embayment ha, been extended and consequently tlunned begmrung m the Late Cretaceou~ but mamly dunng CenozOIc tIme, We suggest here that the uplift of the TransantarctIc Mountam, and the subSIdence of the Ros, Embayment are a result of passIve nftmg governed by a fundamental structural asymmetry defmed by a shallow crustal penetrative detachment zone that dIps \\e,t\\ard beneath the Transantarctic Mountam Front The localizatIOn and asymmetry of thl, detachment and Its unusually deep level expressIOn are attnbuted to a profound crustal arusotropy mhented from an early PalaeOZOIC colliSIOn along the present site of the mountam range
1. Introduction CenozOIc along or Just offshore of the eastern Side of the TAM ThiS Side of the mountams, which The mechamsm of uplIft of the Transantarcllc borders the Ross Embayment, IS step-faulted down Mountams (TAM) remams emgmatlc despite diS towards the coast and has been called the Trans cussIOn about their formatIOn smce the early days antarctIc Mountam Front The TAM Front IS of AntarctIc exploratIOn [1]. The mountams have a bounded to the east by a zone of anomalously thm Simple stratIgraphy compnsmg a Precambnan and crust beneath the Ross Embayment, wrnch has early PalaeozOIC basement unconformably over been the site of accumulatIOn of thick sedimentary lam by sub-honzontal sediments of the Devoman sequences smce the early TertIary or pOSSibly as to Tnasslc Beacon Supergroup Both the basement early as the late MeSOZOIC [4] The mountam front and ItS cover are mtruded by the JurassIc Ferrar marks an abrupt change m crustal thickness that Dolente [2] The crust beneath the moun tams has has long been conSidered to represent the boundary been esllmated to be of only moderate trnckness between East and West Antarctica [3] [3] and unhke most other major CenozOIc moun Llthosphenc stretchmg models [5] have been tam ranges of the world, uplIft was not accompa used to explam the formatIon of sedimentary med by thrustmg, foldmg or andesltIc vo1camsm basms that accompany crustal thmnmg, subSI Rather, uplift has been exclUSively by vertIcal dence and nftmg m the development of passive crustal movement With gentle tIltmg of fault contmental margms These models are aXially blocks ThIS movement has comcided WIth nft-re symmetncal however, and faIl to answer a number lated alkahne vo1camsm actIve from the Late of pomts regardmg basm margm uphft and gen eral basm topography (e.g [6]) In contrast, recent * Present address Department of Earth SCIences, Uruverslty models for the development of the Basm and of Cambndge, Dowrung Street, Cambndge CB2 3EQ, Uruted Range-Colorado Plateau regIOn of North Amenca Kmgdom emphaSize the Importance of structural asymmetry
0012·821X/86/$0350 ;£; 1986 ElseVIer SCIence Publl~hers B V 68 and the role of detachment faultIng durIng con 2. Geology and history of the Transantarctic tInental extenslOn [7-9] These Ideas have more Mountains recently been extended to explaIn the development of nfted contInental margIns [6] Here we propose The TAM form a major transcontInental range a model for the evolutlOn of the TAM-Ross Em that vanes from 100 to 200 km In width and has bayment reglOn which IS SimIlar to, and to a large eievatlOns up to 4500 m (Fig 1) Despite the high extent denved from these recent models for the elevatlOns, the crust beneath the mountaIns, as extenslOnal tectolllcs of the BasIn and Range Pro determIned from gravity modelhng IS only 40-45 VInce km thick [3,11,12], and IS SimIlar to the crustal WhIle there IS a general consensus that the thIckness of the adjacent low-lYIng topographi formatlOn of the TAM-Ross Embayment system cally depressed areas of the East AntarctIc craton has Involved contInental extenslOn, there have been [3,11,13] ThiS contrasts With the Ross Embay no prevlOUS attempts to explaIn the seemIngly ment, a reglOn of thInned contInental crust [3] contrastIng evolutlOn of the TAM and the Ross extendIng about 800 km between the TAM and Embayment, or the cOIncidence of the mountaIn Mane Byrd Land, where the crustal thlckne~s IS range over almost all ItS length With an early between 21 and 30 km [11-13], With the thInnest PalaeozoIc foldbelt (the Ross Orogen) SmIth and crust measured by seismiC refractlOn In McMurdo Drewry [10] attnbute the nse of the TAM to the Sound (Fig 2) [14] In companson wIth the abrupt delayed effects of the overndIng by east AntarctIca 10 km change In crustal thIckness on the western of anomalously hot asthenosphere formed under Side of the Ross Embayment, crustal thIckness west AntarctIca