Early Cretaceous Breakup Research Articles

Provenance of Upper Jurassic–Lower Cretaceous strata in the Mentelle Basin, southwestern Australia, reveals a trans-Gondwanan fluvial pathway

Australia, and East Gondwana more broadly, host extensive Paleozoic–Mesozoic sedimentary basins with thick siliciclastic sequences. These sediments were for the greater part transported by large-scale fluvial systems; however, the spatial and temporal patterns of sediment dispersal remain poorly understood. We investigate the provenance of rift strata in the Mentelle Basin, Western Australia, which were deposited during the Late Jurassic–Early Cretaceous breakup of India from Australia-Antarctica. Monazite U–Pb and zircon U–Pb–Hf isotopic signatures suggest that the Upper Jurassic–Lower Cretaceous syn-rift sediments were supplied predominately by a transcontinental river system draining sediments from interior regions of East Antarctica to the northern Gondwanan passive margin. This fluvial system was focused within the developing rift between India and Australia-Antarctica. Provenance of the transgressive marine post-breakup strata reflects a transition to a proximal sediment source and marks the end of the transcontinental fluvial system after the final breakup of India from Australia-Antarctica in the Valanginian (~136 Ma). Statistical comparison of detrital zircon spectra of Mentelle Basin syn-rift strata with other Australian Paleozoic–Mesozoic rift strata located along our proposed transport route suggests that the transcontinental fluvial pathway was active since the Late Paleozoic. We infer that this fluvial system was one of the principal modes of siliciclastic sediment delivery to rift and passive margin basins of western Australia.

Early Cretaceous subsidence of the Naturaliste Plateau defined by a new record of volcaniclastic-rich sequence at IODP Site U1513

The Naturaliste Plateau is a submarine continental ribbon rifted from the southwest Australian margin during the Early Cretaceous breakup of East Gondwana. It occupied a key position near the juncture of Greater India and the boundary between Australia and Antarctica. However, details of the Early Cretaceous evolution of the plateau are not well known because of limited data. Drilling at Site U1513 during IODP Expedition 369 recovered the first complete Lower Cretaceous succession on the eastern Naturaliste Plateau. The succession includes syn-rift volcanic rocks, Hauterivian to early Aptian volcaniclastic-rich sedimentary rocks, and Albian claystone strata. The 235-m thick volcaniclastic-rich sequence represents the missing post-breakup record in the southwest Australian rifted margin. It spans the transition from syn- to post-rift phase during the final stages of breakup between Greater India and Australia-Antarctica. We report the lithological, petrophysical, geochemical, paleontological, and paleomagnetic characteristics of the sequence, and then synthesize the results to define the Early Cretaceous depositional environment and subsidence history of the Naturaliste Plateau. From the early Hauterivian, weathered volcanic products were eroded and re-deposited locally as a volcaniclastic-rich sequence, with a major contribution from the southern Naturaliste Plateau. The depositional environment evolved from a shelf to upper bathyal condition during the Hauterivian through early Barremian with a decreasing sedimentation rate. This period is defined as a late syn-rift subsidence phase by NW-SE trending extension. After the final breakup with Greater India, the plateau remained at upper bathyal depths with little deposition until the early Aptian. Mid–lower bathyal depths inferred from the Albian claystone strata suggest that the post-rift thermal subsidence commenced during the late Aptian. This two-phase post-rift subsidence reflects the proximity or high temperature of mantle plume, possibly the Kerguelen plume, and its westward migration relative to the southwest Australian rifted margin.

The impact of Early Cretaceous gateway evolution on ocean circulation and organic carbon burial in the emerging South Atlantic and Southern Ocean basins

Organic carbon burial is an important driver of carbon cycle and climate dynamics on geological and shorter time scales. Ocean basins emerging during the Early Cretaceous break-up of Gondwana were primary sites of organic carbon burial, implying that their tectonic and oceanographic evolution may have affected trends and perturbations in global climate via changes in local organic carbon burial. Assessing the role of individual ocean basins in the global carbon-climate context requires a sound understanding of the processes that induced large-scale changes in carbon burial and the timing of these changes. Here we reconstruct the oceanographic evolution, and its links to organic carbon burial, in the Barremian to Albian South Atlantic and Southern Ocean basins, which may have acted as carbon sinks of global importance. Our reconstruction is based on combined seawater neodymium isotope and sedimentological records obtained from multiple deep sea drill sites and a new general circulation model. Deep water circulation within and between those basins was primarily controlled by the opening of the shallow Falkland Plateau Gateway (between ∼118 Ma and ∼113 Ma) and the deep Georgia Basin Gateway (by ∼110 Ma), for which we provide new age constraints based on biostratigraphic and carbon isotope data. The opening of these gateways was accompanied by local to basin-wide decreases in organic carbon burial, suggesting that ocean circulation affected the oxygenation state via changes in deep water ventilation. Although our data do not provide quantitative information on the impact of changes in regional organic carbon burial on the global carbon cycle, the synchronicity between the reduction of organic carbon burial in the South Atlantic basin and global warming during the Early Albian points to a strong causal relationship.

