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When did the Falklands rotate?
Marine and Petroleum Geology 04 "0887# 612Ð625
When did the Falklands rotate< K[ Thomson Department of Geological Sciences\ University of Durham\ South Road\ Durham\ DH0 2LE\ En`land\ U[K[ Received 19 April 0887^ received in revised form 15 June 0887^ accepted 2 July 0887
Abstract Similarities in the styles and relative timings of tectonic events in the Outeniqua Basin\ South Africa and the North Falkland Basin suggest that basin formation in both regions may have preceded rotation of the Falklands microplate[ Contrary to previous models for the break!up of Gondwana\ which suggest Jurassic rotation\ the data implies Valanginian rotation\ contemporaneous with the _rst recorded motion on the Agulhas Falkland Fracture Zone and South Atlantic rifting[ The data also suggests that the formation of the Falkland Plateau Basin may also be a Cretaceous event as opposed to the previously assumed Jurassic age[ Such a model is consistent with new o}shore seismic evidence while the inconclusive nature of the supportive evidence for Jurassic rotation does not exclude later rotation as a possibility[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Keywords] North Falkland Basin] Outeniqua Basin^ Falkland Plateau Basin^ Gondwan break!up
0[ Introduction Although generally accepted as a rotated microplate with closer a.nities with South Africa than South Amer! ica "Adie\ 0841^ Marshall\ 0883a^ Storey et al[\ 0885a\ b^ Curtis + Hyam\ 0887^ Thomson + Underhill\ in press# the exact timing of rotation of the Falklands microplate remains problematic[ Geological and palaeomagnetic constraints clearly illustrate that until at least Early Jur! assic times "089 ma# the Falklands microplate was adjac! ent to the southeastern coast of Natal\ being rotated through approximately 079>"Fig[ 0[ Mitchell et al[\ 0875^ Taylor + Shaw\ 0878^ Curtis + Hyam\ 0887#[ The rotation requires the collapse of the Falkland Plateau Basin with the Falkland microplate originally situated adjacent to the Maurice Ewing Bank\ both being located in the Natal Embayment "Fig[ 0[ Marshall\ 0883a#[ Unfortunately the absence of post!Lower Jurassic sedi! ments on the Falkland Islands means that post!Lower Jurassic palaeomagnetic data is unavailable and the only constraint on the age of rotation is that it must have occurred post!Lower Jurassic[ However\ palaeomagnetic data for the other microplates associated with the break! up of Gondwana "Antarctic Peninsula\ Ellsworth!Whit! more mountains\ Haag Nunataks\ Marie Byrd Land# are
Corresponding author[ Tel[] 9933 080 263 3673^ fax] 9933 080 263 1409
more extensive and clearly indicate rotation from the Middle Jurassic onwards "Grunow et al[\ 0876 and 0880\ Grunow\ 0882a\ b#[ Consequently\ it has been assumed that Falkland microplate rotation occurred at this time "e[g[ Dalziel + Grunow 0881^ Storey\ 0885^ Storey et al[\ 0885b#\ with Marshall "0883a# providing additional geological arguments to support this\ and being incor! porated into break!up models[ However\ as noted by Storey "0885# there is still considerable uncertainty over the number of microplates and their position prior to break!up\ leading to a number of break!up models "Figs 1 and 2[ Dalziel + Grunow\ 0881^ Marshall\ 0883a^ Storey\ 0885^ Storey et al[\ 0885b#\ which in some cases do not _t all the available data "Storey\ 0885#[ Indeed\ Storey "0885# states that the lack of kinematic data for the break!up process means that the published models must be con! sidered as purely speculative[ One common assumption in such models is that the rotation of the Falklands microplate was initiated during the Middle Jurassic and Marshall "0883a# has proposed several reasons to support this[ However\ the data pre! sented in this paper from the Mesozoic basins of the Falklands microplate and South Africa casts doubt on previous speculation regarding the age of rotation and may imply that rotation was later than previously suggested[ This paper will broadly outline the evolution of the Mesozoic basins of the Falklands and South Africa\ discuss the signi_cance of their histories before suggesting an alternative break!up scenario\ how it com!
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Fig[ 0[ Reconstruction placing the Falklands microplate and Maurice Ewing Bank "MEB# in their correct Lower Jurassic position and orientation with respect to Africa[ OB Outeniqua Basin\ NFB North Falkland Basin\ FPB future position of Falkland Plateau Basin\ MR Mozambique Ridge\ NE:NV Natal Embayment:Valley\ KP approximate position of Karoo plume[
pares with previous models and the data on which they were based[ 1[ Syn!rift evolution of the Outeniqua and North Falkland basins 1[0[ Outeniqua Basin The Outeniqua Basin\ situated on the southern con! tinental margin of South Africa\ consists of a number of sub!basins separated by basement highs "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ Superimposed on the older Cape Fold Belt "Lock\ 0867^ Shone et al[\ 0889# there are four major depocentres to the north\ the Bredasdorp\ Pletmos\ Gamtoos + Algoa basins as well as the smaller Infanta
Embayment "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ To the south these sub!basins pass into the bathymetrically deeper Southern Outeniqua Basin\ the southern limit of which is the Agulhas Falkland Fracture Zone "Fig[ 3[ Ben!Avraham et al[\ 0882#[ The Agulhas Falkland Frac! ture Zone strikes northeast!southwest and approaches close to the easternmost sub!basins "Algoa and Gamtoos# before forming the southeastern boundary to the Port Alfred Arch "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ The sub!basins have a common rifting history\ producing complex geometries the severity of which increases to the east[ Initial rifting occurred in the Oxfordian and Kimmeridgian with extensional reactivation of Cape Fold Belt thrusts to produce divergent syn!rift packages
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Fig[ 1[ Gondwana break!up model proposed by Grunow "0882a\ b# and modi_cations proposed in this paper[ "a# Reconstruction of Grunow "0882a\ b# at 064 ma[ "b# Reconstruction of Grunow "0882a\ b# at 044 ma[ "c# Reconstruction of Grunow "0882a\ b# at 029 ma[ "d\ e\ f\ and g# Reconstructions based on Grunow "0882a\ b# with modi_cations proposed in this paper[ "d# 064 ma[ "e# 044 ma[ "f# Just prior to South Atlantic opening "approx 029 ma#[ "g# Just after South Atlantic break!up "approx 029 ma#[ AP Antarctic Peninsula\ EWM Ellsworth Whitmore mountains\ Fl Falkland Islands\ MBL Marine Byrd Land\ SNZ New Zealand\ TI Thurston Island[
thickening towards faults which become more listric in character from west to east "Fig[ 3#[ In addition\ the faults gradually change strike direction from east!west to north! south in the easternmost sub!basins and as the Agulhas Falkland Fracture Zone is approached "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ A second more complex period of tectonism occurred during the Valanginian[ In the western sub!basins "Bre! dasdorp\ Pletmos and Infanta# this period is marked by a second phase of extension with syn!rift packages thick! ening towards normal faults "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ However\ in the Algoa and Gamtoos sub! basins the Valanginian events can be shown to produce a wider range of structural styles[ In the Algoa Basin\ where the swing in strike is less pronounced\ Valanginian tectonism resulted in a second period of extension with divergent sediment packages thickening towards the St
Croix Fault and subsequently truncated by severe post! rift canyon development "Fig[ 3[ Bate + Malan\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ In the Gamtoos Basin the Valanginian tectonism resulted in a variety of structures indicative of both extension and compression[ In the north of the basin east!west trending portions of the Gamtoos Fault experienced rapid exten! sion resulting in substantial fault and fold related top! ography at the latest Valanginian "syn to post!rift# unconformity "Fig[ 3[ Bate + Malan\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ However\ further south where the Gamtoos Fault strikes north!south the same Valanginian event can be shown to have resulted in compressional reactivation with the development of the Gamtoos Anticline in the hangingwall to the Gamtoos Fault "Fig[ 3[ Bate + Malan\ 0881^ McMillan et al[\ 0886#[ In addition\ there is evidence for reactivation of the Recife and St Francis arches at this time "Fig[ 3[ Bate + Malan\ 0881#[
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Fig[ 2[ Gondwana break!up model "Storey\ 0885^ Storey et al[\ 0885b# and modi_cations proposed in this paper[ "a# Pre!Mid!Jurassic reconstruction prior to rotation and translation of crustal blocks[ "b# Initial rifting period\ subsequent rotation of microplates[ "c# Initial drift period of East and West Gondwana[ "d\ e and f# as "a\ b and c# respectively but incorporating the modi_cations proposed in this paper[ AP Antarctic Peninsula\ EWH combined Ellsworth!Whitmore mountains!Haag Nunataks block\ FP Falkland Plateau\ GFS Gastre fault system\ MBL Marie Byrd Land\ NZ New Zealand\ TI Thurston Island\ WSE Weddell Sea Embayment[