In the late Cretaceous The result Increases gradually from the Ross Embayment to Ing heat flow Increase would cause phase changes 35 km under Mane Byrd Land [3,15] In the uppermost contInental mantle, In turn caus Throughout their length In the Ross Sea ~ector lllg uplIft Although a novel Idea, thiS model IS (Fig 1), the TAM expose Precambnan/Cambnan difficult to test by direct observatlOns and does metasedimentary and metavolcalllc rocks, and not address the asymmetncal development of the syntectolllc to post-tectolllc grallltes The bulk of mountaIn range, nor ItS relatlOnshlp to the Ross the grallltOlds form the OrdOVICian Gralllte Embayment and the PalaeozOIc foldbelt Harbour IntruSive SUite, which were Intruded dur In thiS paper we look at flsslOn track data and Ing a tIme of convergence (the Ross Orogeny) [16] review reglOnal geological eVidence which suggest above a westward dippIng subductlOn zone [17,18] that the uplIft of the TAM was coeval With most that lay along the length of the present TAM and of the thInlllng and subsidence of the crust be also parallel to the lIne of the Beardmore Orogen neath the Ross Embayment, and With alkailne of late Precambnan age [16] ThiS metamorphIc volcalllsm along and Just offshore of the TAM and grallltic basement was eroded In the SIlunan Front These lInes of eVidence support the notlOn and early Devolllan to form the Kukn Peneplalll, that both terralllS developed In an extenslOnal whIch IS overlalll by about 25 km of almost regime dunng the CenozoIc We propose that the totally undeformed mid-PalaeozOIc to early extenslOn was governed by a fundamental asym MesozOIC shallow manne and flUVial sediments of metry In the crustal structure defIned by a crustal the Beacon Supergroup [2] Beacon sedlmentatlOn penetrative detachment zone that dips westward ended In the mid-JurassIc and was followed by the beneath the TAM Front The 10cailzatlOn and extensive llltruslOn of tholelltIc magma as Sills and asymmetry of thiS detachment zone IS attnbuted dykes (Ferrar Dolente) and their extruslOn as to pre-existIng alllsotropies In the IIthosphenc lavas (KIrkpatnck Basalt) [2] Beacon strata and structure of the Ross orogen ThiS InterpretatlOn dolente sIlls usually parallel the Kukn Peneplalll, has Important ImpilcatlOns for future work In that which allows the peneplalll to be used as a refer It proVides the first specific and testable model for ence surface for determlllIng post-JurassIc tectolllc the evolutlOn of the TAM-Ross Embayment sys movement Palaeogeographic reconstructlOns of tem the Beacon deposltlOnal baSInS [2] IndICate that the TAM Front had no physlOgraphlc expresslOn from the Devolllan to the Tnasslc, and restnctlOn 69
ROSS ICE SHELF /
CenozoIc Alkaline Volcanics
Beacon Supergroup •o and Ferrar Dolente r::::l Igneous and Metamorphic ~ Basement ·2400 Elevation of Kukn Penaplaln f-f Fault (metres)
300
Fig 1 Index map of the TAM-Ros5. Embayment showmg onshore geology [50], localitIes mentlOned m the text, bathymetry [38,50,51] and sedImentary basms of the Ross Sea [4,21] Po"tIon of the Kukn Peneplam wa, obtamed from [50] and the elevatlOn, from the appropnate U S G S 1 250,000 TopographIc Map' 70
McMurdo Sound
Outline of the Victoria Land Basin x Kukrt HIlls \\ 'I \ and profIle of the Central Rlfl Mt Bird 0 f:;:~::::~:::::::=:~':~:~~::~:'~::"~'~"~'~H~'f'~'~~Tr~~~~~fr~0~m~c~0~op~e:r=an:d:D~a:ve~Y:[~2~1::~~~!!~~~~::::::::;:;Y f D CenozoIc Sediments III CenozoIc Volcanics -;:; -10 ~ Beacon Supergroup and ;; o Ferrar Dolente E E -20 Basement ~ o ::: .:: :::.::. :::. :::.: .::: .. ::: ::, :: ... : ") a -30 ;: : :: : j: :: Kilometres Q) ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ i~ ~ ~ :~ :~ ~ ~' ~ ~ :: ~ :. ~ ~ ~ :~o-?.o .+++ + +.+ +++ ... +++.+ +++ >.+> o 10 20 30 o ::"::::.:::.:::'::.':.,::,::::::::7 -40 t.oo+ •••• <- •••••• +++.+++~ Scale :::::::::.::: .. : •• ...:; +~
1630 E 164"E 166"E 167"E 168"E
