T33D-01 INVITED
Predictions From Numerical Models of Continental Extension Using Ductile Failure.
We present numerical experiments of lithospheric extension using a parameterization for ductile failure in both the brittle and ductile media. We use a ductile fracture criterion based on Freudenthal's critical plastic work done per unit volume. This allows us to model the formation of a semi-brittle media in the continental crust. To be consistent with geological observations ductile fracture is limited to the temperature range corresponding to the onset of quartz and plagioclase plasticity (300?C to 450?C) and the strength of the crust is decrease from that of plagioclase to that of quartz. This approach allowed us to model the evolution of rifted margins and to predict the occurrence of characteristic structures observed at continental margins. These include the presence of a continental block (block H) hanging between seaward dipping normal faults, the observation of exhumed continental crust and mantle along detachment faults working as rolling hinges and the possible influence of a gabbroic lower crust on the evolution of the margin. For initial and boundary conditions similar to that of magma poor margins, we were able to model three consecutive phases of deformations that are consistent with geological and geophysical observations in the Alps, Iberia and Newfoundland. (1) A stretching phase during which deformation is distributed over the whole future subsiding margin and extension is rooted in a semi-brittle layer. (2) A thinning phase when extension is controlled by a system of superimposed concave-downward faults active simultaneously in the brittle upper crust and lower crust/upper mantle (3) An exhumation phase during which serpentinized mantle rocks are exhumed to the seafloor along a downward-concave path and which eventually led to continental break-up and seafloor spreading. We show that by only varying the initial geotherm in the lithosphere we are able to isolate 3 different characteristic styles of extension. An initially hot stretching style which first develops core complexes structures followed by half-grabens similar to those observed in the Basin & Range Province. An initially cold thinning style which leads to the formation of a narrow rift with large rift flank uplift and structures similar to the Rio Grande rift.
T33D-02
The Formation of Parallel-Dipping Normal Faults on Mature Passive Margins: Insights from Numerical and Analogue Models
Observations indicate that extension of the continental upper crust is often accommodated by arrays of normal faults. These faults can have alternating dip directions and define a horst and graben structure or they can all dip in the same direction. Arrays of parallel-dipping normal faults are, for example, observed in the extending domains of offshore Norway, the Basin and Range Province, the Galicia margin west of the Iberian Peninsula, and offshore Angola. We aim to investigate factors and mechanisms that favor the formation of an array of parallel-dipping normal faults over a sequence of horst and graben structures. We focus on the mature stages of passive margins, when gravity spreading and multiphase rifting play a role. We use numerical models and analogue experiments to study fault formation on the scale of the upper crust. The numerical experiments use two-dimensional finite element models that can achieve large deformation with free surface behavior. The analogue experiments are built of (brittle) sand and (viscous) silicone and their internal deformation is visualized in an X-ray scanner. We show that gravity spreading of brittle sediments over a weak layer, such as salt or shales, leads to the development of parallel dipping fault arrays that dip either towards or away from the ocean domain. The dip direction is controlled by the basal shear stress at the brittle-viscous interface. This basal shear stress is in turn controlled by the relative velocities of the brittle and viscous materials and thus by the slope of the layers, their thickness ratio and their rheologies.
