The Ocean-Continent Transition at Rifted Continental Margins: What Is It, How Is It Formed, and How Do We Locate It? II
Presiding: L Lavier, Institute for Geophysics, University of Texas; W Powell, ExxonMobil Upstream Research Company
T42A-01 INVITED 10:30h
Remnants of an Ancient Ocean-Continent Transition Exposed in the Alps: A Window for Understanding Break-up Processes
In the last two decades, remnants of an ancient Ocean-Continent Transition (OCT) of the Alpine Tethys ocean have been mapped in the Alps in SE Switzerland. The most prominent structure within the reconstructed OCT is a detachment system that can be traced from the thinned continental crust, across a Zone of Exhumed Continental Mantle (ZECM) towards oceanic crust. The detachment system is formed by at least two corrugated detachment faults that are overlain by extensional allochthons, tectono-sedimentary breccias and syn-and post-rift sediments. In the ZECM, basaltic magmas grade from T- to N-MORB oceanwards. Near the continental edge they form isolated volcanic bodies emplaced directly onto exhumed mantle whereas further oceanwards they are more voluminous and associated with syn-magmatic high-angle normal faults. In the OCT, lower crustal rocks are rare and where present related to Permian underplated gabbros preserving primary contacts to mantle rocks. In the ZECM, the mantle rocks change from pyroxenite-rich spinel peridotites close to the continent to pyroxenite-poor peridotites impregnated and intruded by asthenospheric melts further oceanwards. Across the whole OCT, the detachment system can be mapped as a damage zone, up to hundred meters thick, with a core zone formed by gouges. The fault rocks record a complex fluid and reaction assisted retrograde deformation history. Maximum temperatures at which the detachment faults were active are 300C in crustal rocks and 600C in mantle rocks. Enrichment of chromium and nickel along the detachment fault in the continental crust may result from fluids derived from a serpentinizing mantle underlying the extending continental crust. High-temperature granulite facies mylonites are found in the lower crustal and mantle rocks, however, their relation to the detachment is not yet fully understood. In the syn-extensional gabbros microstructures reveal a deformation history ranging from syn-magmatic to seafloor conditions. This deformation was acquired during their intrusion into partially serpentinized mantle rocks and subsequent exhumation. U/Pb on zircon and Ar/Ar on phlogopite ages obtained from these gabbros, interpreted as crystallization and cooling ages respectively, range between 165 and 157 Ma, which corresponds to the age of radiolarian cherts, the first sediments sealing oceanic and continental units in the OCT. The occurrence of isolated allochthons of continental origin stranded onto subcontinental mantle is incompatible with mantle exhumation at a mid-ocean ridge. Therefore, the detachment system preserved in the Alps is more likely related to final rifting leading to break-up and onset of seafloor spreading. All observations reported from the Alpine analogues reveal that the detachment system formed as a downward concave fault in a previously rifted crust that was already thinned to less than 10 km, and that serpentinization and magmatic activity were closely related to detachment faulting. Therefore, the thinning of the crust to 10 km and the localization of rifting within the area of final break-up can not be explained by magmatic and/or serpentinization processes, but may be predetermined by inherited heterogeneities within the pre-rift lithosphere. The interaction of tectonic, magmatic and hydrothermal processes during final break-up may explain the previously enigmatic final stage of continental extension and onset of seafloor spreading in magma-poor systems, which appears to be dominated by downward concave faulting.