Fig 2 Crmtal cro" sectIOn through McMurdo Sound on a lIne X- Y a, m of the Beacon Supergroup to broad elongate basms and m the Ross Island-Dry Valleys area (67-80 centred on the present site of the TAM suggests m W m - 2 [24)) These values are conSidered to be an average to slightly reduced crustal thickness at too high and regIOnally extensive to be caused by the tIme of depOSItIon [2) In view of the present the McMurdo VolcanICS alone, suggestmg unusu day smulanty m crustal thIckness beneath the ally high conductIve heat transfer through anoma TAM and the adjacent craton, It IS pOSSible that lously thm lithosphere such as occurs mother the crust beneath the TAM has thickened shghtly modern contmental extenslOnal orogens, such as smce Beacon depOSItIon The tumng of crustal the BaSIn and Range Provmce [24) Unusually thlckemng Will be discussed later high temperatures at shallow levels m the mantle The mid-JurassIc magmatIsm was followed by a beneath the TAM Front are mdlcated by the gap of 160 llllllIon years m the on-land geological breakdown of phologlte to leuclte + glass + record m the Ross Sea sector, the next recorded fostentlc ohvme + feldspar as well as high temper event bemg the eruptIon of alkalme lavas of the ature breakdown reactIOns m spmel seen m xeno McMurdo VolcanIC Group along the TAM Front lIths from the alkalIne volcanICS from Foster Crater These began at least 19 Ma ago [19) and contmue (J A Gamble, personal commUnICatlOn) ThiS fur to the present VolcanIC products older than 30 ther supports the nohon that the contmental htho Ma have been recorded m an offshore dnllhole sphere IS anomalously thIn beneath the mountam [20), and are also suspected m deeper seismic front sequences offshore [21) Alkaline volcanIsm IS con centrated m certam areas along the mountam front 3. Structure and uplift history of the Trans (Fig 1) and extends up to 150 km offshore (e g antarctic Mountains Franklm Island and m the Vlctona Land Basm [21)) The chellllstry of the lavas IS typical of other The pattern of uplift across the TAM IS best contmental nft-related volcanoes such as those m known m south VictorIa Land near McMurdo the East AfrIcan nft and ImplIes crustal extension Sound (Fig 1), where the aXIs of maximum uplIft over thiS perIod [22) lies some 30 km west (mland) of the coast East of Recent extenslOnal tectOnIsm m the Ross Em the aXIS, the amount of uplIft decreases rapidly bayment IS supported by unusually high heat flows through the step-faulted zone of the mountam m the Vlctona Land Basm (83-126 mW m -2 [23)) front (Fig 3) West of the aXIs the amount of 71 Geological Cross Section - Transantarctic Mountains - Dry Valleys Area TransantarctIc Mountain Front o 10 o Igneous and MetamorphIc Basement D Beacon Supergroup D Ferrar Dolerite Fig 3 Geological cross sectIOn of the TAM through the Dry Valleys regIOn on a lme as marked A-B m Fig 1. mcludmg Mt Doorly showmg the step faulted nature of the Transantarctic Mountam Front Faults marked with a questIOn mark mdlcate that the exact pOSitIOn of that fault IS not known Note the gently westerly dip of the Kukn Peneplam and Beacon Supergroup ';(rata, eventually resultmg m their disappearance under the polar ICe-cap Modified after McKelvey and Webb [53] uplIft decreases steadIly, as seen from the gentle strata mcreases towards the polar plateau, dlsap westerly dIP of the Kukn Peneplam The moun peanng beneath Ice 80-200 km mland of the coast tams themselves do not dlmlmsh m elevatIOn how at elevatIOns close to 2 km ThIS sImple structure ever, because the preserved thIckness of Beacon IS eVIdent along most of the length of the moun tams, although m places, for example, the MIller Range m the Beardmore Clacler regIOn and west UPLIFT HISTORY OF THE TAM of the Renmck GlacIer m north Vlctona Land, basement rocks have been uplIfted on the edge of South Victoria Land the polar plateau 2 Apatite fIssIOn track analYSIS on basement rocks from the Dry Valleys of south Vlctona Land has * Kukrl Hills provIded the only quantitative approach so far for Mt Doorly • estImatmg the tImmg and rate of uplIft of the TAM [25-27] The apatite fIssIOn track ages show a strong correlatIOn WIth elevatIOn (FIg 4) and the shape of the age-elevatIOn profIle mdlcates a two stage uplIft hIstory for the TAM The older part of the profIle has a shallower gradIent, whIlst the younger part of the age-elevatIOn profIle for ages less than about 50 Ma, has a much steeper gradI o ent The slope for the older part of the profIle IS g , about 15 m/Ma and was prevIOusly mterpreted as a very slow pre-Eocene uplIft rate [25] However, ((... +, thIS IS now regarded as an artifact of the pre-uplIft thermal profIle (an uplIfted fossIl "partial anneal mg zone"), formed dunng a penod of relative -1~--~~--~--~~---7~--~~~ o 20 40 60 80 100 120 thermal and tectomc stabIlIty establIshed after Apatite FISSion Track Age (Ma) coolIng that followed the JuraSSIC thermal event Fig 4 Apatite fiSSIOn track age versus elevatiOn graph for t"o assocIated WIth the mtrusIOn of Ferrar Dolente samphng profiles m south Vlctona Land [26,27] Error., on ages are plottcd as two standard deViatiOns The upper part of [26,27] The pronounced "break m slope" m