T33D-03
Automated thermo-tectono-stratigraphic basin reconstruction -- Examples from the Norwegian Sea and North Sea
Computational analysis is a powerful tool for understanding and quantifying the evolution of rifted margins and sedimentary basins. Models are, however, only as good as the input and reference data on which they are based. A key fitting parameter in basin modeling is the seismically observed stratigraphy. We present an algorithm for the automated inversion of basin stratigraphy for crustal stretching, mantle stretching, and paleobathimetry. The method is based on the coupling a 2--D thermo--tectono--stratigraphic forward model to an inverse scheme that updates the model parameters. The forward model solves for lithospheric thinning, flexural isostasy, sediment deposition, transient heat flow and mineral phase transformations. Metamorphic phase changes are implemented by precomputing density maps in P--T--X space. The inverse model updates the crustal and mantle thinning factors and paleo-water depth until the input stratigraphy is fitted to a desired accuracy. Both models combined allow for automated forward modeling of the structural and thermal evolution of extensional sedimentary basins. The potential and robustness of this method is demonstrated through two case studies. In the first case study, we apply the presented algorithm to the well studied Viking Graben. This study demonstrates how multiple datasets (e.g. stratigraphy, well temperature, vitrinite reflectance) can be integrated into basin models. The second case study addresses the structural and thermal evolution of the Vøring basin in the Norwegian Sea. A striking feature of the Vøring basin is an uplift event accompanied by erosion during the last rifting phase prior to break-up. We show how the presented algorithm can be used to test different scenarios for this late syn-rift uplift event. Two end-members will be discussed: a 'hot' model where uplift is induced by extreme mantle thinning and a 'cold' model where uplift results from mantle phase transitions.
T33D-04
Cenozoic rifting in the West Antarctic Rift System
The West Antarctic Rift System (WARS) experienced two episodes of Cenozoic rifting. Seafloor spreading at the Adare spreading axis, north of the Ross Sea, from Middle Eocene to Late Oligocene time (43 – 26 Ma), was directly linked with motions within the WARS. For this time interval, marine magnetic anomalies within the Adare Basin and structural features within the Ross Sea constrain the motion between East and West Antarctica. During this episode, widespread intrusive activity took place in the continental part of the rift. Subsequent Late Oligocene until present-day (26 - 0 Ma) extension was characterized by a transition to volcanic activity. Yet, the details of extension during this episode have been poorly resolved. We present preliminary results of new seismic reflection and seafloor mapping data acquired on geophysical cruise 07-01 aboard the R/VIB Nathaniel Palmer in the northern part of the rift. Our results suggest that the style of deformation changed from spreading-related faulting into diffuse normal faulting (tilted blocks) that trend NE-SW with little resultant E-W extension. Recent volcanism is distributed throughout but tends to align with the NE-SW trend, into a localized zone. Formation of the Terror Rift, Ross Sea, within the same time frame suggests that the pole of rotation has changed its position, reflecting a change in the relative magnitudes of tensile stresses along the rift. Moreover, this change was accompanied with a sharp decrease of extension rates.
T33D-05
Velocity Structure of the Rifted Crust in the Northwestern Ross Sea, From Seismic Refraction Data
Extension in the West Antarctic Rift System produced the Transantarctic Mountains, deep sedimentary basins in the Ross Sea, and the Adare Trough spreading center (43 to 26 Ma). The Adare Basin and Northern Basin are located at the northwesternmost extent of this region of deformation, and are generally assumed to be oceanic and continental crust respectively. Their boundary therefore provides an ideal study area for linking the styles of extension in the two types of crust. We process seismic refraction data collected during research cruise NBP0701 to determine 2D crustal velocity models along four seismic lines at the margin of the Adare and Northern Basins. The 48 closely-spaced sonobuoy records included in this study provide continuous refraction data coverage; three of these lines have reversed sonobuoy records. Finite difference modeling of the individual sonobuoys provides accuracy in our interpreted layer velocities, confidence in tracing refracted arrivals back to their associated reflections in the sonobuoy records, and the ability to match these reflected arrivals with the multi- channel seismic reflection data. Preliminary results from the line trending perpendicular to the margin of the Adare and Northern Basins show no change in crustal velocity structure from one basin to the other, with nearly flat velocity contours along the entire line. An apparent velocity of 8000 m/s is observed along this line in the Northern Basin. A comparable layer velocity is not detected in the sonobuoy record shot in the reverse direction, so this velocity could be due to local basement topography. Alternatively, the high velocity may indicate mantle material, and an unusually thin crust at that location. We model structural layers and associated velocities below the sea floor in order to better understand the physical structure and deformational history of the crust in the northwestern Ross Sea. The velocity horizons determined from this data set provide model constraints for a new type of crust that may be formed during rifting, and should be one end-member resulting from the continental rifting process.
