The Ocean-Continent Transition at Rifted Continental Margins: What Is It, How Is It Formed, and How Do We Locate It? IV Posters
Presiding: A Goodliffe, Department of Geological Sciences; V Tymms, Department of Earth and Ocean Sciences,
T53A-01 1330h
A New Starting Point for the History of the Central Atlantic
The African and American margins of the central Atlantic Ocean have a major historical and geological importance: 1- American margins were the place of the most intense geological and geophysical exploration and synthesis related to continental margins (most concepts were defined or tested there); 2- in contrast to the North Atlantic which has a complex rifting history, breakup in the central Atlantic seems to occur in a single phase, at the Trias-Lias boundary, allowing for much easier modelling; 3- contrary to the South and North Atlantic, intracontinental deformation is not so crucial and does not complicate the initial reconstruction; 4- the Central Atlantic is the very place (with the exception of the Alpes) to "read" the breakup history of Pangea and the beginning of the closure of the Thetys Ocean. Since 1986, the breakup story of this region has been largely based on the interpretation of Klitgord & Schouten, which did not take into account the African margin and its magnetic anomalies and salt basins. We propose here a new reconstruction with a new interpretation of these African data. The American margin is characterised by the strong, continuous East Coast Magnetic Anomaly (ECMA) and the Blake Spur Magnetic Anomaly (BSMA). The ECMA, fringed throughout by a salt basin, is thought to represent the continental-oceanic boundary. In the north nevertheless, the salt runs largely over the ECMA. The West African Coast Magnetic Anomaly (WACMA), homologue of the ECMA, is not so well defined due to its weaker amplitude and to a lack of data. Our interpretation is based on all available magnetic data and the following two important points : the similarity in shape of both magnetic anomalies and the position of the Mauritanian salt basin. In contrary to Klitgord & Schouten, who considered the anomaly S in the Morocco Basin as the homologue of the BSMA, this anomaly represents the northern part of the WACMA in our interpretation. This modification has important consequences on the age of the fit which had been supposed to be 175 Ma, by simple extrapolation of the spreading rate between the anomalies M25 and BS to the fit. Our reconstruction at 195 Ma (top Sinemurian) of the Central juxtaposes the ECMA and the WACMA and places side by side the large salt basins of Morocco and Nova Scotia, as well as those of Mauritania and Carolina. The outflowing salt, in the northern part of the area, is interpreted as a large post-rifting slide. Both the ECMA and WACMA coincide with the limit of the salt basin. This fit of the two magnetic anomalies is therefore the paleo-reconstruction of the lower-liasic salt basin, at its maximum extension. Their age is connected to the end of salt deposition, i.e 195 Ma, 20 Ma older than previously thought but in good agreement with the age of the volcanic activity on both sides of the Atlantic ocean (Lower Lias).
T53A-02 1330h
A New Starting point for the History of South and Equatorial Atlantic Oceans
The nature and genesis of the large, thinned transitional zone of the continental passive margins is still a matter of debate. Any further progress in that subject must imply an intregrated structural study of homologous margins, replaced in a very precise pre-opening kinematic reconstruction to constraint horizontal movements. In South and Equatorial Atlantic oceans, the pre-opening misfits problem has been already addressed by several authors and requires an assessment of rigidity of african and/or south american continental plates which border those oceans. Nevertheless the lack of magnetic anomalies, the pre-opening fit of the Equatorial Atlantic ocean is well constrained due to the presence of well-defined oceanic fracture zones, homologous Demerara and Guinea Plateaus, paralellism of the coasts and Kandi and Sobral continental lineations. This contraint compels us to resort to intraplate deformation to close the South Atlantic Ocean. Intregrating all continental deformations of both plates described in the litterature, we propose here the closest pre-opening fit for the Central part of the South Atlantic. This pre-opening fit leaves a large pre-drift thinned basin of several hundred kilometers which cannot be explained by any process which implies more horizontal movement (stretching, simple shear.). South of the Walvis-Rio Grande ridges, the pre-opening fit implies intraplate deformation in Paraña, Solado and Colorado basins (South America) as already suggested by Unternehr et al (1988) and Nurnberg & Muller (1991).