the the proftie repw,ents an uphfted partJal anneahng zone The apatite fISSIOn track age-elevatIOn profIle at 50 Ma "break m slope" of the graph at about 50 Ma represents the marks the base of the uphfted partial annealmg start of uphft of the TAM m thiS area and the gradient of the zone, and therefore dates the start of uplIft of the lower slope represents the average uphft rate The data from the two separate profiles plot almost perfectly together mdlcat TAM mg that they he m 'Imtlar POSitiOnS wlthm the Transantarcltc The stratigraphIc thickness above the base of Mountam Front the uplIfted partial annealIng zone (the "break m 72 slope") has been eStimated [27] from the known sequence of sub-hOrIzontal strata offset by many thickness of granIte, Beacon Supergroup strata small normal faults, some of which displace the and JurassIc dolente plus basalt [28,29], at about sea floor and show a progressive mcrease m dIS 45-5 km, and provides a rrnnImum value for the placement With depth [21] ThiS syn-deposltIonal thIckness pnor to uphft The base of the uphfted faultmg has created a central nft that comcides anneahng zone has an elevatIOn of about 800 m at WIth a bathymetnc trough, along whIch he volcanIC Mt Doorly [27] and from the eastern end of the centres [21], one of whIch (Mt Erebus on Ross Kukn HIlls [26]. By sumrrnng these and subtract Island) IS stIll active SimIlar volcanIC rocks also mg the elevatIOn of the land surface followmg the crop out at places on the western margIn of the extruSIOn of the JurassIc basalts, which although basm and the eastern flank of the TransantarctIc poorly constramed has been estImated at around MountaIns These features all mdlcate that sedi 500 m [27], we obtam a range of 4 8-5 3 km for a mentation and volcanIsm occurred In an exten mInImUm uphft of thIs aXIs smce the JuraSSIC If SIOnal tectOnIC regIme throughout the history of uphft did not begm until 50 Ma, as the age-eleva the baSIn The Central Trough and Eastern BaSIn tIOn curve mdlcates, then the average uphft rate are both shallower (maximum of 7 km of strata) smce that tIme IS about 100 ± 5 m/Ma It IS and Simpler m structure than the Vlctona Land Important to reahse that thiS IS an average rate Basm, they both lack a central nft and any and may well conceal penods of faster and slower volcanIC centres [4,32], but nevertheless also ap uphft, or even subSidence An alternatIve method pear to have formed In an extensIOnal tectOnIC of estImatmg the amount of uphft assumes a regime thermal gradient of 30°C/km pnor to uphft, based The early hIstory of subSidence of the Ross on the present-day gradIent m the DVDP-6 dnll Embayment can only be estabhshed from selsrrnc hole m the Dry Valleys [24] Usmg thIS thermal reflectIOn data as no deep dnlhng has yet been gradient, the depth to the base of the partIal attempted InIhal subSIdence, however, must post anneahng zone hes at about 4 km (a temperature date the Tnasslc Beacon strata that were de of close to 130°C [30]) glVlng some 48 km of pOSited In a trough bounded by hIghlands west of uphft, agam assurrnng that the surface elevatIon the present TransantarCtiC MountaIns [2] The prIor to uphft was 500 m and that the mean oldest strata dated dlfectly were cored by MSSTS-l annual surface temperature close to O°C on the margIn of the Vlctona Land Basm, and are of Late Ohgocene age (30 Ma) [20] The 226 m 4. Structure and subsidence history of the Ross cored by MSSTS-1 IS thought to have penetrated Embayment only UnIts 1 and 2 of the five sedimentary UnIts m the baSIn revealed In selsrrnc traces, represen tmg The Ross Embayment, comprIsmg the Ross Sea only 2 km of the 14 km of sedIment postulated to and Ross Ice Shelf (Fig 1) has a contmental shelf, be present m the central nft [34] At present the on average 500 m deep, diVided by broad north age of older strata IS necessanly conjectural, and south to northeast-southwest trendmg ndges mto the lowest stratIhed UnIt could be Beacon Super three dlstmct sedImentary basms, the Eastern group and Ferrar Group rocks However, re Basm, the Central Trough and the Vlctona Land worked microfossils from the MSSTS-1 core do Basm [4] SeIsrrnc reflectIOn profIles [21,31,32] gIve a gUIde Late Cretaceous and Early CenozoIc show the basmal strata to be largely undeformed foramInifera [35] and palynomorphs [36] Indicate With occaSIOnal normal faultmg Major normal the strata of thIs age may be exposed somewhere faults also mark the western margm of the Embay on the western margm of the Vlctona Land Basm ment and the km have been