1[1[ North Falkland Basin Situated to the north of the Falkland Islands the North Falkland Basin "NFB# consists of a series of sub!basins intimately linked to pre!existing structures currently seen at outcrop on the Falklands "Richards + Fannin\ 0885^ Richards et al[\ 0885^ Thomson + Underhill\ in press#[ The southernmost sub!basins have extension on faults which clearly ~atten at depth\ downthrowing to the nor! theast during two discrete rifting events which produced divergent syn!rift packages thickening towards them "Figs 4 and 5[ Thomson + Underhill\ in press#[ The faults strike WNW!ESE\ parallel to the trend of the Gond! wanan fold and thrust belt exposed on the Falkland Islands\ suggesting signi_cant reactivation of pre!existing structure during basin formation "Richards + Fannin\ 0885^ Richards et al[\ 0885^ Thomson + Underhill\ in press#[ Further north the in~uence of thrust reactivation gradually diminishes as the Main Graben\ which trends
north!south\ becomes dominant[ This is a gradual change from listric faulting to planar normal faulting with the listric faults occasionally seen on seismic data to be o}set by planar normal faults in the region of transition from one structural style to another "Fig[ 6[ Thomson + Underhill\ in press#[ Apart from the southernmost Main Graben the majority of the sub!basin shows no evidence for the in~uence of pre!existing structure and appears to have undergone only one period of extension "Figs 7\ 8 and 09[ Thomson + Underhill\ in press#[ This event correlates with the earlier of the two extensional events seen in the southern sub!basins with the second rift event in the south correlating to passive in_ll of remnant rift topography in the north "Thomson + Underhill\ in press#[ Immediately post!rift the NFB underwent a period of uplift\ tilting the basin down to the south resulting in a forced regressive wedge prograding southwards along the basin axis towards the Falkland Islands "Fig[ 00[ Thomson + Underhill\ in press#[
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Fig[ 3[ Compilation of seismic sections from the Outeniqua Basin "McMillan et al[\ 0886#[ IE Infanta Embayment\ RA Recife Arch\ PET Port Elizabeth Trough\ UT Uitenhage Trough\ PAA Port Alfred Arch[
There is no evidence for signi_cant tectonism preceding rifting and post!dating the Gondwanan Orogenic events seen on the Falkland Islands[ In addition\ the absence of well data from the NFB prevents dating of the events documented above[
2[ Comparisons between the North Falkland and Outeniqua basins The along strike continuations of the Cape Fold Belt and Karoo Basin are generally taken to be found on the
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Fig[ 5[ Listric faulting with thickening of two divergent syn!rift packages towards principal faults[ Southern North Falkland Basin[ Location shown on Fig[ 01[
Falkland Islands once placed in the correct pre break!up position and orientation "Adie\ 0841^ Marshall\ 0883a^ Curtis + Hyam\ 0887#[ Such a position would suggest that if any part of the history of the North Falkland and Outeniqua basins are common then they may be dated as pre break!up and rotation of the Falklands microplate[ The presence of thrusts in both basins with similar pre break!up strikes and two periods of extension could be taken as possible evidence that rifting in the NFB was synchronous with that documented in the Outeniqua Basin and possibly pre!rotational[ However\ for such a premise to stand a break!up model is required which explains] "a# Oxfordian:Kimmeridgian extension in the Outeniqua Basin and NFB prior to rotation of the Falklands microplate[ "b# Valanginian extension in the southern NFB and Out! eniqua Basins[ "c# Valanginian compression along the southern Gam! toos Fault[ "d# No Valanginian tectonism in the northern NFB[ The following sections outline such a model and discuss the wider implications[ 2[0[ Break!up model As the break!up model presented here deals only with events in western Gondwana "South America\ Africa\
Falklands microplate\ Maurice Ewing Bank and Falk! land Plateau Basin#\ the break!up history of eastern Gondwana "Antarctica\ Australia\ India and the associ! ated microplates# will not be discussed in detail[ Instead the modi_cations proposed in this paper for western Gondwana will be incorporated into existing models "Figs[ 1 and 2[ Grunow\ 0882a\ b^ Storey\ 0885^ Storey et al[\ 0885b# with the previously proposed histories for eastern Gondwana left intact[ Palaeomagnetic data from the Falkland Islands dem! onstrates that it was adjacent to South Africa during the Lower Jurassic "Fig[ 0[ Mitchell et al[\ 0875^ Taylor + Shaw\ 0878#[ The data was derived from Lower Jurassic dykes which were associated with the Karoo plume "Cox\ 0881#[ The plume\ centred on Mozambique\ was respon! sible for extensive igneous activity in Africa and Ant! arctica "Burke + Dewey\ 0861^ White + McKenzie\ 0878\ Cox\ 0881# with volcanism occurring between 082 and 051 ma\ pre!dating the opening of the Mozambique and Somali basins and the South Weddell Sea at approxi! mately 069Ð049 ma\ which initiated the separation of East and West Gondwana "Figs 1 and 2[ Martin + Hartnady\ 0875^ Cochran\ 0877^ Grunow\ 0882a\ b^ Storey\ 0885^ Storey et al[\ 0885b#[ South Weddell Sea spreading ceased when North Weddell Sea spreading commenced around 049 ma "Figs 1 and 2[ Grunow\ 0882a\ b\ Storey et al[\ 0885b#[ North Weddell Sea spreading initiated at approxi!
F ig. 5 . Listric faulting with associated synthetic and antithetic faulting. Southern North Falkland Basin. Location shown on Fig. 12.
F ig. 7. Seism ic sectio n fr o m th e N o r th F alk lan d B asin with ear ly listr ic fau ltin g an d later n o r m al faltin g. L o catio n sh o wn o n F ig. 12.
F ig. 8 . Ssismic section from the North Falkland Basin illustrating half graben development associated with planar normal faults. Note that there is only one period of extension. Location shown on Fig. 12.
F ig. 9 . Seismic section from the North Flakland Basin illustrating half graben development associated with planar normal faults. Note that there is only one period of extension. Location shown on Fig. 12.
Fig. 10. Seismic section from the North Falkland Basin illustrating half graben development associated with planar normal faults. Note that there is only one period of extension. Location shown on Fig. 12.
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Fig[ 00[ Seismic section from the North Falkland Basin showing an immediate post!rift forced regressive sequence associated with uplift\ tilting the North Falkland Basin down to the south[ Location shown on Fig[ 01[
mately the same time as the rifting in the Outeniqua Basin "Oxfordian:Kimmeridgian# and by inference the NFB[ Rifting in these basins can be readily explained by ref! erence to plume related break!up processes[ The plume\ by virtue of its thermal anomaly\ would have created a topographic anomaly with a diameter of approximately 1999 km "Fig[ 02[ White + McKenzie\ 0878# with ana! logue modelling "Withjack + Scheiner\ 0871# predicting a radial orientation of extensional structures in such cases and hence northeast!southwest trending faults in the North Falkland and Outeniqua basins unless pre!existing structures were reactivated or the Falklands had already rotated[ Such an orientation closely matches that of the majority of the NFB "Fig[ 0# with the exception of the southern sub!basins which clearly exploited pre!existing structure[ Similarly\ the faulting in the Outeniqua Basin does not follow the predicted trend but again exploits pre!existing structural grain "Fig[ 0#[ This apparent dis! crepancy can be accounted for by the fact that the react! ivated thrusts seen in the Outeniqua and southern North Falkland basins are shallower compared to the northern NFB "i[e[ the down dip direction# and hence easier to reactivate[
The second phase of tectonism in the Outeniqua Basin and presumably the NFB occurred during the Valan! ginian\ post East and West Gondwana separation and contemporaneous with the initiation of South Atlantic break!up and movement of the Maurice Ewing Bank from the Natal Embayment "Figs 1\ 2 and 02[ Fouche et al[\ 0881#[ Assuming that the Falklands had not rotated by this time and that the major extension between the Falkland Islands and Maurice Ewing Bank is Val! anginian then this episode can adequately explain the Valanginian tectonism seen[ Movement of the Falklands microplate:Maurice Ewing Bank along the Agulhas Falkland Fracture Zone had a dextral sense of motion[ The induced stresses associated with this sense of motion would predict extension on structures orientated east! west in the North Falkland and Outeniqua basins and compression on north!south trending structures[ Such a prediction closely matches the evidence from the Out! eniqua Basin where extension occurred on east!west tren! ding faults while the north!south trending Gamtoos Fault experienced inversion "Fig[ 3#[ Furthermore\ the southern NFB would have been orientated east!west at this time and would have undergone a second period of rifting
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Fig[ 01[ Location map for North Falkland Basin seismic sections referred to in this paper[
Fig[ 02[ Proposed model for the timing of rifting in the Outeniqua and North Falkland basins\ rotation of the Falklands microplate and formation of the Falkland Plateau Basin[
"Figs 4 and 5# while the lack of a second rift phase in the majority of the NFB can be explained by the fact that it was orientated almost parallel to the Agulhas Falkland
Fracture Zone and hence unlikely to undergo appreciable movement "Figs 0\ 7\ 8 and 09#[ With initiation of dextral movement on the Agulhas
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Falkland Fracture Zone\ sea!~oor spreading in the South Atlantic\ Natal Valley and possibly the Falkland Plateau Basin it would be possible to rapidly rotate the Falklands into its current position relative to South America "Fig[ 02#[ The cessation of rotation could well be marked by the post!rift uplift of the northern margin of the Falkland Plateau as once docked with South America and clear of South African continental crust the margin would ~exurally unload[ This would result in uplift and erosion of the northern margin with material prograding south! wards down the basin axis towards the Falklands "Fig[ 00#[