Tectonic controls on sediment provenance evolution in rift basins: Detrital zircon U–Pb and Hf isotope analysis from the Perth Basin, Western Australia

The role of tectonics in controlling temporal and spatial variations in sediment provenance during the evolution of extensional basins from initial rifting to continental breakup and passive margin development are not well established. We test the influence of tectonics in a rift basin that has experienced minimal uplift but significant extension throughout its history: the Perth Basin, Western Australia. We use published zircon U–Pb and Hf isotope data from basin inception through to continental drift and complement this with new data from samples deposited synchronously with the continental breakup of eastern Gondwana. Three primary source regions are inferred, namely the Archean Yilgarn Craton to the east, the Paleo- and Mesoproterozoic Albany–Fraser–Wilkes Orogen to the south and east, and the Mesoproterozoic and Ediacaran–Cambrian Pinjarra Orogen underlying the rift basin and comprising the dominant crustal components to the west and southwest. From mid-Paleozoic basin inception to Early Cretaceous breakup of eastern Gondwana, drainage in the Perth Basin was primarily north- to northwest-directed as evidenced by the dominant Mesoproterozoic detrital zircon cargo, paleodrainage patterns and paleocurrent directions. Thus, provenance was primarily parallel to the rift axis and perpendicular to the extension direction, particularly during periods of thermal subsidence. During episodes of mechanical extension, detrital zircon ages are polymodal and consistently dominated by Paleo- and Mesoproterozoic grains derived from the Albany–Fraser–Wilkes Orogen, but with significant Archean and Neoproterozoic inputs from the rift margins. It is inferred that during mechanical extension the rate of subsidence exceeded sediment supply, which generated basin-margin scarps and enhanced direct input from the rift shoulders. Detrital zircon spectra from temporally-equivalent samples at the rift margin and in the rift axis reveal that distinct sedimentary routing operated on the flanks. In summary, sediment provenance in the Perth Basin (and probably other rift basins) is tectonically controlled by: (1) extension direction, (2) episodes of mechanical extension (rift) or thermal subsidence (post-rift), and (3) proximity to rift axis or rift margin.

The role of break-up localization in microcontinent separation along a strike-slip margin: the East India–Elan Bank case study

Abstract The Elan Bank microcontinent was separated from East India during the Early Cretaceous break-up. The crustal architecture and rifting geometry of East India and the Elan Bank margins document that the early break-up between India and Antarctica was initiated in the eastern portions of the Cauvery and Krishna–Godavari rift zones, and in the southern portion of Elan Bank. However, the westwards break-up propagation along the Krishna–Godavari Rift Zone continued even after the break-up in the overstepping portion of the Cauvery Rift Zone. Eventually, the western propagating end of the Krishna–Godavari Rift Zone became hard-linked with the failed western portion of the Cauvery Rift Zone by the dextral Coromandel transfer fault zone. Consequently, the break-up location between India and Antarctica shifted from its initial to its final location along the northern portion of the Elan Bank formed by the western Krishna–Godavari Rift Zone. The competition between the two rift zones to capture continental break-up and asymmetric ridge propagation resulted in a ridge jump and the Elan Bank microcontinent release. Supplementary material: Supplementary figures are available at http://www.geolsoc.org.uk/SUP18864

Quantitative sonic transit time analysis defines multiple Permian-Cretaceous exhumation events during the breakup of Gondwana