reported from Cape EroSIOn along thIS margm north of McMurdo Surpnse [33], and at Mt Doorly a serIes of step Sound has exposed the oldest sedImentary forma faults records a total dIsplacement of about 1 km tIon of the basm (sedimentary UnIt 5) upturned [27] agamst the TAM Front [34], whIch could, If ex The structure of the Victona Land Basm, whIch posed to the south, have been the source of the hes adjacent to the TransantarctIc Mountams, IS oldest reworked matenal We conclude therefore partIcularly Important It consists of a 14 km thIck that supsldence m the Vlctona Land Basm prob- 73 ably began m the Late Cretaceous/Early Ceno most subsIdence, post-date the Late OlIgocene An ZOIC, that the greatest amount of subsIdence oc extensIOnal tectomc regIme has persIsted through curred durmg the CenozoIc, and that subsIdence out sedImentatIOn, and In the western part of the and sedImentatIOn has contmued to the present Ross Embayment thIs sedIme alkalIne volcamsm day The western margm of the basm at MSSTS-1 ExtenSIve dIsruptIOn of the sequences by growth expenenced steady subsIdence through the Late faults above the anomalously thm crust m the OlIgocene and MIOcene followed by more rapId VIctona Land Basm [21] also suggests that SUbSI subsIdence and then 1 km of uplIft m the PlIocene dence there was coeval WIth crustal thmmng [20], but thIS cannot be taken to represent the basm as a whole 5. Model for extensional asymmetry in the TAM The Central Trough IS the least well known of Ross Embayment the three basms, but m the central part more than 2200 m of pre-Late OlIgocene sedIments are con Contmental extenSIOn has tradItIonally been re sIdered to occur beneath 3800 m of younger sedI garded as an aXIally symmetncal process mvolvmg ments [32] The Eastern Basm and the Central pure-shear thInnIng of the ductIle lIthosphere and Trough are separated by a basement hIgh, the lIstnc normal faultmg of the bnttle lIthosphere northward extenSIOn of whIch forms the Iselm WhIle such extenSIOn models, espeCIally those m Bank MultIchannel seIsmIC reflectIOn data across volvmg depth-dependent extenSIOn, have success the IselIn Bank show the mCISIOn of basement by fully accounted for subsIdence hlstones of exten two north-south trendmg graben covered by a thIn SIOnal terrams such as nfted contmental margms, sedImentary sectIOn [31] they cannot account for the asymmetry on a re Dnllmg close to the margm of the Eastern gIOnal scale of a number of modern extensIOnal Basm also proVIdes a chronology for basm SUbSI orogens, such as the claSSIC Basm and Range-Col dence The western margm, at DSDP SIte 270, was orado Plateau system Moreover, aXIally symmet above sea level for mIllIons of years, long enough ncal models mvolvmg depth-dependent extenSIOn for a well developed palaeosol to form on the are often geometncally unacceptable because they basement gneISS [37] It subsIded below sea level generally do not perrmt the constructIOn of bal m the Late OlIgocene to depths of several hundred anced cross-sectIOns (1 e mass IS not conserved). metres and was uplIfted and eroded m the PlIo The sIgmfIcance of large-scale structural asymme cene [38] Net subsIdence over the 26 Ma penod try dunng the evolutIOn of the Basm and Range of sedImentatIOn was about 1 km In the central Provmce, fust suggested by Wermcke [7], has re part of the basm eqUIvalent strata (late OlIgocene cently been hIghlIghted by publIcatIOn of a num and MIOcene) are about 37 km thIck and are ber of studIes [6,8,40,41] The govermng structures underlam by an older sequence more than 2200 m dunng the evolutIOn of the Basm and Range ex thIck [32] LIke MSSTS-1, DSDP-270 also con tenSIOnal orogen are low-angle normal shear zones tams reworked Late Cretaceous forammIfera [35], or detachment faults whIch show regIOnally con suggestmg that the deepest sedImentary succeSSIOn SIstent movement sense here, lIke that of the VIctona Land Basm, IS of The chronology of uplIft of the TAM and sub thIS age as well A Late Cretaceous age for the sIdence of the Ross Embayment dIscussed In the oldest nft-related strata m the Ross Sea IS also prevIOUS sectIOns suggests that the uplIft of the conSIstent WIth the oldest strata m the Great South TAM IS mtImately aSSOCIated WIth thmnmg and Basm of New Zealand [39], now 3000 km, but subsIdence of the Ross Embayment, both terrams then only 500 km, to the north [39] are related to the TAM extenSIOnal orogen that In summary, the floor of the Ross Embayment has developed smce the Late Cretaceous/Early has sunk at least 10 km adjacent to the