3[ Discussion The break!up model described in this paper di}ers markedly from previous proposals "e[g[ Marshall\ 0883a^ Storey\ 0885^ Storey et al[\ 0885b#[ However\ unlike pre! vious models it can be shown to be consistent with o}! shore seismic data from the NFB and Outeniqua basins[ The similarity of rifting events is key to this model[ If rifting can be shown to have the same timing in the North Falkland and Outeniqua basins then it could be taken as partial proof for the model[ Although not conclusive the question would arise as to how two basins could have rifted simultaneously with similar structural styles when one had previously rotated through 019> to become an integral part of South America with the remaining 59> of rotation due to separation of South America from Africa "Mitchell et al[\ 0875^ Taylor + Shaw\ 0878#[ The lack of rotational structures within the NFB and along the microplate margins has been taken to imply that rotation must have pre!dated extension in the NFB[ Consequently\ the lack of such evidence could imply that the model described in this paper is invalid[ However\ there is also no evidence for rotation both onshore and o}shore which post!dates Gondwanan orogenic events and pre!dates rifting[ If the same logic is applied then there is no structural evidence for rotation before\ during or after formation of the NFB[ Consequently\ the only quantitative evidence is the palaeomagnetic analysis of Mitchell et al[ "0875# and Taylor + Shaw "0878#[ Marshall "0883a# has suggested that the palaeomagnetic evidence from the Falkland Islands is consistent with Jurassic rotation[ Although this may be the case\ the reliability of the palaeomagnetic evidence is still open to question[ The Lower Jurassic dykes of the Falkland Islands have provided inconsistent results with the north!south tren! ding dykes of West Falkland "Fig[ 0# indicating rotation of 079> while the Cape Orford dykes suggest 89> and the east!west trending dykes in the Cape Meredith region providing no usable results "Mitchell et al[\ 0875^ Taylor + Shaw\ 0878#[ The radiometric dating is also incon! clusive with a single K!Ar date from the Cape Meredith dykes "081209 ma^ Cingolana + Varela\ 0865#\ two
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acceptable 39Ar!28Ar dates of around 089 ma from the Cape Orford and Cape Meredith dykes "Mussett + Taylor\ 0883# and more recently two K!Ar dates from the Cape Meredith dykes of 06526 ma and 05125 ma "Thistlewood et al[\ 0886#[ These dates suggest that the Cape Meredith dykes\ for which there is no palaeo! magnetic data\ are probably the same age as the Cape Orford dykes and if the palaeomagnetic results from Cape Orford are correct then the Cape Meredith dykes probably rotated 89>[ However\ the radiometric dates do not allow the age of the north!south trending dykes of West Falkland to be known with certainty[ If they are the same age as the Cape Orford dykes then the palaeo! magnetic data is confusing and Marshall|s "0883a# suggestion of partial rotation during dyke emplacement is a possibility[ However\ the limited and inconsistent nature of this data suggests that further work needs to be carried out to con_rm Marshall|s "0883a# hypothesis and consequently other interpretations of the data\ such as that presented in this paper cannot be excluded[ The model presented in this paper provides a radical re! interpretation not only of the evolution of the Falkland Islands but also of the Falkland Plateau Basin[ Marshall "0883a# suggested that the similarity in the length of the Agulhas Falkland Fracture Zone on both the continental margin of South Africa and the northern margin of the Falkland Plateau implies that the Falklands must have been close to its present position relative to South Amer! ica prior to separation of South America and Africa during the Valanginian[ Combined with the widely accepted age for the formation of the Falkland Plateau Basin "Early Jurassic^ Lorenzo + Mutter\ 0877#\ and the fact that rotation of the Falklands could not have occurred until it had cleared the Mozambique Ridge: Agulhas Plateau\ led Marshall "0883a# to the conclusion that rotation was achieved during the Middle and Upper Jurassic[ Consequently\ the model presented in this paper needs to address these points[ The suggestion of Valanginian rotation in this paper implies that the Falkland Plateau Basin did not exist until this time so that the North Falkland and Outeniqua basins were continuous[ It also suggests that the _rst signi_cant movement in the Agulhas Falkland Fracture Zone was Valanginian and not Jurassic[ The incon! sistencies in the palaeomagnetic data "Mitchell et al[\ 0875^ Taylor + Shaw\ 0878# which have been taken by Marshall "0883a# to suggest Jurassic rotation could be used as su.cient cause to suggest the alternative view that they only imply post!Lower Jurassic rotation which requires further clari_cation[ Consequently the roles of the Agulhas Falkland Fracture Zone and the Falkland Plateau Basin become key to the problem[ Valanginian tectonism in the basins of South Africa provides the earl! iest available evidence for movement on the Agulhas Falkland Zone "c[f[ Ben!Avraham et al[\ 0882#[ If this is the case then the Falklands could not have rotated prior
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to this date and the Falkland Plateau Basin could not have existed[ However\ Marshall "0883a# cites evidence from the Gastre Fault System of South America "Fig[ 02[ Rapela + Pankhurst\ 0881# to support earlier movement on the Agulhas Falkland Fracture Zone[ Rapela + Pan! khurst "0881# suggest that the Gastre Fault System is the {most suitable candidate| for being the extension of the Agulhas Falkland Fracture Zone and provide evidence for Early and Middle Jurassic dextral movement on it while Storey "0885# has suggested that on kinematic grounds it was unlikely to be the main fault system opera! ting at this time[ It is also important to note that Rapela + Pankhurst "0881# state that there are no geological constraints on the magnitude of movement on the Gastre Fault System and that there is also evidence for Late Mesozoic:Early Cenozoic movement[ Consequently\ the evidence for movement on the Agulhas Falkland Frac! ture Zone based on this argument can not be taken as conclusive proof[ Furthermore\ the San Jorge Basin "Fig[ 02[ Fitzgerald et al[\ 0889^ Keeley + Light\ 0882# is docu! mented to have undergone two periods of rifting the _rst of which is Early to Middle Jurassic\ coincident with the movement of the Gastre Fault System "Rapela + Pankhurst\ 0881#\ which due to the proximity of the basin to the Gastre Fault could suggest that the events were linked[ The second rifting event in the San Jorge Basin is Valanginian "Fitzgerald et al[\ 0889# and may suggest that movement on the Gastre Fault System occurred at this time[ Consequently\ a case could be made that there may be an undocumented Valanginian dextral event on the Gastre Fault\ and if the arguments of Storey "0885# regarding the signi_cance of the fault can be dismissed\ then this event may have been the most signi_cant and thus compatible with the timing of signi_cant movement on the Agulhas Falkland Fracture Zone outlined in this paper[ These arguments clearly illustrate that linking the Agulhas Falkland Fracture Zone to the Gastre Fault System is still open to question and requires further inves! tigation[ The inconclusive nature of the results suggests that the only hard evidence for the timing of Agulhas Falkland Fracture Zone evidence is that it was Val! anginian unless data from the Falkland Plateau Basin indicates otherwise[ Rifting in the Falkland Plateau Basin is generally accepted to have occurred during the Lower and Middle Jurassic "Marshall\ 0883a#[ This is based on DSDP holes 216\ 229 and 400 which con_rm the presence of 159 m of Middle and Upper Jurassic sediments resting on Pre! cambrian basement "Fig[ 03#\ the wells being situated on the western ~ank of the basin "i[e[ Maurice Ewing Bank#[ Lorenzo + Mutter "0877# tied 03\999 km of single and multichannel seismic data to the DSDP holes to derive a seismo!stratigraphic breakdown for the basin which suggests rifting occurred prior to the deposition of the Late Jurassic sediments\ the rift onset marker "U0^ Figs 03 and 04[ Lorenzo + Mutter\ 0877# being an uncon!