SUMMARY The Perth Basin in southwestern Australia has an extended history involving multiple regional unconformity-forming events from the Permian to Cretaceous. The central and southern Perth Basin is the closest basin to the relict triple junction of eastern Gondwana and comprises a complete Permian to Recent stratigraphy, thus recording the full history of the breakup events. We use sonic transit time analysis to quantify the magnitudes of net exhumation and the minimum differences in net exhumation across different time intervals (here called ‘interval exhumation’) for four stratigraphic periods from 37 wells. We were able to quantify the minimum interval exhumation of the Permian–Triassic, Triassic–Jurassic, Early Cretaceous breakup and post-Early Cretaceous events. The Permian– Triassic and Triassic–Jurassic events recorded spatially varied exhumation, up to 1000 m, across sub-basins. These localized variations are caused primarily by reverse (re-) activation of NW- and N-striking faults in the Permian–Triassic and Triassic–Jurassic events, respectively. The Valanginian breakup unconformity (~133 Ma) records approximately 400 m of basin-wide interval exhumation during the breakup of Gondwana, which implies a change to relatively uniform exhumation on a regional scale. Using published uplift rates for volcanic and non-volcanic passive margins, estimates of the time required for 400 m of exhumation vary from 6 to 20 Ma, respectively. A volcanic margin is far more likely given that post-breakup sedimentation commenced 2–7 Ma after breakup. Lastly, post-breakup interval exhumation ranges from 0 to 800 m. The highest values are in the hangingwall blocks of faults. Up to 200 m may be locally caused by reverse fault re-activation due to the present-day compressional stress state of Australia. The remainder is attributed to regional exhumation caused by dynamic topography in the last 50 Ma.

Structural and stratigraphic evolution of the Iberia–Newfoundland hyper-extended rifted margin: a quantitative modelling approach

Abstract We investigate the evolution of the Iberia–Newfoundland margin from Permian post-orogenic extension to Early Cretaceous break-up. We used a Quantitative Basin Analysis approach to integrate seismic stratigraphic interpretations and drill-hole data of two representative sections across the Iberia–Newfoundland margin with kinematic models for lithospheric thinning and subsequent flexural readjustment. We model the distribution of extension and thinning, palaeobathymetry, crustal structure, and subsidence and uplift history as functions of space and time. We start our modelling following post-orogenic extension, magmatic underplating and thermal re-equilibration of the Permian lithosphere. During the Late Triassic–Early Jurassic, broadly distributed, depth-independent lithospheric extension evolved into Late Jurassic–Early Cretaceous depth-dependent thinning as crustal extension progressed from distributed to focused deformation. During this time, palaeobathymetries rapidly deepened across the margin. Modelling of the southern and northern profiles highlighted the rapid development of crustal deformation from south to north over a 5–10 myr period, which accounts for the rapid change in Tithonian–Valanginian, deep- to shallow-water sedimentary facies between the Abyssal Plain and the adjacent Galicia Bank, respectively. Late-stage deformation of both margins was characterized by brittle deformation of the remaining continental crust, which led to exhumation of subcontinental mantle and, eventually, continental break-up and seafloor spreading.

New exploration opportunities in the offshore Houtman and Abrolhos sub-basins, northern Perth Basin, WA

The 2013 Acreage Release Areas W13-19 and W13-20 in the offshore northern Perth Basin, Western Australia, cover more than 19,000 km2 in parts of the Houtman, Abrolhos, Zeewyck and Gascoyne sub-basins. The Release Areas are located adjacent to WA-481-P, the only active offshore exploration permit in the Perth Basin, granted to joint venture partners Murphy Australia Oil Pty Ltd, Kufpec Australia Pty Ltd and Samsung Oil and Gas Australia Pty Ltd in August 2012. Geoscience Australia recently undertook a regional prospectivity study in the area as part of the Australian Government’s Offshore Energy Security Program, which provides fresh insights into basin evolution and hydrocarbon prospectivity. A sequence stratigraphic framework, based on new biostratigraphic sampling and interpretation, and an updated tectonostratigraphic model, using multiple 1D burial history models for Permian to Cenozoic sequences, have been developed. New geochemical studies of key offshore wells demonstrate that the oil-prone source interval of the Lopingian–Lower Triassic Hovea Member extends regionally offshore into the Abrolhos Sub-basin and potentially as far as the Houtman Sub-basin. This is supported by fluid inclusion data that provide evidence for palaeo-oil columns within Permian reservoirs in wells from the Abrolhos Sub-basin. Oil trapped in fluid inclusions in Houtman-1 can be linked to Jurassic source rocks, suggesting that multiple petroleum systems are effective in the Release Areas. The presence of active petroleum systems is also supported by the results of a recent marine survey. Potential seepage sites on the seafloor over reactivated faults correlate with hydroacoustic flares, pockmarks and dark colored viscous fluids that were observed over the areas. This may indicate an active modern-day petroleum system in the Houtman Sub-basin. Finally, a trap integrity analysis was undertaken to mitigate exploration risks associated with trap failure during Early Cretaceous breakup and provides a predictive approach to prospect assessment. These results provide strong support for the presence of active petroleum systems in the offshore northern Perth Basin and upgrade the prospectivity of the Release Areas.