T53A-03 1330h
Late Jurassic - Early Cretaceous tectonic reconstructions of the Central and South Atlantic Oceans
We use satellite-derived gravity anomalies and open-file marine magnetic data over the Central and South Atlantic Oceans to calculate new Euler poles and rotation angles for the relative motion between North America and Africa, and between South America and Africa. For the Central Atlantic, new reconstruction poles were calculated for Chrons M0, M4, M25 and M40 (approximately 121 Ma, 126 Ma, 154 Ma and 167 Ma). We correlate the prominent Blake Spur Magnetic Anomaly (BSMA) and East Coast Magnetic Anomaly (ECMA) with similar magnetic anomalies, called S1 and S3 respectively, over the African flank of the central Atlantic. Magnetic anomalies over the western and eastern flanks of the South Atlantic, inboard of the M4 anomalies, are analogous to the BSMA-S1 and ECMA-S3 anomaly pairs over the Central Atlantic. New reconstruction poles were calculated for Chrons M0 and M4 in the South Atlantic as well as these prominent anomaly pairs, which might be produced by the earliest oceanic rocks accreted along the margins as rifting ended and seafloor spreading began (i.e., "break-up" anomalies). Results of our kinematic analysis: 1) improve the overall fit between South America and Africa; 2) confirm an eastward ridge jump in the Central Atlantic prior to 167 Ma (Chron 40); and 3) suggest a westward ridge jump in the Central Atlantic between 164 Ma to 159 Ma (Chron 38 to Chron 32).
T53A-04 1330h
Crustal Thickness Mapping of the Rifted Margin Ocean-Continent Transition using Satellite Gravity Inversion Incorporating a Lithosphere Thermal Correction
A new method of inverting satellite gravity at rifted continental margins to give crustal thickness, incorporating a lithosphere thermal correction, has been developed which does not use a priori information about the location of the ocean-continent transition (OCT) and provides an independent prediction of OCT location. Satellite derived gravity anomaly data (Sandwell and Smith 1997) and bathymetry data (Gebco 2003) are used to derive the mantle residual gravity anomaly which is inverted in 3D in the spectral domain to give Moho depth. Oceanic lithosphere and stretched continental margin lithosphere produce a large negative residual thermal gravity anomaly (up to -380 mgal), which must be corrected for in order to determine Moho depth. This thermal gravity correction may be determined for oceanic lithosphere using oceanic isochron data, and for the thinned continental margin lithosphere using margin rift age and beta stretching estimates iteratively derived from crustal basement thickness determined from the gravity inversion. The gravity inversion using the thermal gravity correction predicts oceanic crustal thicknesses consistent with seismic observations, while that without the thermal correction predicts much too great oceanic crustal thicknesses. Predicted Moho depth and crustal thinning across the Hatton and Faroes rifted margins, using the gravity inversion with embedded thermal correction, compare well with those produced by wide-angle seismology. A new gravity inversion method has been developed in which no isochrons are used to define the thermal gravity correction. The new method assumes all lithosphere to be initially continental and a uniform lithosphere stretching age is used corresponding to the time of continental breakup. The thinning factor produced by the gravity inversion is used to predict the thickness of oceanic crust. This new modified form of gravity inversion with embedded thermal correction provides an improved estimate of rifted continental margin crustal thinning and an improved (and isochron independent) prediction of OCT location. The new method uses an empirical relationship to predict the thickness of oceanic crust as a function of lithosphere thinning factor controlled by two input parameters: a critical thinning factor for the start of ocean crust production and the maximum oceanic crustal thickness produced when the thinning factor = 1, corresponding to infinite lithosphere stretching. The disadvantage of using a uniform stretching age corresponding to the age of continental breakup is that the inversion fails to predict increasing thermal gravity correction towards the ocean ridge and incorrectly predicts thickening of oceanic crust with decreasing oceanic age. The new gravity inversion method has been applied to N. Atlantic rifted margins. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, ConocoPhillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, A Chappell, J Eccles, R Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & R Spitzer.