formity separating Precambrian basement from Late Jur! assic sediments and showing signi_cant o}sets across normal faults[ In addition\ Marshall "0883a\ b# highlights a signi_cant increase in thermal maturity across the Neo! comian unconformity "U1^ Lorenzo + Mutter\ 0877# which separates signi_cantly thermally more mature Jurassic sediments from the overlying Cretaceous and younger strata[ The U1 marker is also associated with signi_cant normal and reverse faulting on the northern margin of the Maurice Ewing Bank which with the associ! ated folding that resulted has been attributed to move! ment on the Agulhas Falkland Fracture Zone[ Although the data from the Falkland Plateau Basin may seem conclusive for a Jurassic age of formation and hence Jurassic movement on the Agulhas Falkland Fracture Zone and rotation of the Falklands microplate some caution is required[ Firstly\ the seismo!stratigraphic interpretation of Lorenzo + Mutter "0877# may not be a unique solution to the rather poor quality\ widely!spaced data with limited stratigraphic control[ As the DSDP holes tied to the seismic data are located on the Maurice Ewing Bank it is possible that the interpretation in the main depocentre of the Falkland Plateau Basin may be incorrect[ Indeed\ it is signi_cant to note that in their recent interpretation\ Richards et al[ "0885#\ who used a signi_cantly higher quality dataset than was available to Lorenzo + Mutter "0877#\ acknowledge on their pub! lished interpreted seismic sections the potential for incor! rect stratigraphic assignment of seismic picks[ Furthermore\ the published sections of Lorenzo + Mut! ter "0877# contain some potential for reinterpretation[ Fig[ 04a shows a line drawing published by Lorenzo + Mutter "0877# in which the U1 pick geometry shown could equally suggest that the U1 pick is simply down! thrown across the normal fault[ This allows the possibility that the section can be interpreted with the U1 pick representing the rift onset "Fig[ 04b# as predicted by the alternative break!up model presented here[ If this can be done\ and the revised U1 marker still correlates with the U1 marker on the northern ~ank of the Maurice Ewing Bank where normal and reverse faulting and associated folding can be found\ then the basin could be interpreted as forming simultaneously with the _rst signi_cant move! ment of the Agulhas Falkland Fracture Zone in the Val! anginian[ In such a case the signi_cant unconformity in DSDP holes 216\ 229 and 400 is not that between the basement and the Jurassic but the Neocomian event which would indicate rifting during the Valanginian[ With regard to the signi_cant increase in thermal maturity across the Neocomian unconformity "Marshall\ 0883a\ b# this can be explained by rift ~ank uplift and erosion of footwall highs during rifting of the Falkland Plateau Basin and possibly uplift of the Maurice Ewing Bank associated with strike!slip movement on the Agul! has Falkland Fracture Zone[ The arguments presented thus far suggest that
K[ Thomson:Marine and Petroleum Geolo`y 04 "0887# 612Ð625 Fig[ 03[ Correlation of the major tectonic and volcanic events associated with Gondwana break!up stratigraphy of the Falkland Plateau Basin and tectonic events in the Outeniqua\ North Falkland\ San Jorge and Malvinas basins[
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Fig[ 04[ "a# Line drawing from the Falkland Plateau basin[ Drawing and annotation after Lorenzo and Mutter "0877#[ Highlighted region refers to potential normal o}set of U1 "Neocomian unconformity# marker suggested in this paper to indicate possible Late Jurassic:Early Cretaceous rifting as opposed to the generally accepted interpretation of Early Jurassic rifting[ "b# Alternative interpretation of "a# with the major rifting event being associated with the U1 marker of Lorenzo and Mutter "0877#[
although Jurassic rotation of the Falklands may be valid there is still enough uncertainty to allow alternative models[ Valanginian rotation appears to be a valid alter! native providing extension in the Falkland Plateau Basin can occur at the same time[ The similarity in the lengths of the Agulhas Falkland Fracture Zone in the conjugate margins which Marshall "0883a# suggests is indicative of rotation being complete prior to the Valanginian does not preclude this[ The similarity in length depends cru! cially on how much the Falkland Plateau Basin extended and consequently it is not impossible to arrive at the current lengths for the Agulhas Falkland Fracture Zone by invoking Valanginian extension[ With regard to the suggestion that Falkland Plateau Basin extension must pre!date rotation "Marshall\ 0883a# so that the Falklands are clear of the Mozambique Ridge:Agulhas Plateau\ this is only correct until spreading commences in the Natal Valley[ Once Valanginian spreading in the Natal Valley commenced the need for Falkland Plateau Basin exten! sion to provide all the movement of the Falklands micro! plate past the Agulhas Plateau is no longer valid[ It is possible to move the Falklands microplate into a position
where rotation can occur through a combination of Falk! land Plateau Basin extension and Natal Valley spreading\ potentially with Falkland Plateau Basin extension con! tinuing for some time after the Falklands was free to rotate "Figs 02 and 03#[ Despite these arguments there is one crucial piece of structural evidence missing[ If the Falklands had rotated then there should be structural evidence for it docking with South America[ In the case of this model\ Val! anginian inversion of the Malvinas Basin and possibly the San Jorge Basin would be expected "Figs 02 and 03#[ Unfortunately\ there is no evidence for this "Fitzgerald et al[\ 0889^ Keeley + Light\ 0882#[ However\ as the other models for Gondwanan break!up which incorporate Jur! assic rotation and docking "e[g[ Marshall\ 0883a^ Storey\ 0885^ Storey et al[\ 0885b#\ and hence inversion\ cannot provide evidence for this along the Argentine margin "Keeley + Light\ 0882# the lack of suitable evidence to support docking cannot be used to dismiss the model proposed in this paper without dismissing the others[ With the uncertainties discussed in this paper and as stratigraphic evidence for contemporaneous rifting in the
K[ Thomson:Marine and Petroleum Geolo`y 04 "0887# 612Ð625
North Falkland and Outeniqua basins would not provide conclusive proof then further constraints are required on when the Falklands rotated[ As drilling of the Falkland Plateau Basin is unlikely to take place for the foreseeable future\ thus removing the possibility of dating rifting in the main depocentre\ the only quantitative test available if similar ages of rifting are found is palaeomagnetism[ The increased stratigraphy available from the NFB would allow palaeomagnetic analysis of orientated core to take place thus constraining post!Lower Jurassic rotation and consequently validating either previously proposed models\ this model or even di}erent models[
4[ Conclusions 0[ Evidence from the Outeniqua Basin suggests that rift! ing in NFB may have a similar history to South Africa[ 1[ The model predicts Oxfordian:Kimmeridgian rifting in both areas followed by a complex tectonic event producing extension and compression due to dextral motion on the Agulhas Falkland Fracture Zone dur! ing the Valanginian[ 2[ If correct these assumptions suggest that the Falklands could not have rotated prior to the Valanginian\ the earliest known period of motion on the Agulhas Falk! land Fracture Zone[ In addition\ the model requires the Falkland Plateau Basin to have undergone sig! ni_cant extension as this time and not during the Jur! assic[ 3[ For such a model to be valid three key tests can be carried out[ Firstly\ extension in the NFB needs to be of the same age as the Outeniqua Basin[ Although not conclusive proof it is the most readily available test[ The age of rifting in the Falkland Plateau Basin would need to be constrained by further drilling in the main depocentre to con_rm that the most signi_cant phase of rifting is Valanginian and not Jurassic[ Finally\ palaeomagnetic analysis of orientated core from the NFB provides the most likely method of determining when the Falklands microplate actually rotated[
Acknowledgements The author acknowledges the Falkland Islands Government and Spectrum Energy and Information Technology Ltd[ for access to the Falklands data on which this study was based[ Michael Johnson "Spectrum# is especially thanked for his encouragement\ constructive comments and for facilitating the release of data[ SOEKOR are thanked for their cooperation with this project through the release of data and permission to publish[ In particular\ the assistance of David Broad\ Steve Mills\ Chris Davies\ Eric Jungslager and Ian McLa! chlan is acknowledged[ John Underhill and Brian Turner
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are thanked for their encouragement and constructive comments on the paper[
References Adie\ R[ J[ "0841#[ The position of the Falkland Islands in a recon! struction of Gondwanaland[ Geolo`ical Ma`azine\ 78\ 390Ð309[ Bate\ K[ J[\ + Malan\ J[ A[ "0881#[ Tectonostratigraphic evolution of the Algoa\ Gamtoos and Pletmos Basins[ O}shore South Africa[ In de Wit\ M[ J[ and Ransome\ I[ G[ D[ "Eds#\ Inversion Tectonics of the Cape Fold Belt\ Karoo and Cretaceous Basins of Southern Africa[ 50Ð62[ Balkema\ Rotterdam[ Ben!Avraham\ Z[\ Hartnady\ C[ J[ H[\ + Malan\ J[ A[ "0882#[ Early tectonic extension between the Agulhas Bank and the Falkland Plateau due to the rotation of the Lafonia microplate[ Earth and Planetary Science Letters\ 006\ 32Ð47[ Broad\ D[ S[\ + Mills\ S[ R[ "0882#[ South Africa o}ers exploratory potential in a variety of basins[ Oil and Gas Journal\ December 5th\ 27Ð33[ Burke\ K[\ + Dewey\ J[ F[ "0861#[ Plume!generated triple junctions] key indicators in applying plate tectonics to old rocks[ Journal of Geolo`y\ 70\ 395Ð322[ Cingolana\ C[ A[\ + Varela\ R[ "0865#[ Investigaciones geoloicas y geocronologicas en el extremo sur de la Isla Gran Malvina\ sector de Cabo Belgrano "Cabo Meredith#\ Islas Malvinas[ Actas 5 Con`reso Geolo`ico Ar`entino\ 0\ 346Ð379[ Cochran\ J[ R[ "0877#[ The Somali Basin\ Chain Ridge and the origin of the north Somali Basin gravity and geoid low[ Journal of Geophysical Research\ 82\ 00874Ð01997[ Cox\ K[ G[ "0881#[ Karoo igneous activity\ and early stages of the break!up of Gondwanaland[ In Storey\ B[ C[\ Alabaster\ T[ and Pankhurst\ R[ J[ "Eds#\ Ma`matism and the Causes of Continental Break!up[ Journal of the Geolo`ical Society\ London\ Special Pub! lication\ 57\ 026Ð037[ Curtis\ M[ L[\ + Hyam\ D[ M[ "0887#[ Late Palaeozoic to Mesozoic structural evolution of the Falkland Islands] a displaced segment of the Cape Fold Belt[ Journal of the Geolo`ical Society\ London\ 044\ 004Ð018[ Dalziel\ I[ W[ D[\ + Grunow\ A[ M[ "0881#[ Late Gondwanide tectonic rotations within Gondwanaland[ Tectonics\ 00\ 592Ð595[ Fitzgerald\ M[ G[\ Mitchum\ R[ M[\ Uliana\ M[ A[\ + Biddle\ K[ T[ "0889#[ Evolution of the San Jorge Basin[ American Association of Petroleum Geolo`ists Bulletin\ 63\ 768Ð819[ Fouche\ J[\ Bate\ K[ J[\ + van der Merwe\ R[ "0881#[ Plate tectonic setting of the Mesozoic Basins\ southern o}shore\ South Africa] A review[ In de Wit\ M[ J[\ + Ransome\ I[ G[ D[ "Eds#\ Inversion Tectonics of the Cape Fold Belt\ Karoo and Cretaceous Basins of Southern Africa[ "22Ð34#[ Balkema\ Rotterdam[ Grunow\ A[ M[\ Dalziel\ I[ W[ D[\ + Kent\ D[ V[ "0876#[ Mesozoic evolution of West Antarctica and the Weddel Sea Basin] New palaeomagnetic constraints[ Earth and Planetary Science Letters\ 75\ 05Ð15[ Grunow\ A[ M[\ Kent\ D[ V[\ + Dalziel\ I[ W[ D[ "0880#[ New palaeo! magnetic data from Thurston Island and their implications for the tectonics of West Antarctica[ Journal of Geophysical Research\ 85\ 06824Ð06843[ Grunow\ A[ M[\ "0882a#[ New Palaeomagnetic data from the Antarctic Peninsula and their tectonic implications[ Journal of Geophysical Research\ 87\ 02704Ð02722[ Grunow\ A[ M[\ "0882b#[ Creation and destruction of Weddell Sea ~oor in the Jurassic[ Geolo`y\ 10\ 536Ð549[ Keeley\ M[ L[\ + Light\ M[ P[ R[ "0882#[ Basin evolution and pros! pectivity of the Argentine continental margin[ Journal of Petroleum Geolo`y\ 05\ 340Ð353[ Lock\ B[ E[ "0867#[ The Cape Fold Belt of South Africa^ tectonic control
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of sedimentation[ Proceedin`s of the Geolo`ists Association\ 78\ 152Ð 170[ Lorenzo\ J[ M[\ + Mutter\ J[ C[ "0877#[ Seismic stratigraphy and tectonic evolution of the Falkland:Malvinas Plateau[ Revista Brasileira de Geociencas 07\ 080Ð199[ Marshall\ J[ E[ A[ "0883a#[ The Falkland Islands] a key element in Gondwana paleogeography[ Tectonics\ 02\ 388Ð403[ Marshall\ J[ E[ A[ "0883b#[ The Falkland Islands and the fragmentation of Gondwana] implications for hydrocarbon exploration in the Falkland Plateau[ Marine and Petroleum Geolo`y 00\ 520Ð525[ Martin\ A[ K[\ + Hartnady\ C[ J[ H[ "0875#[ Plate tectonic development of the South West Indian Ocean] a revised reconstruction of East Antarctica and Africa[ Journal of Geophysical Research\ 80\ 3656Ð 3675[ McMillan\ I[ K[\ Brink\ G[ J[\ Broad\ D[ S[\ + Maier\ J[ J[ "0886#[ Late Mesozoic Sedimentary Basins o} the South Coast of South Africa[ In R[ C[ Selley "Ed[#\ African basins[ Sedimentary Basins of the World\ 2[ Elsevier Science B[ V[ "208Ð265#[ Amsterdam[ Mitchell\ C[\ Taylor\ G[ K[\ Cox\ K[ G[\ + Shaw\ J[ "0875#[ Are the Falkland Islands a rotated microplate< Nature\ 208\ 020Ð023[ Mussett\ A[ E[\ + Taylor\ G[ K[ "0883#[ 39Ar!28Ar ages for dykes from the Falkland Islands with implications for the break!up of southern Gondwana[ Journal of the Geological Society\ London\ 040\ 68Ð70[ Rapela\ C[ W[\ + Pankhurst\ R[ J[ "0881#[ The granites of northern Patagonia and the Gastre Fault System in relation to the break!up of Gondwana[ In Storey\ B[ C[\ Alabaster\ T[\ + Pankhurst\ R[ J[ "Eds[#\ Ma`matism and the Causes of Continental Break!up[ Journal of the Geolo`ical Society\ London[ Special Publication\ 57\ 198Ð119[ Richards\ P[ C[\ + Fannin\ N[ G[ T[ "0885#[ Geology of the North Falkland Basin[ Journal of Petroleum Geolo`y\ 19\ 054Ð72[ Richards\ P[ C[\ Gatli}\ R[ W[\ Quinn\ M[ F[ Williamson\ J[ P[\ + Fannin\ N[ G[ T[ "0885#[ The geological evolution of the Falkland Islands continental shelf[ In Storey\ B[ C[\ King\ E[ C[\ + Livermore\ R[ A[ "Eds[#\ Weddell sea Tectonics and Gondwana Break!up[ Journal of the Geolo`ical Society\ London[ Special Publication 097\ 094Ð017[
Shone\ R[ W[\ Nolte\ C[ C[\ + Booth\ P[ W[ K[ "0889#[ Pre!Cape rocks of the Gamtoos area*a complex tectonostratigraphic package pre! served as a host block[ Transactions of the Geolo`ical Society of South Africa\ 82\ 505Ð513[ Storey\ B[ C[ "0885#[ Microplates and Mantle Plumes in Antarctica[ Terra Antarctica\ 2\ 80Ð091[ Storey\ B[ C[\ King\ E[ C[\ + Livermore\ R[ A[ "0885a#[ Weddell Sea Tectonics and Gondwana Break!up[ Journal of the Geolo`ical Society\ London[ Special Publication 097[ Storey\ B[ C[\ Vaughan\ A[ M[\ + Millar\ I[ L[ "0885b#[ Geodynamic evolution of the Antarctic Penninsula during Mesozoic times and its bearing on Weddell Sea history[ In Storey\ B[ C[\ King\ E[ C[\ + Livermore\ R[ A[ "Eds[#\ Weddell sea Tectonics and Gondwana Break!up[ Journal of the Geolo`ical Society\ London[ Special Pub! lication 097\ 76Ð092[ Taylor\ G[ K[\ + Shaw\ J[ "0878#[ The Falkland islands] new palaeo! magnetic data and their origin as a displaced terrane from southern[ In Hillhouse\ J[ W[ "Ed[#\ Deep structure and past kinematics of accreted terranes[ Geophysical Mono`raph\ 49\ IUGG Vol[ 4\ 48Ð 61[ Thistlehood\ L[\ Leat\ P[ T[\ Millar\ I[ L[ Storey\ B[ C[\ + Vaughan\ A[ P[ M[ "0886#[ Basement geology and Palaeozoic!Mesozoic ma_c dykes from the Cape Meredith Complex\ Falkland Islands] a record of repeated intracontinental extension[ Geolo`ical Ma`azine\ 023\ 244Ð256[ Thomson\ K[\ + Underhill\ J[ R[ "in press#[ Frontier exploration in the South Atlantic] Structural prospectivity in the North Falkland Basin[ American Association of Petroleum Geolo`ists Bulletin[ Withjack\ M[ O[\ + Scheiner\ C[ "0871#[ Fault patterns associated with domes*An experimental and analytical study[ American Associ! ation of Petroleum Geolo`ists Bulletin\ 55\ 291Ð205[ White\ R[ S[\ + McKenzie\ D[ P[ "0878#[ Magmatism at rift zones] the generation of volcanic continental margins and ~ood basalts[ Journal of Geophysical Research 83\ 6574Ð6618[
When did the Falklands rotate< K[ Thomson Department of Geological Sciences\ University of Durham\ South Road\ Durham\ DH0 2LE\ En`land\ U[K[ Received 19 April 0887^ received in revised form 15 June 0887^ accepted 2 July 0887
Abstract Similarities in the styles and relative timings of tectonic events in the Outeniqua Basin\ South Africa and the North Falkland Basin suggest that basin formation in both regions may have preceded rotation of the Falklands microplate[ Contrary to previous models for the break!up of Gondwana\ which suggest Jurassic rotation\ the data implies Valanginian rotation\ contemporaneous with the _rst recorded motion on the Agulhas Falkland Fracture Zone and South Atlantic rifting[ The data also suggests that the formation of the Falkland Plateau Basin may also be a Cretaceous event as opposed to the previously assumed Jurassic age[ Such a model is consistent with new o}shore seismic evidence while the inconclusive nature of the supportive evidence for Jurassic rotation does not exclude later rotation as a possibility[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Keywords] North Falkland Basin] Outeniqua Basin^ Falkland Plateau Basin^ Gondwan break!up
0[ Introduction Although generally accepted as a rotated microplate with closer a.nities with South Africa than South Amer! ica "Adie\ 0841^ Marshall\ 0883a^ Storey et al[\ 0885a\ b^ Curtis + Hyam\ 0887^ Thomson + Underhill\ in press# the exact timing of rotation of the Falklands microplate remains problematic[ Geological and palaeomagnetic constraints clearly illustrate that until at least Early Jur! assic times "089 ma# the Falklands microplate was adjac! ent to the southeastern coast of Natal\ being rotated through approximately 079>"Fig[ 0[ Mitchell et al[\ 0875^ Taylor + Shaw\ 0878^ Curtis + Hyam\ 0887#[ The rotation requires the collapse of the Falkland Plateau Basin with the Falkland microplate originally situated adjacent to the Maurice Ewing Bank\ both being located in the Natal Embayment "Fig[ 0[ Marshall\ 0883a#[ Unfortunately the absence of post!Lower Jurassic sedi! ments on the Falkland Islands means that post!Lower Jurassic palaeomagnetic data is unavailable and the only constraint on the age of rotation is that it must have occurred post!Lower Jurassic[ However\ palaeomagnetic data for the other microplates associated with the break! up of Gondwana "Antarctic Peninsula\ Ellsworth!Whit! more mountains\ Haag Nunataks\ Marie Byrd Land# are
Corresponding author[ Tel[] 9933 080 263 3673^ fax] 9933 080 263 1409
more extensive and clearly indicate rotation from the Middle Jurassic onwards "Grunow et al[\ 0876 and 0880\ Grunow\ 0882a\ b#[ Consequently\ it has been assumed that Falkland microplate rotation occurred at this time "e[g[ Dalziel + Grunow 0881^ Storey\ 0885^ Storey et al[\ 0885b#\ with Marshall "0883a# providing additional geological arguments to support this\ and being incor! porated into break!up models[ However\ as noted by Storey "0885# there is still considerable uncertainty over the number of microplates and their position prior to break!up\ leading to a number of break!up models "Figs 1 and 2[ Dalziel + Grunow\ 0881^ Marshall\ 0883a^ Storey\ 0885^ Storey et al[\ 0885b#\ which in some cases do not _t all the available data "Storey\ 0885#[ Indeed\ Storey "0885# states that the lack of kinematic data for the break!up process means that the published models must be con! sidered as purely speculative[ One common assumption in such models is that the rotation of the Falklands microplate was initiated during the Middle Jurassic and Marshall "0883a# has proposed several reasons to support this[ However\ the data pre! sented in this paper from the Mesozoic basins of the Falklands microplate and South Africa casts doubt on previous speculation regarding the age of rotation and may imply that rotation was later than previously suggested[ This paper will broadly outline the evolution of the Mesozoic basins of the Falklands and South Africa\ discuss the signi_cance of their histories before suggesting an alternative break!up scenario\ how it com!