East Indian margin evolution and crustal architecture: integration of deep reflection seismic interpretation and gravity modelling

Abstract The segmented East Indian continental margin developed after the Early Cretaceous break-up from Antarctica. Its continental crust terminates abruptly without considerable thinning along the Coromondal strike-slip segment and thins considerably before it terminates in the orthogonal rifting segments. The segments have an exhumed continental mantle corridor oceanwards of them. This, proto-oceanic crust, corridor varies in width from segment to segment, indicating a relationship with varying break-up-controlling tectonics of the adjacent margin segments. The top of the proto-oceanic crust is imaged by a higher reflectivity zone, while its base does not have any distinct signature. A contorted system of reflectors represents its internal structure. Its gravity signature is a longer-wavelength anomaly with peak values up to 30 mGal less negative than surrounding values. Its magnetic signature is represented by a positive anomaly with peak values of 0–56 nT. Wide proto-oceanic segments are adjacent to margin segments that are preceded by the orthogonally rifting Cauvery, Krishna–Godavari and Mahanadi rift zones. A narrow proto-oceanic segment is adjacent to the margin segment initiated by the dextral Coromondal transfer zone. A combination of seismic interpretation and gravity/magnetic forward modelling indicates that proto-oceanic crust is most probably composed of lower crust slivers and unroofed hydrated upper mantle, being formed between the late rifting and the organized sea-floor spreading.

Episodic burial and exhumation in NE Brazil after opening of the South Atlantic

Research Article| May 01, 2012 Episodic burial and exhumation in NE Brazil after opening of the South Atlantic Peter Japsen; Peter Japsen † 1Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark †E-mail: pj@geus.dk Search for other works by this author on: GSW Google Scholar Johan M. Bonow; Johan M. Bonow 1Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen, Denmark Search for other works by this author on: GSW Google Scholar Paul F. Green; Paul F. Green 2Geotrack International, 37 Melville Road, Brunswick West, Victoria 3055, Australia Search for other works by this author on: GSW Google Scholar Peter R. Cobbold; Peter R. Cobbold 3Géosciences (UMR6118), Centre National de la Recherche Scientifique (CNRS) et Université de Rennes1, 35042 Rennes, France Search for other works by this author on: GSW Google Scholar Dario Chiossi; Dario Chiossi 4Statoil do Brasil, Praia de Botafoga 300, 22250-040 Rio de Janeiro, Brazil Search for other works by this author on: GSW Google Scholar Ragnhild Lilletveit; Ragnhild Lilletveit 5Statoil Angola Team, Grenseveien 21, 4035 Stavanger, Norway Search for other works by this author on: GSW Google Scholar Luciano P. Magnavita; Luciano P. Magnavita 6Petrobras, Avenida República do Chile 330, Rio de Janeiro, 20031-170, Brazil Search for other works by this author on: GSW Google Scholar Augusto Pedreira Augusto Pedreira 7Geological Survey of Brazil (CPRM), Avenida Ulysses Guimaraes 2862, Salvador, 41213-000, Brazil Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (5-6): 800–816. https://doi.org/10.1130/B30515.1 Article history received: 25 Mar 2011 rev-recd: 29 Jun 2011 accepted: 13 Jul 2011 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter Japsen, Johan M. Bonow, Paul F. Green, Peter R. Cobbold, Dario Chiossi, Ragnhild Lilletveit, Luciano P. Magnavita, Augusto Pedreira; Episodic burial and exhumation in NE Brazil after opening of the South Atlantic. GSA Bulletin 2012;; 124 (5-6): 800–816. doi: https://doi.org/10.1130/B30515.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract It is a common assumption that elevated passive continental margins have remained high since rifting and breakup. Here, we show that the Atlantic margin of NE Brazil has undergone a more complex history. Our synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) After Early Cretaceous breakup, the margin under went burial beneath a thick sedimentary cover; (2) uplift episodes in the Campanian and Eocene led to almost complete removal of these deposits; (3) the resulting large-scale, low-relief erosion surface (peneplain) was deeply weathered and finally reburied at the Oligocene-Miocene transition; and (4) Miocene uplift and erosion produced a new, lower-level peneplain by incision of the uplifted and re-exposed Paleogene peneplain. Previous studies have identified aspects of this interpretation, but we have defined the absolute timing and magnitude of discrete events of burial and exhumation that followed Early Cretaceous rifting and Eocene–Oligocene peneplanation. We suggest that a late sedimentary cover protected Paleogene weathering profiles until the present day. The uplift phases in Brazil are synchronous with uplift phases in Africa and the Andes. The Andean phases coincided with rapid convergence on the western margin of South America, and the Campanian uplift coincided with a decline in spreading rate at the Mid-Atlantic Ridge. Consequently, we suggest that both vertical movements and lateral changes in the motion of the plates have a common cause, which is lateral resistance to plate motion. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrochemistry of Mesozoic mafic intrusions related to the Paraná Magmatic Province, Uruguay