T53A-05 1330h
Crustal Structure in the Southern Rockall Trough from Satellite Gravity Data: Evidence for Sea-floor Spreading
The southern Rockall Trough south of 57 N has previously been interpreted as either an intra-continental rift floored with highly extended continental crust, or a failed oceanic rift formed by Cretaceous sea floor spreading. Satellite gravity, bathymetry data and seismic estimates of sediment thickness are used to derive crustal basement thickness for the southern Rockall Trough and adjacent regions using a gravity inversion method incorporating a correction for the large negative thermal gravity component present in oceanic and stretched continental lithosphere. The marine Bouguer anomaly, derived from satellite free air gravity (Sandwell & Smith 1997) and Gebco 2003 bathymetry data, is inverted using the method of Oldenberg (1974), incorporating an iteratively applied thermal anomaly correction, to give Moho depth. For oceanic crust the thermal anomaly correction is calculated using isochron ages (Muller et al. 1997) and for continental crust from the beta stretching factors resulting from gravity derived crustal basement thickness and an assumed rift age. When sediment thickness and volcanic addition are assumed to be zero, the resulting upper bound of crustal thickness from the gravity inversion is as little as 10 km in the southern Rockall Trough. A segmented axial thickening of the crust at the centre of the Rockall Trough is predicted, between the Barra volcanic ridge and the Anton Dohrn seamount and is interpreted as having a volcanic origin. Inclusion of a sediment thickness correction in the gravity inversion further reduces predicted crustal thickness. A pseudo-sediment-thickness map has been constructed from the available wide-angle data and incorporated in the gravity inversion. The addition of up to 5.5 km of sediment in the gravity inversion reduces the upper bound of crustal thickness to less than 3 km in some locations. The segmented axial thickening and thin crust shown by the gravity inversion, the lack of intra-basinal faulting, and the volcanic origin for the axis shown by normal incidence seismic data, are consistent with a sea-floor spreading origin for the southern Rockall Trough and not formation by intra-continental rifting. We investigate the formation of the southern Rockall Trough using SfMargin, a new model of continental lithosphere thinning leading to continental breakup and sea-floor spreading initiation. Comparisons of the geometry of the southern Rockall Trough predicted by SfMargin with that observed are consistent with a short period (20Ma) of slow Cretaceous sea-floor spreading, followed by thermal subsidence to present day. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, ConocoPhillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, A Chappell, J Eccles, R Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & R Spitzer.
T53A-06 1330h
Basement Structure and Sediment Architecture in the Southern Iberia Abyssal Plain
Deep-sea drilling and geophysical surveys at the Southern Iberia Abyssal Plain (SIAP) have revealed the existence of an up to 130 km wide Ocean-Continent-Transition (OCT). This OCT presents geophysical characteristics that match neither those of oceanic crust nor those of a thinned continental crust. This zone is characterized by weak and non-linear magnetic anomalies, variable topography, particular seismic velocities structures with strong gradients and no clear Moho reflection. New models assume that the OCT is formed by mantle exhumation along downward concave faults, an idea that is also supported by numerical modelling and observations in ancient margins exposed in collisional orogens. This model has considerable implications for the sedimentary architecture and the internal structure and nature of basement rocks in the OCT which are not yet tested. In order to test this model we mapped seismic formations and reflection geometries in the basement and in the sedimentary cover, and distinguished five basement domains based on seismic characteristics throughout the abyssal plain using seismic reflection lines Lusigal 12, Sonne, and CAM. The results of this work enabled us to describe the 3D sedimentary architecture in the OCT and to define its relation to basement structures. Of particular interest is the distribution of the lowermost formation overlying the basement, which is dated, based on correlation with DSDP Site 398 further to the east, as Valanginian-Hauterivian to late Aptian. This formation shows tilted reflector sequences thickening onto the footwall in the northwest whereas comparable structures are not observed towards the south and in the adjacent basins further continentwards. This observation indicates the existence of growth structures in the OCT and suggests a migration of the tectonic activity in the OCT towards the northwest. A second aim of the study was to define the internal structure of the OCT which is so far unconstrained. Because inherited heterogeneities or compositional variations within the basement of the OCT are likely to control the distribution of the deformation within the overlying sediments during tectonic inversion, we mapped the strain distribution within the post-rift sediments and compared it with the distribution of the five basement domains that were defined based on the seismic characteristics. Preliminary results show that: (1) reactivation structures are localized in the SIAP within a zone overlying basement with well-defined seismic characteristics, and (2) deformation occurs within the OCT and not at its transition to continental or oceanic crust. These results suggest that the OCT can not be considered as a homogeneous domain and may give some insights on where and how subduction may form during reactivation of passive margins.