S9153Ð7061:87:,*see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved PII] S 9 1 5 3 Ð 7 0 6 1 " 8 7 # 9 9 9 4 9 Ð 5
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Fig[ 0[ Reconstruction placing the Falklands microplate and Maurice Ewing Bank "MEB# in their correct Lower Jurassic position and orientation with respect to Africa[ OB Outeniqua Basin\ NFB North Falkland Basin\ FPB future position of Falkland Plateau Basin\ MR Mozambique Ridge\ NE:NV Natal Embayment:Valley\ KP approximate position of Karoo plume[
pares with previous models and the data on which they were based[ 1[ Syn!rift evolution of the Outeniqua and North Falkland basins 1[0[ Outeniqua Basin The Outeniqua Basin\ situated on the southern con! tinental margin of South Africa\ consists of a number of sub!basins separated by basement highs "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ Superimposed on the older Cape Fold Belt "Lock\ 0867^ Shone et al[\ 0889# there are four major depocentres to the north\ the Bredasdorp\ Pletmos\ Gamtoos + Algoa basins as well as the smaller Infanta
Embayment "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ To the south these sub!basins pass into the bathymetrically deeper Southern Outeniqua Basin\ the southern limit of which is the Agulhas Falkland Fracture Zone "Fig[ 3[ Ben!Avraham et al[\ 0882#[ The Agulhas Falkland Frac! ture Zone strikes northeast!southwest and approaches close to the easternmost sub!basins "Algoa and Gamtoos# before forming the southeastern boundary to the Port Alfred Arch "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ The sub!basins have a common rifting history\ producing complex geometries the severity of which increases to the east[ Initial rifting occurred in the Oxfordian and Kimmeridgian with extensional reactivation of Cape Fold Belt thrusts to produce divergent syn!rift packages
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Fig[ 1[ Gondwana break!up model proposed by Grunow "0882a\ b# and modi_cations proposed in this paper[ "a# Reconstruction of Grunow "0882a\ b# at 064 ma[ "b# Reconstruction of Grunow "0882a\ b# at 044 ma[ "c# Reconstruction of Grunow "0882a\ b# at 029 ma[ "d\ e\ f\ and g# Reconstructions based on Grunow "0882a\ b# with modi_cations proposed in this paper[ "d# 064 ma[ "e# 044 ma[ "f# Just prior to South Atlantic opening "approx 029 ma#[ "g# Just after South Atlantic break!up "approx 029 ma#[ AP Antarctic Peninsula\ EWM Ellsworth Whitmore mountains\ Fl Falkland Islands\ MBL Marine Byrd Land\ SNZ New Zealand\ TI Thurston Island[
thickening towards faults which become more listric in character from west to east "Fig[ 3#[ In addition\ the faults gradually change strike direction from east!west to north! south in the easternmost sub!basins and as the Agulhas Falkland Fracture Zone is approached "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ A second more complex period of tectonism occurred during the Valanginian[ In the western sub!basins "Bre! dasdorp\ Pletmos and Infanta# this period is marked by a second phase of extension with syn!rift packages thick! ening towards normal faults "Fig[ 3[ Bate + Malan\ 0881^ Fouche et al[\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ However\ in the Algoa and Gamtoos sub! basins the Valanginian events can be shown to produce a wider range of structural styles[ In the Algoa Basin\ where the swing in strike is less pronounced\ Valanginian tectonism resulted in a second period of extension with divergent sediment packages thickening towards the St
Croix Fault and subsequently truncated by severe post! rift canyon development "Fig[ 3[ Bate + Malan\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ In the Gamtoos Basin the Valanginian tectonism resulted in a variety of structures indicative of both extension and compression[ In the north of the basin east!west trending portions of the Gamtoos Fault experienced rapid exten! sion resulting in substantial fault and fold related top! ography at the latest Valanginian "syn to post!rift# unconformity "Fig[ 3[ Bate + Malan\ 0881^ Broad + Mills\ 0882^ McMillan et al[\ 0886#[ However\ further south where the Gamtoos Fault strikes north!south the same Valanginian event can be shown to have resulted in compressional reactivation with the development of the Gamtoos Anticline in the hangingwall to the Gamtoos Fault "Fig[ 3[ Bate + Malan\ 0881^ McMillan et al[\ 0886#[ In addition\ there is evidence for reactivation of the Recife and St Francis arches at this time "Fig[ 3[ Bate + Malan\ 0881#[
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Fig[ 2[ Gondwana break!up model "Storey\ 0885^ Storey et al[\ 0885b# and modi_cations proposed in this paper[ "a# Pre!Mid!Jurassic reconstruction prior to rotation and translation of crustal blocks[ "b# Initial rifting period\ subsequent rotation of microplates[ "c# Initial drift period of East and West Gondwana[ "d\ e and f# as "a\ b and c# respectively but incorporating the modi_cations proposed in this paper[ AP Antarctic Peninsula\ EWH combined Ellsworth!Whitmore mountains!Haag Nunataks block\ FP Falkland Plateau\ GFS Gastre fault system\ MBL Marie Byrd Land\ NZ New Zealand\ TI Thurston Island\ WSE Weddell Sea Embayment[
1[1[ North Falkland Basin Situated to the north of the Falkland Islands the North Falkland Basin "NFB# consists of a series of sub!basins intimately linked to pre!existing structures currently seen at outcrop on the Falklands "Richards + Fannin\ 0885^ Richards et al[\ 0885^ Thomson + Underhill\ in press#[ The southernmost sub!basins have extension on faults which clearly ~atten at depth\ downthrowing to the nor! theast during two discrete rifting events which produced divergent syn!rift packages thickening towards them "Figs 4 and 5[ Thomson + Underhill\ in press#[ The faults strike WNW!ESE\ parallel to the trend of the Gond! wanan fold and thrust belt exposed on the Falkland Islands\ suggesting signi_cant reactivation of pre!existing structure during basin formation "Richards + Fannin\ 0885^ Richards et al[\ 0885^ Thomson + Underhill\ in press#[ Further north the in~uence of thrust reactivation gradually diminishes as the Main Graben\ which trends
north!south\ becomes dominant[ This is a gradual change from listric faulting to planar normal faulting with the listric faults occasionally seen on seismic data to be o}set by planar normal faults in the region of transition from one structural style to another "Fig[ 6[ Thomson + Underhill\ in press#[ Apart from the southernmost Main Graben the majority of the sub!basin shows no evidence for the in~uence of pre!existing structure and appears to have undergone only one period of extension "Figs 7\ 8 and 09[ Thomson + Underhill\ in press#[ This event correlates with the earlier of the two extensional events seen in the southern sub!basins with the second rift event in the south correlating to passive in_ll of remnant rift topography in the north "Thomson + Underhill\ in press#[ Immediately post!rift the NFB underwent a period of uplift\ tilting the basin down to the south resulting in a forced regressive wedge prograding southwards along the basin axis towards the Falkland Islands "Fig[ 00[ Thomson + Underhill\ in press#[
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Fig[ 3[ Compilation of seismic sections from the Outeniqua Basin "McMillan et al[\ 0886#[ IE Infanta Embayment\ RA Recife Arch\ PET Port Elizabeth Trough\ UT Uitenhage Trough\ PAA Port Alfred Arch[
There is no evidence for signi_cant tectonism preceding rifting and post!dating the Gondwanan Orogenic events seen on the Falkland Islands[ In addition\ the absence of well data from the NFB prevents dating of the events documented above[
2[ Comparisons between the North Falkland and Outeniqua basins The along strike continuations of the Cape Fold Belt and Karoo Basin are generally taken to be found on the
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Fig[ 5[ Listric faulting with thickening of two divergent syn!rift packages towards principal faults[ Southern North Falkland Basin[ Location shown on Fig[ 01[
Falkland Islands once placed in the correct pre break!up position and orientation "Adie\ 0841^ Marshall\ 0883a^ Curtis + Hyam\ 0887#[ Such a position would suggest that if any part of the history of the North Falkland and Outeniqua basins are common then they may be dated as pre break!up and rotation of the Falklands microplate[ The presence of thrusts in both basins with similar pre break!up strikes and two periods of extension could be taken as possible evidence that rifting in the NFB was synchronous with that documented in the Outeniqua Basin and possibly pre!rotational[ However\ for such a premise to stand a break!up model is required which explains] "a# Oxfordian:Kimmeridgian extension in the Outeniqua Basin and NFB prior to rotation of the Falklands microplate[ "b# Valanginian extension in the southern NFB and Out! eniqua Basins[ "c# Valanginian compression along the southern Gam! toos Fault[ "d# No Valanginian tectonism in the northern NFB[ The following sections outline such a model and discuss the wider implications[ 2[0[ Break!up model As the break!up model presented here deals only with events in western Gondwana "South America\ Africa\
Falklands microplate\ Maurice Ewing Bank and Falk! land Plateau Basin#\ the break!up history of eastern Gondwana "Antarctica\ Australia\ India and the associ! ated microplates# will not be discussed in detail[ Instead the modi_cations proposed in this paper for western Gondwana will be incorporated into existing models "Figs[ 1 and 2[ Grunow\ 0882a\ b^ Storey\ 0885^ Storey et al[\ 0885b# with the previously proposed histories for eastern Gondwana left intact[ Palaeomagnetic data from the Falkland Islands dem! onstrates that it was adjacent to South Africa during the Lower Jurassic "Fig[ 0[ Mitchell et al[\ 0875^ Taylor + Shaw\ 0878#[ The data was derived from Lower Jurassic dykes which were associated with the Karoo plume "Cox\ 0881#[ The plume\ centred on Mozambique\ was respon! sible for extensive igneous activity in Africa and Ant! arctica "Burke + Dewey\ 0861^ White + McKenzie\ 0878\ Cox\ 0881# with volcanism occurring between 082 and 051 ma\ pre!dating the opening of the Mozambique and Somali basins and the South Weddell Sea at approxi! mately 069Ð049 ma\ which initiated the separation of East and West Gondwana "Figs 1 and 2[ Martin + Hartnady\ 0875^ Cochran\ 0877^ Grunow\ 0882a\ b^ Storey\ 0885^ Storey et al[\ 0885b#[ South Weddell Sea spreading ceased when North Weddell Sea spreading commenced around 049 ma "Figs 1 and 2[ Grunow\ 0882a\ b\ Storey et al[\ 0885b#[ North Weddell Sea spreading initiated at approxi!
F ig. 5 . Listric faulting with associated synthetic and antithetic faulting. Southern North Falkland Basin. Location shown on Fig. 12.
F ig. 7. Seism ic sectio n fr o m th e N o r th F alk lan d B asin with ear ly listr ic fau ltin g an d later n o r m al faltin g. L o catio n sh o wn o n F ig. 12.
F ig. 8 . Ssismic section from the North Falkland Basin illustrating half graben development associated with planar normal faults. Note that there is only one period of extension. Location shown on Fig. 12.