The Cuaró Formation is part of the sequence of Mesozoic mafic intrusions related to the Early Cretaceous break-up of Gondwana and represents the southernmost occurrences within the Paraná Magmatic Province in Uruguay. We present field data, petrography and lithogeochemical results regarding these dike swarms and sills that crop out in the southern extreme of the Paraná Basin. Dolerites and sills mainly exhibit glomeroporphyritic textures; the phenocrysts consist of plagioclase, clinopyroxene, relicts of olivine and titaniferous magnetite. Bulk-rock geochemical analyses allowed their classification as low-Ti subalkaline tholeiitic basalts and andesitic basalts. Trace element data indicated that the protoliths of these intrusions include the subcontinental lithospheric mantle, as is generally recognized for other Gondwana-related continental flood basalt provinces.

Early Aptian (Early Cretaceous) freshwater bivalves from the Australian–Antarctic rift, southeast Victoria

Australian Mesozoic freshwater and estuarine bivalves belonging to the Hyriidae, Glauconomidae and Sphaeriidae have been described from only a few localities in Queensland, New South Wales and South Australia, revealing a low-diversity fauna. The hyriids Megalovirgus flemingi McMichael, 1956, and Megalovirgus stirlingi sp. nov. are among the first Cretaceous freshwater bivalves to be described from Victoria and were recovered from upper Strezlecki Group (lower Aptian) strata, ca 4 km southwest of Inverloch, Gippsland Basin. These strata represent major fluvial flood deposits that accumulated in the Australia–Antarctica rift during the Early Cretaceous breakup of eastern Gondwana. The distribution of Megalovirgus flemingi in the Cretaceous of both New Zealand and Victoria indicates close links in the freshwater faunas between Zealandia and southeastern Australia during the Early Cretaceous.

Deep crustal structure of the sheared South African continental margin: first results of the Agulhas-Karoo Geoscience Transect

The southern margin of South Africa developed as a consequence of shear motion between the African and South American plates along the Agulhas-Falkland Transform Fault during the Early Cretaceous break-up of Gondwana. The Agulhas-Karoo Geoscience Transect crosses this continental margin, and includes two combined offshore-onshore seismic reflection/refraction profiles. We present results from the western offshore profile. Using ocean-bottom seismometers and a dense airgun shot pattern, a detailed image of the velocity – depth structure of the margin from the Agulhas Passage to the Agulhas Bank was derived. Modelling reveals crustal thicknesses of between 7 km and 30 km along the profile. The upper crust has P-wave velocities of between 5.6 and 6.6 km/s and the lower crust has velocities that lie between 6.4 and 7.1 km/s. Uppermost mantle velocities range from 7.8 to 8.0 km/s. The 52 km wide continent-ocean-transition zone, where the Moho rises steeply, occurs at the Agulhas-Falkland Fracture Zone. Beneath the Southern Outeniqua Basin and the Diaz Marginal Ridge, a zone of relatively low velocities (~5 km/s) with a thickness of up to 3 km, can be discerned in the upper crust. We interpret this zone as an old basin filled with pre-break-up metasediments, which may be related to the Cape Fold Belt. We suggest that the origin of the Diaz Marginal Ridge is bound up with the tectonic history of this basin as it exhibits a similar velocity structure. Almost no stratified sediment cover exists in the Agulhas Passage because of strong erosion due to ocean currents. The velocity structure and seismic reflection results indicate the presence of alternating layers of volcanic flows and sediments, with a mean velocity of about 4 km/s. We suggest these volcanic flows were an accompaniment of possible re-activation events of parts of the Agulhas-Falkland Fracture Zone. Tectonic motion seems to be sub vertical instead of strike-slip along the re-activated part of the fracture zone. Therefore, a relation to the uplift of the South African crust may be possible.