T53A-07 1330h
Deep Crustal Strucure of the Southern Morrocan Margin from Reflection and Wide-angle Seismic Data
During the DAKHLA cruise, a joint project of Ifremer, the University of Brest, the University of Lisbon and Total, a total 1500 km of seismic reflection and wide-angle profiles were acquired off the southern Morrocan margin. Two profiles parallel to the margin and two profiles perpendicular to the margin were equipped with ocean-bottom seismometers (OBS). Both profiles perpendicular to the margin were additionally equipped with landstations in the prolongation of the marine profiles. A vibroseis study was carried out on land to determine the shallow structure underneath the landstations. The main aims of the cruise were to image the deep structure of the margin and to characterize the nature of the transition zone between the continental and oceanic crust. A profile perpendicular to the margin extended 200 km out onto oceanic crust and reveals a change in the roughness of the oceanic crust around anomaly M25. Modeling of the reflection and wide-angle seismic data reveals a 10 km deep sedimentary basin including two high velocity carbonate layers. The crustal thinning from 30 km at the continental part to 7 km in the oceanic part occurs over a 100 km wide zone. The continental crust is divided into two distinct layers of 12 and 18 km thickness. Oceanic crust east of the M25 magnetic anomaly displays higher velocities in layer 3 than west of the magnetic anomaly. No layer of anomalously high velocities between 7.2 and 8.0 km/s, which could be interpreted as underplate or serpentinized upper mantle material, has been detected at the ocean continent transition. The change in velocity suggests a possible link to changes in accretionary processes of the oceanic crust. A comparison with wide-angle models (SISMAR cruise) from the northern Morrocan margin shows similar crustal thickness but a wider ocean - continent transition zone. As in the DAKHLA velocity models no high velocity layer corresponding to either serpentinized upper mantle or underplating has been imaged in the SISMAR velocity model.
T53A-08 1330h
Seismic Stratigraphy Study in the Southern Sudanese Red Sea (Tokar Delta)
The Red Sea is a rift or graben, which separates the Arabian and African plates. It extends from the Gulf of Aden in the south (lat 12° 40' N) to the Gulf of Suez and Aqaba in the north (lat 27° 50' N). The study area comprises about 2500sq.Kms. From the seismic data acquired by the Oil Companies (Chevron 1975-76 Total 1980 and IPC 1992), thirty-two seismic lines were selected to study he stratigraphy and sedimentation of the area. Synthetic seismograms of Suakin-1, Bashayer-1A and Bashayer-2A wells are also used to assistant the data interpretation. We have found that the stratigraphy and sedimentation of the Sudanese Red Sea can be placed into four major tectonic phases, namely, Pre-rifting stratigraphy, Syn-Rift Pre-Salt Stratigarphy, Salt Phase and Syn-Rift Post Salt Stratigraphy. According to our seismic stratigraphy interpretation, the lower most sequence (Mukawar, Hamamit and Mughersum group) overlying the basement complex is relatively uniform throughout the area. The evaporate sequence deposited during the cycle of sea level rise and fall, indicating a change in climate and prolonged period of desiccation. This sequence is composed of Belayim and Dungunab Formation. The Dungunab Formation is a lactic glide surface. The third sequence contains three system tracts. The lowermost is a low stand and the middle is a transgressive, while the upper is a high stand system tract. This sequence is known as the Zeit Formation. The uppermost sequence is a thick sedimentary unit, which is composed mainly of a mixture of clastic and carbonates of Abu Shagara Group. The region witness extensive studies concerning the behavior of the tectonism and the evolution of the Red Sea and its adjoining. Tokar Delta is a part of a NW-SE trending fault-controlled sedimentary basin.