F ig. 9 . Seismic section from the North Flakland Basin illustrating half graben development associated with planar normal faults. Note that there is only one period of extension. Location shown on Fig. 12.
Fig. 10. Seismic section from the North Falkland Basin illustrating half graben development associated with planar normal faults. Note that there is only one period of extension. Location shown on Fig. 12.
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Fig[ 00[ Seismic section from the North Falkland Basin showing an immediate post!rift forced regressive sequence associated with uplift\ tilting the North Falkland Basin down to the south[ Location shown on Fig[ 01[
mately the same time as the rifting in the Outeniqua Basin "Oxfordian:Kimmeridgian# and by inference the NFB[ Rifting in these basins can be readily explained by ref! erence to plume related break!up processes[ The plume\ by virtue of its thermal anomaly\ would have created a topographic anomaly with a diameter of approximately 1999 km "Fig[ 02[ White + McKenzie\ 0878# with ana! logue modelling "Withjack + Scheiner\ 0871# predicting a radial orientation of extensional structures in such cases and hence northeast!southwest trending faults in the North Falkland and Outeniqua basins unless pre!existing structures were reactivated or the Falklands had already rotated[ Such an orientation closely matches that of the majority of the NFB "Fig[ 0# with the exception of the southern sub!basins which clearly exploited pre!existing structure[ Similarly\ the faulting in the Outeniqua Basin does not follow the predicted trend but again exploits pre!existing structural grain "Fig[ 0#[ This apparent dis! crepancy can be accounted for by the fact that the react! ivated thrusts seen in the Outeniqua and southern North Falkland basins are shallower compared to the northern NFB "i[e[ the down dip direction# and hence easier to reactivate[
The second phase of tectonism in the Outeniqua Basin and presumably the NFB occurred during the Valan! ginian\ post East and West Gondwana separation and contemporaneous with the initiation of South Atlantic break!up and movement of the Maurice Ewing Bank from the Natal Embayment "Figs 1\ 2 and 02[ Fouche et al[\ 0881#[ Assuming that the Falklands had not rotated by this time and that the major extension between the Falkland Islands and Maurice Ewing Bank is Val! anginian then this episode can adequately explain the Valanginian tectonism seen[ Movement of the Falklands microplate:Maurice Ewing Bank along the Agulhas Falkland Fracture Zone had a dextral sense of motion[ The induced stresses associated with this sense of motion would predict extension on structures orientated east! west in the North Falkland and Outeniqua basins and compression on north!south trending structures[ Such a prediction closely matches the evidence from the Out! eniqua Basin where extension occurred on east!west tren! ding faults while the north!south trending Gamtoos Fault experienced inversion "Fig[ 3#[ Furthermore\ the southern NFB would have been orientated east!west at this time and would have undergone a second period of rifting
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Fig[ 01[ Location map for North Falkland Basin seismic sections referred to in this paper[
Fig[ 02[ Proposed model for the timing of rifting in the Outeniqua and North Falkland basins\ rotation of the Falklands microplate and formation of the Falkland Plateau Basin[
"Figs 4 and 5# while the lack of a second rift phase in the majority of the NFB can be explained by the fact that it was orientated almost parallel to the Agulhas Falkland
Fracture Zone and hence unlikely to undergo appreciable movement "Figs 0\ 7\ 8 and 09#[ With initiation of dextral movement on the Agulhas
K[ Thomson:Marine and Petroleum Geolo`y 04 "0887# 612Ð625
Falkland Fracture Zone\ sea!~oor spreading in the South Atlantic\ Natal Valley and possibly the Falkland Plateau Basin it would be possible to rapidly rotate the Falklands into its current position relative to South America "Fig[ 02#[ The cessation of rotation could well be marked by the post!rift uplift of the northern margin of the Falkland Plateau as once docked with South America and clear of South African continental crust the margin would ~exurally unload[ This would result in uplift and erosion of the northern margin with material prograding south! wards down the basin axis towards the Falklands "Fig[ 00#[
3[ Discussion The break!up model described in this paper di}ers markedly from previous proposals "e[g[ Marshall\ 0883a^ Storey\ 0885^ Storey et al[\ 0885b#[ However\ unlike pre! vious models it can be shown to be consistent with o}! shore seismic data from the NFB and Outeniqua basins[ The similarity of rifting events is key to this model[ If rifting can be shown to have the same timing in the North Falkland and Outeniqua basins then it could be taken as partial proof for the model[ Although not conclusive the question would arise as to how two basins could have rifted simultaneously with similar structural styles when one had previously rotated through 019> to become an integral part of South America with the remaining 59> of rotation due to separation of South America from Africa "Mitchell et al[\ 0875^ Taylor + Shaw\ 0878#[ The lack of rotational structures within the NFB and along the microplate margins has been taken to imply that rotation must have pre!dated extension in the NFB[ Consequently\ the lack of such evidence could imply that the model described in this paper is invalid[ However\ there is also no evidence for rotation both onshore and o}shore which post!dates Gondwanan orogenic events and pre!dates rifting[ If the same logic is applied then there is no structural evidence for rotation before\ during or after formation of the NFB[ Consequently\ the only quantitative evidence is the palaeomagnetic analysis of Mitchell et al[ "0875# and Taylor + Shaw "0878#[ Marshall "0883a# has suggested that the palaeomagnetic evidence from the Falkland Islands is consistent with Jurassic rotation[ Although this may be the case\ the reliability of the palaeomagnetic evidence is still open to question[ The Lower Jurassic dykes of the Falkland Islands have provided inconsistent results with the north!south tren! ding dykes of West Falkland "Fig[ 0# indicating rotation of 079> while the Cape Orford dykes suggest 89> and the east!west trending dykes in the Cape Meredith region providing no usable results "Mitchell et al[\ 0875^ Taylor + Shaw\ 0878#[ The radiometric dating is also incon! clusive with a single K!Ar date from the Cape Meredith dykes "081209 ma^ Cingolana + Varela\ 0865#\ two
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acceptable 39Ar!28Ar dates of around 089 ma from the Cape Orford and Cape Meredith dykes "Mussett + Taylor\ 0883# and more recently two K!Ar dates from the Cape Meredith dykes of 06526 ma and 05125 ma "Thistlewood et al[\ 0886#[ These dates suggest that the Cape Meredith dykes\ for which there is no palaeo! magnetic data\ are probably the same age as the Cape Orford dykes and if the palaeomagnetic results from Cape Orford are correct then the Cape Meredith dykes probably rotated 89>[ However\ the radiometric dates do not allow the age of the north!south trending dykes of West Falkland to be known with certainty[ If they are the same age as the Cape Orford dykes then the palaeo! magnetic data is confusing and Marshall|s "0883a# suggestion of partial rotation during dyke emplacement is a possibility[ However\ the limited and inconsistent nature of this data suggests that further work needs to be carried out to con_rm Marshall|s "0883a# hypothesis and consequently other interpretations of the data\ such as that presented in this paper cannot be excluded[ The model presented in this paper provides a radical re! interpretation not only of the evolution of the Falkland Islands but also of the Falkland Plateau Basin[ Marshall "0883a# suggested that the similarity in the length of the Agulhas Falkland Fracture Zone on both the continental margin of South Africa and the northern margin of the Falkland Plateau implies that the Falklands must have been close to its present position relative to South Amer! ica prior to separation of South America and Africa during the Valanginian[ Combined with the widely accepted age for the formation of the Falkland Plateau Basin "Early Jurassic^ Lorenzo + Mutter\ 0877#\ and the fact that rotation of the Falklands could not have occurred until it had cleared the Mozambique Ridge: Agulhas Plateau\ led Marshall "0883a# to the conclusion that rotation was achieved during the Middle and Upper Jurassic[ Consequently\ the model presented in this paper needs to address these points[ The suggestion of Valanginian rotation in this paper implies that the Falkland Plateau Basin did not exist until this time so that the North Falkland and Outeniqua basins were continuous[ It also suggests that the _rst signi_cant movement in the Agulhas Falkland Fracture Zone was Valanginian and not Jurassic[ The incon! sistencies in the palaeomagnetic data "Mitchell et al[\ 0875^ Taylor + Shaw\ 0878# which have been taken by Marshall "0883a# to suggest Jurassic rotation could be used as su.cient cause to suggest the alternative view that they only imply post!Lower Jurassic rotation which requires further clari_cation[ Consequently the roles of the Agulhas Falkland Fracture Zone and the Falkland Plateau Basin become key to the problem[ Valanginian tectonism in the basins of South Africa provides the earl! iest available evidence for movement on the Agulhas Falkland Zone "c[f[ Ben!Avraham et al[\ 0882#[ If this is the case then the Falklands could not have rotated prior
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to this date and the Falkland Plateau Basin could not have existed[ However\ Marshall "0883a# cites evidence from the Gastre Fault System of South America "Fig[ 02[ Rapela + Pankhurst\ 0881# to support earlier movement on the Agulhas Falkland Fracture Zone[ Rapela + Pan! khurst "0881# suggest that the Gastre Fault System is the {most suitable candidate| for being the extension of the Agulhas Falkland Fracture Zone and provide evidence for Early and Middle Jurassic dextral movement on it while Storey "0885# has suggested that on kinematic grounds it was unlikely to be the main fault system opera! ting at this time[ It is also important to note that Rapela + Pankhurst "0881# state that there are no geological constraints on the magnitude of movement on the Gastre Fault System and that there is also evidence for Late Mesozoic:Early Cenozoic movement[ Consequently\ the evidence for movement on the Agulhas Falkland Frac! ture Zone based on this argument can not be taken as conclusive proof[ Furthermore\ the San Jorge Basin "Fig[ 02[ Fitzgerald et al[\ 0889^ Keeley + Light\ 0882# is docu! mented to have undergone two periods of rifting the _rst of which is Early to Middle Jurassic\ coincident with the movement of the Gastre Fault System "Rapela + Pankhurst\ 0881#\ which due to the proximity of the basin to the Gastre Fault could suggest that the events were linked[ The second rifting event in the San Jorge Basin is Valanginian "Fitzgerald et al[\ 0889# and may suggest that movement on the Gastre Fault System occurred at this time[ Consequently\ a case could be made that there may be an undocumented Valanginian dextral event on the Gastre Fault\ and if the arguments of Storey "0885# regarding the signi_cance of the fault can be dismissed\ then this event may have been the most signi_cant and thus compatible with the timing of signi_cant movement on the Agulhas Falkland Fracture Zone outlined in this paper[ These arguments clearly illustrate that linking the Agulhas Falkland Fracture Zone to the Gastre Fault System is still open to question and requires further inves! tigation[ The inconclusive nature of the results suggests that the only hard evidence for the timing of Agulhas Falkland Fracture Zone evidence is that it was Val! anginian unless data from the Falkland Plateau Basin indicates otherwise[ Rifting in the Falkland Plateau Basin is generally accepted to have occurred during the Lower and Middle Jurassic "Marshall\ 0883a#[ This is based on DSDP holes 216\ 229 and 400 which con_rm the presence of 159 m of Middle and Upper Jurassic sediments resting on Pre! cambrian basement "Fig[ 03#\ the wells being situated on the western ~ank of the basin "i[e[ Maurice Ewing Bank#[ Lorenzo + Mutter "0877# tied 03\999 km of single and multichannel seismic data to the DSDP holes to derive a seismo!stratigraphic breakdown for the basin which suggests rifting occurred prior to the deposition of the Late Jurassic sediments\ the rift onset marker "U0^ Figs 03 and 04[ Lorenzo + Mutter\ 0877# being an uncon!