Modeling petroleum generation in the Paleozoic of the Carnarvon Basin, Western Australia: Implications for prospectivity

Maturity and petroleum generation modeling of the Paleozoic succession in the Carnarvon Basin shows that most potential source rock intervals reached their maximum generation migration during the Carboniferous–Permian and could have charged traps developed during rifting in the middle Carboniferous to Early Permian except for the Peedamullah Shelf, where generation peaked during the Cretaceous. However, identification of such traps is challenging because it is difficult to differentiate between the deformation associated with middle Carboniferous–Early Permian rifting and that of the Early Cretaceous breakup rifting. Given the presence of suitable reservoir and sealing units in most of the Paleozoic, the prime risks for this section are the volume of available source rock, trap integrity because of the long period of preservation required, and relative timing of generation vs. trap formation, such that charging of younger traps requires secondary migration.The best Paleozoic oil-prone source beds identified in the Carnarvon Basin are thin beds in carbonate-dominated Silurian and Devonian units on the Gascoyne Platform, but Devonian source beds are restricted to the northern parts of the platform. The maturity of these units progressively increases from immature in the south-southeast to mature in the north-northwest, following increasing depth of burial in that direction. The best gas-prone source beds lie within the Lower Permian of the Merlinleigh Subbasin, and their maturity ranges from immature along the margins of the subbasin to overmature toward the center. Within the Upper Permian, the best source beds for oil and gas are in the Peedamullah Shelf, where they range from immature in the southeast to mature in the northwest.Commercial Paleozoic petroleum has yet to be discovered in the Paleozoic of the Carnarvon Basin, but the assessment of this part of the basin is limited because few, if any, of the exploration wells drilled to date were valid tests of these objectives.

Structure and tectonics of the Agulhas-Falkland fracture zone

The ∼1200-km-long Agulhas-Falkland Transform developed during the Early Cretaceous break-up of West Gondwanaland. On the African Plate, the continent bordering the Agulhas Fracture Zone extends between the Agulhas Bank and the South Tugela reentrant. It is divided into four distinct parts: Mallory Trough segment (I), Diaz Ridge segment (II), East London segment (III) and Durban segment (IV), from southwest to northeast. Each segment differs from the ohters in its physiography and in the nature of the continent-ocean crustal boundary. In segment I, a wedge-like, eastward-narrowing deep-water basin separates the steep continental slope from a marginal fracture ridge of probable volcanic origin along the transform fault trace. Along segment II the South African continental slope is deeply embayed and a great thickness of sedimentary strata has been ponded behind a buried ridge along the northern side of the fracture-zone trace, above a probable fragment of oceanic crust representing a northerly extension of the Jurassic Falkland Plateau Basin. In segment III, the continental margin is steep, especially along the middle slope; it lies directly along the transform trace, but conspicuously rugged along the continental slope, due to the effects of submarine canyoning and sediment slumping. In segment IV, the continent-ocean boundary also lies directly along the transform fault trace, and the margin shows evidence of seaward tilting above landward-dipping faults along the middle or lower slope. The segmentation is inherited from Jurassic structural fabrics formed prior to Early Cretaceous transform motion, when tectonic rotation of microplates caused rifting and partial oceanization along the future Agulhas-Falkland megafault trajectory.

Further evidence for the Early Cretaceous breakup of Gondwanaland off southwestern Australia

Magnetic anomalies adjacent to the northwest margin of the Naturaliste Plateau are identified as Early Cretaceous reversals M-0 through M-4. These lineations indicate that northwest—southeast sea-floor spreading began off the present Naturaliste Plateau circa 118 m.y. B.P., 8 m.y. later than along the adjacent continental margin off Perth. The Naturaliste and Perth lineation sequences are offset 130 km dextrally by a fracture zone which passes near DSDP Site 257. The data indicate that an intracratonic small ocean basin existed seaward of Perth from about 126 to 118 m.y. B.P.

Early Cretaceous breakup of Gondwanaland off western Australia

Magnetic lineations mapped between the Wallaby and Exmouth plateaus off western Australia in the eastern Indian Ocean are identified as Early Cretaceous reversals M-0 to M-4 and some older part of the Early Cretaceous reversal sequence. They formed at the same plate boundary as the anomalies to the south in the Perth abyssal plain and, likewise, date the Early Cretaceous breakup of eastern Gondwanaland at between 120 and 135 m.y. The presence of these lineations implies that the “Greater India” or Tibetan part of Gondwanaland lay adjacent to western Australia as far north as the Exmouth plateau prior to breakup of the supercontinent.