T53A-09 1330h
Geological Constraints on the Evolution of the Angolan Margin Based on Reflection and Refraction Seismic Data (Za°Ango project)
Deep penetration multi-channel reflection and OBS wide-angle seismic data from the Congo-Angola margin were collected in 2000 during the ZaiAngo cruise (Ifremer and Total). These data help constrain the deep structure of the non-volcanic continental margin, the geometry of the pre-salt sediment layers and the geometry of the Aptian salt layer. Dating the deposition of the salt relative to the chronology of the margin formation is an issue of fundamental importance for reconstructing the evolution of the margin and for the understanding of the crustal thinning processes. The data show that the crust thins abruptly, from a 30 - 40km thickness to less than 10km, over a lateral distance of less than 50km. The transitional domain is a 180km wide basin with a thickness lower than 7 km. The pre-salt sediment layering within this basin is parallel to the base of the salt and hardly affected by tectonic deformation. In addition, the presence of a continuous salt cover, from the continental platform down to the presumed oceanic boundary, provides indications on the conditions of salt deposition that constrain the geometry of the margin at that time. These crucial observations imply shallow deposition environments during the rifting and suggest that vertical motions prevailed - compared to horizontal motions - during the formation of the basin.
T53A-10 1330h
Modelling of Continental Lithosphere Breakup and Rifted Margin Formation in Response to an Upwelling Divergent Flow Field Incorporating a Temperature Dependent Rheology
We numerically model continental lithosphere deformation leading to breakup and sea floor spreading initiation in response to an imposed upwelling and divergent flow field applied to continental lithosphere and asthenosphere. The model is used to predict rifted continental margin lithosphere thinning and temperature structure. Model predictions are compared with observed rifted margin structure for four diverse case studies. Prior to application of the upwelling divergent flow field the continental lithosphere is undeformed with a uniform temperature gradient. The upwelling divergent flow field is defined kinematically using boundary conditions consisting of the upwelling velocity Vz at the divergence axis and the half divergence rate Vx . The resultant velocity field throughout the continuum is computed using finite element (FE) code incorporating a Newtonian temperature dependent rheology. The flow field is used to advect the continental lithosphere material and lithospheric and asthenospheric temperatures. Viscosity structure is hence modified and the velocities change correspondingly in a feedback loop. We find the kinematic boundary conditions Vz and Vx to be of first order importance. A high Vz/Vx (greater than10), corresponding to buoyancy assisted flow, leads to minimal mantle exhumation and a well defined continent ocean transition consistent with observations at volcanic margins. For Vz/Vx near unity, corresponding to plate boundary driven divergence, mantle exhumation over widths of up to 100 km is predicted which is consistent with observations at non-volcanic margins. The FE method allows the upwelling velocity Vz to be propagated upwards from the top of the asthenosphere to the Earth's surface without the requirement of imposing Vx. When continental breakup is achieved the half divergence velocity Vx can be applied at the lithosphere surface and the upwelling velocity Vz left free. We find this time and space dependent set of boundary conditions is more plausible than a constant corner flow type solution and predicts levels of depth dependent stretching and continent ocean transitions consistent with observation. Depth dependent lithosphere stretching, which is observed at rifted continental margins, is predicted to occur before continental breakup and sea-floor spreading initiation. The model may be used to predict surface heat flow and bathymetry, and to provide estimates of melt production rates and cumulative thickness. We compare model predictions with observed margin structure for four diverse rifted margins: the Lofoten Margin (a mature volcanic margin), Goban Spur (a mature non-volcanic margin), the Woodlark Basin (a neotectonic young ocean basin) and the Faroe-Shetland Basin (a failed attempt at continental breakup). This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, ConocoPhillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, A Chappell, J Eccles, R Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & R Spitzer.