formity separating Precambrian basement from Late Jur! assic sediments and showing signi_cant o}sets across normal faults[ In addition\ Marshall "0883a\ b# highlights a signi_cant increase in thermal maturity across the Neo! comian unconformity "U1^ Lorenzo + Mutter\ 0877# which separates signi_cantly thermally more mature Jurassic sediments from the overlying Cretaceous and younger strata[ The U1 marker is also associated with signi_cant normal and reverse faulting on the northern margin of the Maurice Ewing Bank which with the associ! ated folding that resulted has been attributed to move! ment on the Agulhas Falkland Fracture Zone[ Although the data from the Falkland Plateau Basin may seem conclusive for a Jurassic age of formation and hence Jurassic movement on the Agulhas Falkland Fracture Zone and rotation of the Falklands microplate some caution is required[ Firstly\ the seismo!stratigraphic interpretation of Lorenzo + Mutter "0877# may not be a unique solution to the rather poor quality\ widely!spaced data with limited stratigraphic control[ As the DSDP holes tied to the seismic data are located on the Maurice Ewing Bank it is possible that the interpretation in the main depocentre of the Falkland Plateau Basin may be incorrect[ Indeed\ it is signi_cant to note that in their recent interpretation\ Richards et al[ "0885#\ who used a signi_cantly higher quality dataset than was available to Lorenzo + Mutter "0877#\ acknowledge on their pub! lished interpreted seismic sections the potential for incor! rect stratigraphic assignment of seismic picks[ Furthermore\ the published sections of Lorenzo + Mut! ter "0877# contain some potential for reinterpretation[ Fig[ 04a shows a line drawing published by Lorenzo + Mutter "0877# in which the U1 pick geometry shown could equally suggest that the U1 pick is simply down! thrown across the normal fault[ This allows the possibility that the section can be interpreted with the U1 pick representing the rift onset "Fig[ 04b# as predicted by the alternative break!up model presented here[ If this can be done\ and the revised U1 marker still correlates with the U1 marker on the northern ~ank of the Maurice Ewing Bank where normal and reverse faulting and associated folding can be found\ then the basin could be interpreted as forming simultaneously with the _rst signi_cant move! ment of the Agulhas Falkland Fracture Zone in the Val! anginian[ In such a case the signi_cant unconformity in DSDP holes 216\ 229 and 400 is not that between the basement and the Jurassic but the Neocomian event which would indicate rifting during the Valanginian[ With regard to the signi_cant increase in thermal maturity across the Neocomian unconformity "Marshall\ 0883a\ b# this can be explained by rift ~ank uplift and erosion of footwall highs during rifting of the Falkland Plateau Basin and possibly uplift of the Maurice Ewing Bank associated with strike!slip movement on the Agul! has Falkland Fracture Zone[ The arguments presented thus far suggest that
K[ Thomson:Marine and Petroleum Geolo`y 04 "0887# 612Ð625 Fig[ 03[ Correlation of the major tectonic and volcanic events associated with Gondwana break!up stratigraphy of the Falkland Plateau Basin and tectonic events in the Outeniqua\ North Falkland\ San Jorge and Malvinas basins[
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K[ Thomson:Marine and Petroleum Geolo`y 04 "0887# 612Ð625
Fig[ 04[ "a# Line drawing from the Falkland Plateau basin[ Drawing and annotation after Lorenzo and Mutter "0877#[ Highlighted region refers to potential normal o}set of U1 "Neocomian unconformity# marker suggested in this paper to indicate possible Late Jurassic:Early Cretaceous rifting as opposed to the generally accepted interpretation of Early Jurassic rifting[ "b# Alternative interpretation of "a# with the major rifting event being associated with the U1 marker of Lorenzo and Mutter "0877#[
although Jurassic rotation of the Falklands may be valid there is still enough uncertainty to allow alternative models[ Valanginian rotation appears to be a valid alter! native providing extension in the Falkland Plateau Basin can occur at the same time[ The similarity in the lengths of the Agulhas Falkland Fracture Zone in the conjugate margins which Marshall "0883a# suggests is indicative of rotation being complete prior to the Valanginian does not preclude this[ The similarity in length depends cru! cially on how much the Falkland Plateau Basin extended and consequently it is not impossible to arrive at the current lengths for the Agulhas Falkland Fracture Zone by invoking Valanginian extension[ With regard to the suggestion that Falkland Plateau Basin extension must pre!date rotation "Marshall\ 0883a# so that the Falklands are clear of the Mozambique Ridge:Agulhas Plateau\ this is only correct until spreading commences in the Natal Valley[ Once Valanginian spreading in the Natal Valley commenced the need for Falkland Plateau Basin exten! sion to provide all the movement of the Falklands micro! plate past the Agulhas Plateau is no longer valid[ It is possible to move the Falklands microplate into a position
where rotation can occur through a combination of Falk! land Plateau Basin extension and Natal Valley spreading\ potentially with Falkland Plateau Basin extension con! tinuing for some time after the Falklands was free to rotate "Figs 02 and 03#[ Despite these arguments there is one crucial piece of structural evidence missing[ If the Falklands had rotated then there should be structural evidence for it docking with South America[ In the case of this model\ Val! anginian inversion of the Malvinas Basin and possibly the San Jorge Basin would be expected "Figs 02 and 03#[ Unfortunately\ there is no evidence for this "Fitzgerald et al[\ 0889^ Keeley + Light\ 0882#[ However\ as the other models for Gondwanan break!up which incorporate Jur! assic rotation and docking "e[g[ Marshall\ 0883a^ Storey\ 0885^ Storey et al[\ 0885b#\ and hence inversion\ cannot provide evidence for this along the Argentine margin "Keeley + Light\ 0882# the lack of suitable evidence to support docking cannot be used to dismiss the model proposed in this paper without dismissing the others[ With the uncertainties discussed in this paper and as stratigraphic evidence for contemporaneous rifting in the
K[ Thomson:Marine and Petroleum Geolo`y 04 "0887# 612Ð625
North Falkland and Outeniqua basins would not provide conclusive proof then further constraints are required on when the Falklands rotated[ As drilling of the Falkland Plateau Basin is unlikely to take place for the foreseeable future\ thus removing the possibility of dating rifting in the main depocentre\ the only quantitative test available if similar ages of rifting are found is palaeomagnetism[ The increased stratigraphy available from the NFB would allow palaeomagnetic analysis of orientated core to take place thus constraining post!Lower Jurassic rotation and consequently validating either previously proposed models\ this model or even di}erent models[
4[ Conclusions 0[ Evidence from the Outeniqua Basin suggests that rift! ing in NFB may have a similar history to South Africa[ 1[ The model predicts Oxfordian:Kimmeridgian rifting in both areas followed by a complex tectonic event producing extension and compression due to dextral motion on the Agulhas Falkland Fracture Zone dur! ing the Valanginian[ 2[ If correct these assumptions suggest that the Falklands could not have rotated prior to the Valanginian\ the earliest known period of motion on the Agulhas Falk! land Fracture Zone[ In addition\ the model requires the Falkland Plateau Basin to have undergone sig! ni_cant extension as this time and not during the Jur! assic[ 3[ For such a model to be valid three key tests can be carried out[ Firstly\ extension in the NFB needs to be of the same age as the Outeniqua Basin[ Although not conclusive proof it is the most readily available test[ The age of rifting in the Falkland Plateau Basin would need to be constrained by further drilling in the main depocentre to con_rm that the most signi_cant phase of rifting is Valanginian and not Jurassic[ Finally\ palaeomagnetic analysis of orientated core from the NFB provides the most likely method of determining when the Falklands microplate actually rotated[
Acknowledgements The author acknowledges the Falkland Islands Government and Spectrum Energy and Information Technology Ltd[ for access to the Falklands data on which this study was based[ Michael Johnson "Spectrum# is especially thanked for his encouragement\ constructive comments and for facilitating the release of data[ SOEKOR are thanked for their cooperation with this project through the release of data and permission to publish[ In particular\ the assistance of David Broad\ Steve Mills\ Chris Davies\ Eric Jungslager and Ian McLa! chlan is acknowledged[ John Underhill and Brian Turner
624
are thanked for their encouragement and constructive comments on the paper[
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