TAM, and CenOZOIC The pronounced asymmetry of thIS 6 or 7 km In basms farther east Reworked mIcro orogen IS hIghlIghted by the contrasts m the evolu fossIls and seIsmIC stratIgraphy suggests that most tIOn of the two terraInS The TAM have under subSIdence m the VIctona Land Basm took place gone about 5 km of uplIft m the last 50 Ma, m the Late Cretaceous/Early CenozoIc, but m the whereas the Ross Embayment has undergone Eastern Basm most sedIment, and presumably equally dramatIc subsIdence WIth the formatIOn of 74 basms, 6 km or more thIck smce the Late Creta stram may be locally enhanced by the mfluence of ceous/Early CenozoIc Moreover, whIle the crust magmatIc addItIons underplatmg the base of the beneath the TAM has remamed of constant thIck crust MagmatIsm dunng the JuraSSIC may also ness, or more probably been slIghtly thIckened have contnbuted to magmatIc accretIon and hence smce deposItIOn of the Beacon Supergroup, the crustal thickenmg In terms of volume and distn contmental basement beneath the Ross Embay butIOn, the JuraSSIC Igneous event was of greater ment had been thmned Usmg a one-layer exten sIgmficance than the CenOZOIC event, but a nft sIOnal model we can estImate crustal thmmng m system eVIdently dId not form dunng the JuraSSIC the Ross Embayment by assummg It had a crustal [44], and the apparent lagk of any great thIckness thIckness of 35-40 km pnor to nftmg (smular to of sedIments older than CenozOIc m the Ross that m East AntarctIca) and companng thIS wIth Embayment mdicate that lIttle or no slgmflcant the present-day average crustal thIckness of 25-30 extenSIOn took place before thIS tIme It IS, how km over the wIdth of the Embayment (800 km) ever, dIffIcult to aportlOn the amount of under The total crustal extensIOn determmed m thIS way platmg caused by the JuraSSIC versus the CenOZOIC IS about 25-30%, or close to 200 km In the events Our observatIOns here are a fIrst-order Victona Land Basm actual extensIOn may be as look at what IS a complex problem Nevertheless It much as 50%, wIth pOSSIbly as much as 70% IS lIkely that some of the mferred crustal thlcken beneath the central nft mg beneath the TAM (smce the tIme of Beacon TheoretIcal models for the evolutIOn of exten depOSItIon) has occurred dunng uplIft of the sIOnal terrams show that the fIrst-order changes m moun tams The correlatIOn of volcamsm m the surface elevatIOn reflect the relatIve changes m Dry Valleys-Royal Society Range regIOn With thIckness between the crust and subcrustal htho greater uplIft does proVide circumstantIal eVidence sphere [5,42,43] Where extensIOnal stram IS for magmatIc underplatmg dunng the CenozOIc equally partItIOned between the crust and sub compared to the ImmedIate north or south of thIS crustal lIthosphere, subSIdence and basm develop area It should be noted, however, that mother ment results [5] SubSIdence IS greatly enhanced If areas of hIgh Kukn Peneplam elevatIon relatIve to extensIOnal stram IS greater m the crust Prefer surroundmg regIOns, for example Just north of the entIal partItIOn of stram mto the subcrustal htho Beardmore GlaCIer (Fig 1), CenozoIc volcamsm IS sphere results m mmImal subSIdence and may, m notably lackmg Nevertheless, the above model the case where lIttle or no stram IS accommodated can account for the magmtude, tImmg and rate of m the crust, result m uplIft The contrastmg uplIft uplIft of the TAM compared to subSidence of and subSIdence histones across the TAM Front basms withm the Ross Embayment In addItIon, may therefore be VIewed as a consequence of a we note that graVIty models for the structure of regIOnal vanatIOn m the depth at whIch exten the Ross Embayment [34] are consistent WIth the SIOnal stram has been accommodated, WIth exten model proposed here for the TAM-Ross Embay SIOn partItIOned largely mto the crust beneath the ment system Ross Embayment and mto the subcrustal htho The mferred regIOnal vanatIOn m the depth of sphere beneath the TAM stram accommodatIOn across the TAM Front pre We can use the Kukn Peneplam as a reference cludes an aXIally symmetncal model for the Ross surface for a fust-order approXImatIOn of the Embayment-TAM extenSIOnal orogen Rather, It amount of uplIft that an area has undergone, even suggests that at least wIthm the upper lIthosphere, though there must have been slIght vanatIOns m stram has been localIsed along a detachment or the ongmal elevatIOns of thIS eroSIOn surface along shear zone whIch dips westward from the Ross the length of the TAM correspondmg to localIzed Embayment and penetrates the Moho below