Lisa M. Gahagan

Digital isochrons of the world's ocean floor

We have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self-consistent set of global isochrons and associated plate reconstruction poles. The age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models. We have constructed an age grid with error estimates for each grid cell as a function of (1) the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid, (2) the distance of a given grid cell to the nearest magnetic anomaly identification, and (3) the gradient of the age grid: i.e., larger errors are associated with high age gradients at fracture zones or other age discontinuities. Future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere, the heat loss of the Earth, ridge-push forces through time, asymmetry of spreading, and providing constraints for seismic tomography and mantle convection models.

Plate tectonic reconstructions of the Cretaceous and Cenozoic ocean basins

Abstract In this paper we present nine reconstructions for the Mesozoic and Cenozoic, based on previously published sea-floor spreading isochrons ∗ . The purpose of this study was 1. (1) to determine if the isochrons could be refitted to produce accurate plate tectonic reconstructions 2. (2) to identify areas of apparent mismatch between magnetic isochrons as a focus for further investigations, and 3. (3) to test the capabilities and accuracy of interactive computer graphic methods of plate tectonic reconstruction. In general, Tertiary and Late Cretaceous isochrons could be refitted with little overlap and few gaps; however, closure errors were apparent in the vicinity of the Bouvet and Macquarie triple junctions. It was not possible to produce Early Cretaceous reconstructions that were consistent with the previously published isochrons. In this paper we also propose that the Late Cretaceous and Early Tertiary plate reorganizations observed in the Indian Ocean were the result of the progressive subduction of an intra-Tethyan rift system.

Evolution of Cenozoic seaways in the circum-Antarctic region

Abstract A complete circum-Antarctic seaway did not open until both the South Tasman Rise cleared the Oates Land coast of East Antarctica and Drake Passage opened between the southern tip of South America and the northern end of the Antarctic Peninsula. Major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the South Tasman Rise and Antarctica as very close to the Eocene–Oligocene boundary, with an unrestricted opening deeper than 2000 m dating from ∼32 Ma. Timing of the opening of Drake Passage is more circumstantial because the exact motions of certain micro-continental fragments are not known. The motion of Africa with respect to South America as well as the motion of East Antarctica with respect to Africa are well constrained for the Cenozoic. These major plate motions are used with the reasonable assumption of no Cenozoic motion of the Antarctic Peninsula with respect to East Antarctica to constrain the location of the Antarctic Peninsula with respect to the southern tip of South America for the critical period of late Eocene to late Oligocene. Uncertainty of motion of the South Georgia and South Orkney microcontinents and other possible continental fragments make an exact time for opening of Drake Passage difficult to ascertain. Even so, the early Oligocene position of the Antarctic Peninsula with respect to South America requires a through-going, deep-water seaway to have been open at Drake Passage prior to 28 Ma, even given the unconstrained motion of various high-standing crustal fragments in the Scotia Sea. With reasonable assumptions concerning motion of the crustal fragments in the western and central Scotia Sea, it is likely that Drake Passage or passage through Powell Basin was open to deep water circulation by ∼31±2 Ma.

Paleozoic Laurentia-Gondwana interaction and the origin of the Appalachian-Andean mountain system

Laurentia, the rift-bounded Precambrian nucleus of North America, may have broken out from a Neoproterozoic supercontinent between East and West Gondwana. Several lines of evidence suggest that the Appalachian margin of Laurentia subsequently collided with the proto-Andean margin of the amalgamated Gondwana supercontinent in different relative positions during early and mid-Paleozoic time, in route to final docking against northwest Africa to complete the assembly of Pangea. Hence the Appalachian and Andean orogens may have originated as a single mountain system. The overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the Paleozoic Era. Laurentia-Gondwana collisions may help to explain contemporaneous unconformities in the Paleozoic sedimentary cover of the Laurentian, Gondwanan, and Baltic cratons.

Laurentia‐Kalahari Collision and the Assembly of Rodinia

The Llano Orogenic Belt along the present southern margin of Laurentia, regarded as continuation of the Grenvillian Orogen along the eastern Laurentian margin and exposed in basement uplifts in central and western Texas, records an ∼300‐m.yr. history of orogenesis culminating in arc‐continent and continent‐continent collision between ∼1150 and 1120 Ma and continuing until ∼980 Ma. The shape of the orogen and kinematics of the contractional deformation along the belt, together with the high‐P metamorphic conditions attained, indicate that a previously unidentified craton served as an indentor. It is paleomagnetically acceptable for the Kalahari Craton of southern Africa to have been opposed to this margin and within ∼1500 km of present‐day central Texas at ∼1100 Ma. Moreover, the Kalahari Craton is the correct size, and the structural and metamorphic evolution of the 1200–950 Ma Namaqua‐Natal Orogenic Belt that wraps around its present southern margin is compatible with that craton having been the indentor. The ocean basin that closed between the Laurentia and Kalahari Cratons would have been comparable to the present Pacific, with island arc/terrane accretion occurring during the Mesoproterozoic along opposing active convergent margins. The coeval 1.1 Ga Keeweenawan and Umkondo magmatic provinces of Laurentia and Kalahari, respectively, are associated with rifts at a high angle to the Llano and Namaqua Orogens. The rifts are interpreted as the result of collision‐generated extensional stresses within the two cratons. The voluminous mafic igneous rocks in both provinces, however, may reflect contemporaneous plume activity. Our reconstruction for 1.1 Ga provides a testable model for the Llano Orogenic Belt of Texas and the Namaqua Orogenic Belt of southwestern Africa as opposite sides of a Himalayan‐type collisional orogen, with the Natal Belt of southeastern Africa and the originally continuous Maudheim Belt of East Antarctica as a related Indonesian‐type ocean‐continent convergence zone. This reconstruction leads to a refinement of the paleogeography of Rodinia, with the Kalahari Craton in a position isolated from both the East Antarctic and Rio de la Plata Cratons by oceanic lithosphere. It also provides the first model for the assembly of that hypothetical early Neoproterozoic supercontinent. At least four separate cratonic entities appear to have collided along three discrete segments of the apparently anastomosing global network of “Grenvillian” orogens: the type‐Grenville Belt of eastern North America and counterparts in South America, the Llano‐Namaqua Belt, and the Eastern Ghats‐Albany/Fraser Belt of India‐East Antarctica and Australia. Over the remarkably short interval of ∼200 m.yr., this first‐order composite collisional event resulted in the amalgamation of most of Earth’s continental lithosphere and defined the close of the Mesoproterozoic Era.

Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic

Abstract This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential. Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by ∼180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the Permian–Cretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the ocean–continent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift. During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was ∼33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature. There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper Jurassic–Cretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality. In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the Reconcavo–Tucano–Jatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge. Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.

Plate-tectonic Evolution and Paleogeography of the Circum-Carpathian Region

Sixteen time interval maps were constructed that depict the latest Precambrian to Neogene plate-tectonic configuration, paleogeography, and lithofacies of the circum-Carpathian area. The plate-tectonic model used was based on PLATES and PALEOMAP software. The supercontinent Pannotia was assembled during the latest Precambrian as a result of the Pan-African and Cadomian orogenies. All Precambrian terranes in the circum-Carpathian realm belonged to the supercontinent Pannotia, which, during the latest Precambrian–earliest Cambrian, was divided into Gondwana, Laurentia, and Baltica. The split of Gondwana during the Paleozoic caused the origin of the Avalonian and then Gothic terranes. The subsequent collision of these terranes with Baltica was expressed in the Caledonian and Hercynian orogenies. The terrane collision was followed by the collision between Gondwana and the amalgamation of Baltica and Laurentia known as Laurussia. The basement of most of the plates, which was an important factor in the Mesozoic–Cenozoic evolution of the circum-Carpathian area, was formed during the late Paleozoic collisional events. The older Cadomian and Caledonian basement elements experienced Hercynian tectonothermal overprint. The Mesozoic rifting events resulted in the origin of oceanic-type basins like Meliata and Pieniny along the northern margin of the Tethys. The separation of Eurasia from Gondwana resulted in the formation of the Ligurian–Penninic–Pieniny Ocean as a continuation of the Central Atlantic Ocean and as part of the Pangean breakup tectonic system. During the Late Jurassic–Early Cretaceous, the Outer Carpathian rift developed. The latest Cretaceous–earliest Paleocene was the time of the closure of the Pieniny Ocean. The Adria–Alcapa terranes continued their northward movement during the Eocene–early Miocene. Their oblique collision with the North European plate led to the development of the accretionary wedge of the Outer Carpathians and foreland basin. The northward movement of the Alpine segment of the Carpathian–Alpine orogen has been stopped because of the collision with the Bohemian Massif. At the same time, the extruded Carpatho-Pannonian units were pushed to the open space toward the bay of weak crust filled up by the Outer Carpathian flysch sediments. The separation of the Carpatho-Pannonian segment from the Alpine one and its propagation to the north were related to the development of the north–south dextral strike-slip faults. The formation of the Western Carpathian thrusts was completed by the Miocene. The thrust front was still progressing eastward in the Eastern Carpathians. The Carpathian loop, including the Pieniny Klippen structure, was formed. The Neogene evolution of the Carpathians resulted also in the formation of the genetically different sedimentary basins. The various basins were formed because of the lithospheric extension, flexure, and strike-slip-related processes.

Tectonic fabric map of the ocean basins from satellite altimetry data

Abstract Satellite altimetry data provide a new source of information on the bathymetry of the ocean floor. The tectonic fabric of the oceans (i.e., the arrangement of fracture zones, ridges, volcanic plateaus and trenches) is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements. SEASAT and GEOSAT altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features. The uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship-track data. This map is also the first global map to incorporate the results of the GEOSAT mission, and as a result, new tectonic features are revealed at high southerly latitudes. This map permits the extension of many tectonic features well beyond what was previously known. For instance, various fracture zones, such as the Ascension, Tasman, and Udintsev fracture zones, can be extended much closer to adjacent coninental margins. The tectonic fabric map also reveals many features that have not been previously mapped. These features include extinct ridges, minor fracture zone lineations and seamounts. In several areas, especially across aseismic plateaus or along the margins of the continents, the map displays broad gravity anomalies whose origin may be related to basement structures.

Present-day Plate Boundary Digital Data Compilation

The PLATES Project compiled digital data representing the present-day plate boundaries. This report includes the digital data in three formats: GMT, KMZ and ArcGIS shapefiles.

Ontong Java and Kerguelen Plateaux: Cretaceous Icelands?

Abstract Together with Iceland, the two giant oceanic plateaux, Ontong Java in the western Pacific and Kerguelen/Broken Ridge in the Indian Ocean, are accumulations of mafic igneous rock which were not formed by ‘normal’ seafloor spreading. We compare published geochronological, crustal structure, and subsidence results with tectonic fabric highlighted in new satellite-derived free-air gravity data from the three igneous provinces, and conclude that existing evidence weighs lightly against the Ontong Java and Kerguelen plateaux originating at a seafloor spreading centre.