T42A-02 11:00h
A Sharp Continent-Ocean Transition in the Area of the Canary Islands: Evidence From Upper Mantle and Lower Crustal Xenoliths
Our present information on passive margins rests almost exclusively on seismic and density data. An important exception is the west Iberia margin where petrological and geochemical information on crustal and mantle rocks have been made available through drilling experiments. In order to increase our information about, and understanding of, passive margins and their mode of formation, more information on crustal and mantle rocks along different types of passive margins are needed. In the area of the Canary Islands such information has been obtained through the study of mantle and deep crustal xenoliths brought to the surface by basaltic magmas. In-situ laser ablation (LA) ICP-MS mineral analyses have enabled us to "see through" the effects of the Canary Islands event and obtain robust information about the original (pre-Canarian) chemical character of the crust and upper mantle on which these islands are built. Our studies show that the lithosphere beneath the Canary Islands originated as highly refractory N-MORB type oceanic mantle overlain by highly refractory N-MORB crust. Both the lithospheric mantle and lower crust have been metasomatized to different degrees by a variety of fluid and melts. The enriched material is commonly concentrated along grain boundaries and cracks through mineral grains, suggesting that the metasomatism is relatively recent, and is thus associated with the Canary Islands magmatism. The original, strongly depleted trace element patterns and the low 87Sr/86Sr isotopic ratios typical of the oceanic lithosphere are preserved in the minerals in the least metasomatized rocks (e.g. LaN/LuN<0.1 in orthopyroxene and 87Sr/86Sr=0.7027-0.7029 in clinopyroxene in mantle xenoliths). The compositions of the most depleted gabbro samples from the different islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. Furthermore, we have found no evidence of continental material that might reflect attenuated continental lithosphere in this area. The easternmost Canary Islands, Fuerteventura and Lanzarote, appear to overlap the lower part of the continental slope of Africa. The presence of normal oceanic lithosphere beneath these islands implies that the continent-ocean transition in the Canary Islands area must be relatively sharp, in contrast to the passive non-volcanic margin further north along the coast of Morocco, along the Iberia peninsula, and in many other areas. Our data also contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean.
T42A-03 11:15h
Depth-Imaging the Transfer Boundary Between the Galicia Bank and Iberia Abyssal Plain Rifted Margin Segments, Integrating MCS and OBS Data
We present multi-channel seismic reflection data and coincident wide-angle ocean-bottom seismometer (OBS) data from the non-volcanic rifted margin west of Iberia. Profile ISE-9 runs 270 km north-south, 200 km west of the Portuguese coast, imaging the physiographic transition between Galicia Bank in the north (500-3000 m water depths) and the Iberia Abyssal Plain in the south (4000-5500 m water depths). Underlying Galicia Bank the crust is 13-16 km thick (from OBS modeling), indicating a continental origin with relatively minor thinning. To the south, from the flank of Galicia Bank into the Iberia Abyssal Plain, the crust thins significantly, reaching a thickness of less than 1 km in a region where crust has been previously characterized as transitional between oceanic and continental, and where mantle in places may have been exhumed. Atop Galicia Bank, 10-20 km-wide sedimentary basins are separated by near-seafloor basement highs. In contrast, sediments in the Iberia Abyssal Plain are laterally continuous and up to 4500 m thick. Relatively flat-lying stratigraphy unconformably overlies tilted stratigraphy, but there is no apparent thickening of any sedimentary packages. Strong intra-crustal reflections, occasionally coinciding with Moho, suggest complex extensional faulting along the margin apparently orthogonal to the widely accepted east-west direction of rifting. We have migrated and depth -converted the reflection data utilizing a combination of stacking velocities (for lateral resolution) and OBS-modeled velocities (for time-depth accuracy) as a starting velocity field. The resulting seismic profile accurately depicts the structural geometries along strike of the margin, but the relationship of these faults to the west-dipping normal faults of intersecting profiles remains ambiguous. Two possibilities could explain the apparent south-dipping faults: 1) they are southwest-dipping faults which have been imaged obliquely by north-south and west-east profiles, or 2) they are south-dipping faults distinct from the west-dipping faults. In either case, crustal extension oblique or perpendicular to the east-west rifting is not necessarily indicative of full lithospheric extension driven by asthenospheric flow. Instead, gravity-induced mass wasting may cause the north-south, rift-axis-parallel component of extension.