the Devoman basms [2] In thIS way, It can be seen western boundary of the TAM Front (Fig 5) It IS from FIg 1 that the Dry Valleys-Royal SOCIety thIS shallow dIppmg detachment zone whIch al Range regIOn (an area of greater peneplam eleva lows for the translatIOn m the depth of extenSIOnal tIOn than to the north or south) IS COInCIdent WIth stram accommodatIOn from the crustal levels m a concentratIOn of CenozOIc volcamcs It IS POSSI the Ross Embayment to subcrustal levels beneath ble therefore that uplIft mduced by extenSIOnal the TAM Front WhIle mdependent eVIdence for 75 VictOria land Transantarcllc Central Trough Eastern Basin MBl Mountains Basin Depth (km) late Cretaceous (7) o and CenozoIc sediments o Upper crust 80 o lower crust Magmatic accretion (underplating) 120 D Subcrustal hthosphere Asthenosphere 160 o 300 I Scale (km) 200 Fig 5 Conceptual model of asymmetnc extensIOn In the TAM-Ross Embayment ,howlng the Inferred detachment zone bencdth the TAM and pos~lble detachments beneath thc Ro5.S Embaymcnt The pOSitIOn of the asthenm,phere-subcrustal hthmpherc bounddry IS e'>lnnated uSing the approach of Turcotte [42] to calculate uphft and wbSldence as a functIOn of rclahve thlnmng of crust ver,us ,ubcrustal hthosphere Stram In the subcru,tal hthm,phere IS modelled as ductJle pure-~hear flow Imhal hthm,phenc thlCkne,>, IS as,umed to be 300 km [54] Pm,lhons of MSSTS-l and DSDP-270 dnllhole, arc dtagrammdtlc MBI_ = Manc Byrd Land the eXIstence of such a structure IS not aVailable, tenSIOnal regIme over 3000 km strongly supports a we note that the mferred pOSItIOn and asymmetry paSSIve rather than an actlve mechamsm [42] for of thIs structure parallels the structural gram and CenozOIC extensIOnal tectomsm m the TAM The asymmetry m the metamorphIc basement of the confmement of the Ferrar magmatlsm to a hnear TAM-Imparted dunng early PalaeOZOIC subduc zone along the SIte of the TAM has also been tion suggested to be a result of reactlvatIOn of pre-ex WhIle regIOnal shear zones often follow pre-ex Istmg zones of weakness the Beardmore and Ross Istmg structural gram or hnes of mhented weak orogemc belts [44] Indeed, the mferred west dl ness [7,45], major faults do not always reactIvate ppmg asymmetry of the regIOnal detachment old thrusts and m some extensIOnal belts the sense structure and parallel nature of the structural gram of asymmetry IS OppOSIte to these old planes of of the CenozOIC, MesozOIC, PalaeozOIC and Pre weakness (e g [46]) However, the sense of asym cambnan tectomc events IS rather too SimIlar to metry m the TAM-Ross Embayment IS the same be mere comcldence as the postulated detachment zone and therefore WhIle the eXIstence of a smgle, shallow west thIs extensIOnal orogen could be due to reactiva dIppmg, crustal penetratlve detachment zone may tIon of a fundamental hthosphenc weakness gen account for the asymmetry m the western Ross erated dunng early PalaeozOIC convergence The Embayment and TAM, It seems unhkely that such comcidence of a PalaeozOIC foldbelt WIth an ex- a structure could accommodate the estimated 200 76 km of extensIOn across the Ross Embayment TAM smce the Early CenozOic Less well con without complete separatIOn of the contmental stramed IS the tmllng of the subSidence of basms crust The dip of the detachment zone can be wlthm the Ross Embayment The sedimentary se either mferred from the angle of the Moho be quences there, although not well dated, mdlcate neath the mountam front obtamed from gravity that subSidence may have begun m the late Creta models [47] as 20-30°, or be calculated by assum ceous, With major sediment accumulatIOn smce mg It runs from the base of the crust under the the early CenozoIc The model proposed here to TAM to near the top of the basement high be explam the contemporaneous but contrastmg hls tween the Vlctona Land Basm and Central Trough, tones of these two terrams IS based on recent m which case an angle of about 10° IS obtamed studies of contmental extensIOn zones, m particu We note that detachments modelled for the Basm lar wlthm the Basm and Range Provmce It fur and Range Provmce have shallow regIOnal Imtlal ther emphaSizes the Importance of asymmetry m dips of 10-30° [8] These detachment or shear the mterpretatlOn of contmental extensIOn zones tend to shallow or steepen due to a number We mvoke regIOnal vanatlOn m depth of exten of factors [8] but usually do not follow regIOnal SIOnal stram to account for the contrastmg uphft sub-honzontal boundanes due to changmg rheo subSidence of the TAM-Ross Embayment system logical