T53A-11 1330h
The Margins of the Eastern Woodlark Basin: The Birth and Death of an Ocean Basin
In Oct/Nov 2004 we surveyed the margins of the eastern Woodlark Basin on the R/V Kilo Moana, revealing details of its earliest rifting history and modern subduction. Initial stretching affected the borders of the Solomon Sea basin to the north and the Louisiade Plateau to the south. The NE end of the Pocklington Trough, formerly interpreted as a paleo-trench, is a segmented rift >200 km long that borders the south side of a 25-40-km-wide region of faulted blocks contiguous with the Pocklington Rise. The oldest oceanic crust in the Woodlark Basin formed during the reversed interval prior to magnetic chron 3. Spreading propagated westward to 158o and then virtually simultaneously to 157o. Spreading at 156o and 155o began prior to and during, respectively, chron 2A. The transform at ~155o 10' developed after seafloor spreading had begun, cutting through rifted crust and truncating the northern limb of an overlapping spreading center. The strike-slip fault system separating the Solomon Sea and the eastern Woodlark Basin includes two major strands that splay to the NE and bound a widening wedge of intervening crust of the northern Woodlark Rise that is stretched along NW-trending normal faults. A small, right-stepping pull-apart basin on the northern fault provides further evidence for the right-lateral motion on this transform fault system. A new magnetization inversion and magnetic chron identification have allowed a revised interpretation of the triple junction region adjacent to the Solomon Islands. Although spreading may have ceased, Simbo Island sits on Brunhes ages crust. The current Simbo Transform Fault, linking the eastern end of segment 5 with either the Simbo spreading segment or the trench, appears to comprise right-stepping en echelon faults offsetting short volcanic/spreading segments. A volcano in the southern San Cristobal Trench axis is partially subducted beneath the forearc SW of the Russell Islands, uplifting the lower slope. A reentrant in, and uplift of, the lower forearc slope south of the eastern tip of Tetipare appears to have been caused by more complete subduction of another volcano. Further NW, subduction of the rifted northern margin fault blocks has created a serrated trench axis, uplifted the Treasury Islands and collapsed adjacent portions of the forearc.
T53A-12 1330h
The rifting to spreading transition in the Central Mariana Trough
The Mariana backarc basin is an actively opening basin in the western Pacific formed by the rifting of an earlier arc massif at about 6 Ma and subsequent seafloor spreading beginning at about 3-4 Ma. Bordered to the east by the active Mariana Arc and to the west by the remnant Mariana Ridge, the basin is 250 km wide at its widest at 180 north. In 2002, a marine geophysical survey on the R/V Maurice Ewing (EW0202 and EW0203) to study the MARGINS subduction factory in the area between 14-190 north and 141-1490 east, included a portion of the conjugate backarc margins. Concurrent multi-channel seismic reflection, gravity, magnetics, and multibeam bathymetry data were collected during the cruises. The objective of this study is to create an image of the seafloor and sub-seafloor in order to characterize the location and describe the nature of the transition between rifted arc crust and newly accreted oceanic crust. Seismic profiles across the western margin of the basin, where sediment cover is thin, show an abrupt transition from rifting to seafloor spreading. This transition appears to take place over as little as one (shallow angle) normal fault spanning a distance of about 3 km. On the eastern margin, where sediment from the active arc blankets the seafloor, the nature of the transition is less clear in the seismic data, but it is apparent that it takes place over a much greater distance. Magnetic and isostatically balanced gravity models provide insights into the crustal structure and first order compositional changes across both margins. Magnetic modeling differentiates the highly magnetized oceanic crust from relatively poorly magnetized rifted arc crust. We present detailed cross-sectional models across both the eastern and western margins. Combining these geophysical methods provides a powerful tool for locating the transition from rifted arc crust to seafloor spreading.