T42A-04 11:30h
Observations From the Alpine Tethys and the Iberia/Newfoundland Margins Pertinent to the Interpretation of Magma-Poor Rifted Margins
Although the Iberia/Newfoundland and Alpine Tethys margins are of different age and ultimately had a different fate, they share remarkable similarities. These similarities permit us to compare direct observation and unlimited sampling of the ancient Alpine margins with the drill-hole and geophysical data from the present-day Iberia/Newfoundland margins. This exercise results in new concepts for the near surface response of lithospheric rupturing at magma-poor rifted margins. Rifting at these two pairs of margins can be described by three modes of extension: a stretching mode, a thinning mode and an exhumation mode. Each mode is characterized by a particular isostatic response to extension, its basin architecture and fault geometry, and the bulk rheological evolution of the extending lithosphere. Initial rifting was controlled by the stretching mode. Deformation in the upper crust and upper mantle was decoupled along mylonitic shear zones in the middle crust, rift-shoulder uplift was moderate to weak, and basins were distributed across the whole subsiding margin. During an advanced stage of rifting, extension became localized in the future distal margin and was controlled by the thinning mode. Extension in the upper crust and upper mantle were coupled along major mylonitic shear zones, rift-shoulder uplift was very pronounced and resulted in sub-aerial exposure of parts of the future distal margin. During this stage, the crust in the future distal margin was thinned by more than 15 km although no evidence for upper crustal extension is found in the stratigraphic record. Final rifting was localized in the previously thinned crust and was controlled by the exhumation mode. During this stage, downwards-concave faults exhumed lower crustal and mantle rocks to the seafloor leading to a tens of kilometres wide Zone of Exhumed Continental Mantle (ZECM). Despite of the high extension, these faults did not produce a major fault bounded topography. This final stage of rifting was assisted by serpentinization and magmatism, but in contrast to the following seafloor spreading, the continental lithosphere was not yet broken apart and the asymmetry of the extending system affected the whole distal margin. The rift-evolution observed along the Iberia/Newfoundland and Alpine Tethys margins shows a change in the mode of extension from initially distributed and decoupled (extension mode), to localized, coupled and asymmetric (thinning and exhumation mode), to final localization in a Mid Ocean Ridge. The change in the mode of deformation can be interpreted to reflect an evolution of the bulk rheology of the extending lithosphere. Initial rifting appears to be controlled by inherited heterogeneities and recrystallization processes in mylonitic shear zones (stretching and thinning mode) whereas later rifting and continental rupturing may be controlled by hydration (serpentinization), magmatic and thermal weakening (exhumation mode). Since both serpentinization and magmatism developed during a late stage of rifting within an already thinned crust, they may control the break-up of the continental lithosphere but not the localization of rifting within the area of the future break-up. The localization of rifting within the area of final break-up has therefore to be explained by inherited heterogeneities or another weakening process within the extending lithosphere. At other margins, these modes may interact in a different way depending on the pre-rift conditions and the evolution of the rheology during rifting.
T42A-05 11:45h
Modeling Polyphase Rifting in Magma Poor Margins
In the past few years there has been an increase in the acquisition of higher-resolution data from passive continental margins and rift basins. This has been concurrent with the development of numerical techniques to dynamically model lithospheric deformation. These developments give us the tools to peer into the rheological evolution and the physical processes involved in controlling continental break-up. We present numerical experiments of lithospheric extension in which the initial thermal and lithological structure of the lithosphere is constrained by geological reconstructions of the Alpine Tethyan and Iberia/Newfounland conjugate margins. Geological reconstructions points to three consecutive phases or modes of deformations that spans initial break-up of the continent to the initiation of an ocean basin. (1) A stretching mode during which the deformation is distributed over several basins. The crust is initially 30 km thick and in each of these basins the rate of subsidence is greater than that the flank uplift. (2) A thinning mode during which the crust is thinned down to 10 km. Little or no evidence of upper crustal extension is found in the stratigraphic record. (3) An exhumation mode during which downward-concave faults exhumed lower crustal and mantle rocks to the seafloor. During these 3 phases the rift flanks have undergone little or no uplift. This suggests that the strength of the lithosphere decreased all along the evolution of the rift. We find that in order to consistently model the rift evolution during the first two stages, we need to simulate the progressive weakening of the continental crust with the formation of an attenuated middle crustal layer composed of anorthosite and quartz. This layer essentially decouples upper/middle crust from lower crust and mantle. During the first stage the presence of a preexisting strong gabbroic layer (Ivrea lower crustal body) in the lower crust helps distribute the deformation by strengthening the crust. In the second stage this lower crust allow for the coupling of the crust and mantle along a system a downward concave faults that thins the crust with no distributed faulting. During the exhumation phase the downward concave fault exhumes lower crust and mantle. In that last phase serpentinization provides a mechanism to weaken (attenuate) the lithospheric mantle.