properhes between different crustal layers and suggest that the fundamental controllmg The Idea of bemg able to apply a regIOnal dip to a structure for the development of thiS extem.lOnal detachment or shear zone and that tills shear orogen IS a shallow westerly dlppmg detachment surface can penetrate rheological boundanes has fault zone which has allowed translatIOn of exten been con fumed [45] SIOnal stram from crustal levels m the Ross Em These regIOnal angles of dip of the detachment, bayment to subcrustal levels beneath the TAM together With the 10 km reductIOn m crustal thick The proposed detachment fault IS most likely ness suggests that the detachment fault here IS comcldent With an old plane of weakness due to unhkely to accommodate much more than 50-100 early PalaeozOiC subductIOn, and Implies passive km of extensIOn The posslblhty of Similar detach rather than active nftmg ExtenSIOn, of the order ments, perhaps not as major, m the central and of 200 km across the Ross Embayment IS sug eastern Ros~ Embayment IS, however, suggested gested, but cannot be accommodated on the one by the geometry of the basement m thiS regIOn detachment fault beneath the TAM Front, so It IS Profound geometnc slmtlanty to the Basm and suggested that other detachment faults eXist under Range Provmce [8,40] may be observed With the Basm and Range style morphology m the rest mendlOnal trendmg sedimentary basms separated of the Ross Embayment to the east of the Vlctona by relahvely narrow highs exposmg metamorpillc Land Basm (Fig 5) These detachment faults basement Indeed, It IS conceivable that these should overlap through any given vertical lme to basement highs are analogous to the Cordilleran allow for the pOSSibilIty that stram IS accomodated Core complexes m the Basm and Range Provmce entirely wlthm the detachment zone and that the which are exposed between shallow dlPpmg de crustal "lozenges" (Fig 5) that he between the tachment zones [8,9,48] Furthermore, crustal thm detachment zones are essentially stram free mng due to low-angle detachment faultmg may Hmz and Block [32] show that any deformatIOn have resulted m widespread tectomc unroofmg m the Central Trough and Eastern Basm of the wlthm the Ross Embayment, the presence of calc Ross Sea must predate the Late Oligocene, as slhcate gneiss dnlled at DSDP-270 [37], bemg sediments thought to be that age and younger are consistent with thiS model undeformed The Vlctona Land Basm IS, however, an active tectomc basm, and multIchannel seismiC 6. Conclusion data from here suggest two penods of nftmg tentatIvely dated as late MesozOiC and then The development of the TAM and the Ross Paleogene(?) to recent [34] Sea-floor magnetic Embayment has been shown to be broadly con studies [49] suggest that relative displacement be temporaneous FISSIOn track datmg studies show tween East and West AntarctIca took place some that there has been about 5 km of uplIft m the time from the Late Cretaceous to Late Eocene but 77 that the tnlling and amount of dIsplacement IS R 0 Oliver, P R James and J B Jago, eds, pp 533-538, uncertam What IS clear both from the geophysIcal Austrahan Academy of SCience, Canberra, ACT, 1983 and geologICal eVIdence summansed here, IS that 5 D McKenzie, Some remarks on the development of sedi mentary basllls, Earth Planet SCI Lett 40,25-32, 1978 from the Late Cretaceous/Early CenozoIc the Ross 6 G S Lister, M A Ethendge and P A Symonds, Detach Embayment has been the sIte of extenslOn and ment faultlllg and the evolutIOn of pa551ve contillental basm formatlOn, the TAM has been the locus of margllls, Geology 14, 246-250, 1986 uphft and that these two events are related 7 B Werrucke, Low-angle normal faults III the Ba51ll and Whlle the model presented here for the evolu Range Provillce nappe tectorucs III an extendlllg orogen, Nature 291, 645-648, 1981 tlOn of the TAM-Ross Embayment system IS 8 B Wermcke, Uruform-sense normal Simple shear of the largely conceptual, It does have Important conse contillental lith05phere, Can J Earth SCI 22, 108-125, quences for the directlOn of future research m the 1985 reglOn allowmg for the refmement of mput param 9 B Wernicke and B C Burchflcl, Modes of extemlOnal eters as well as a test of the baSIC Idea The tectomcs, J Struct Geol 4,105-115,1982 10 A G Smith and D J Drewry, Delayed phase change due to stratlgraphy and age of the sedIments of the Ross hot asthenosphere causes Transantarcl!c uplift", Nature Sea basms, espeCIally the Victona Land Basm, 309, 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