T33D-06
New investigations of an ultra-slow continental rifting revealed from hydroacoustic monitoring in the Bransfield Strait, Antarctica
From November 2005 to November 2006, we have operated an Autonomous Underwater Hydrophone (AUH) array in the Bransfield Strait and the Drake Passage, Antarctica. The array takes advantage of the efficient propagation of sound in the oceans to detect, locate and analyze the temporal and spatial distribution of small- to moderate-size earthquakes along the South Shetland Islands, Bransfield Strait and Scotia Sea. During the mooring period, a total of 1,294 earthquakes occurred in the Bransfield back-arc basin, while 2,514 total earthquakes were located from throughout the region including the South Shetland Trench and the Drake Passage. Earthquakes in the Bransfield concentrated in five main swarms, four of which occurred in August 2006. Two swarms at Edifice A near Deception Island and the Three-Sisters show a temporal propagation of seismicity (1~2 km/hr) that may be caused by magmatic activity. The other three swarms at Hook Ridge and the northeast rift zones show no propagation and may represent tectonic rifting without a magmatic component. This is consistent with a transition from volcanic to fault driven rifting from the SW to NE within the Bransfield. During August 2006, the entire length of the Bransfield back-arc was seismically active. In addition to Bransfield seismicity, the hydrophone array detected dozens of earthquakes from the South Shetland Trench and an intense swarm of earthquakes at an unidentified mid-plate location in the Drake Passage. Given the extinction of the Phoenix-Antarctic Ridge (Lawver et al., 1996), seismicity in the trench and back-arc is likely a result of slab rollback. Earthquake swarms, combined with the ~10 mm/yr spreading rate from GPS measurement (Dietrich et al., 2001), demonstrate that the Bransfield Strait is in an ultra-slow continental rifting regime.
T33D-07
Crustal structure of the Morrocan margin from wide-angle and reflection seismic data
Two deep seismic cruises were conducted as joint projects between Ifremer, Total, and the Universities of Brest, El Jadida and Lisbon to constrain the deep crustal structure of the Morrocan continental margin. During the SISMAR cruise four combined wide-angle and reflection seismic profiles were acquired on the northern Morrocan passive margin. Two profiles were shot parallel and two profiles perpendicular to the margin. During the DAKHLA cruise, a total 1500 km of seismic reflection and wide-angle profiles were acquired off the southern Morrocan margin. Modelling of the reflection and wide-angle seismic data from the SISMAR survey images the thick sedimentary cover of the margin, which is locally perturbed by salt tectonics. The sedimentary basin thickens from 1.5 km on normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. The model reveals basement structures including a few tilted fault blocks and a transition zone to thin oceanic-type crust. The crust thins from 35 km underneath the continent to about 7 km at the western end of the profile. Modelling of the seismic data from the DAKHLA cruise 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. Oceanic crust east of the M25 magnetic anomaly displays higher velocities in layer 3 than west of the magnetic anomaly. This change in velocity suggests a possible link to changes in accretionary processes of the oceanic crust. A comparison of wide-angle models from the northern and southern experiment show similar continental crustal thickness and structure in both regions, but a wider ocean - continent transition zone in the south. No high velocity layer corresponding to either serpentinized upper mantle or underplating has been imaged in either margin transect.
T33D-08
Constraints on post Mid-Jurassic basin evolution in the North Sea from 3D numerical modelling of basin initiation and subsidence.
The North Sea sedimentary basin contains more than 3km of post Mid-Jurassic sediments. These are located in a trilete graben system consisting of the Moray Firth and the Viking and Central grabens, but also in a broad region surrounding the grabens, corresponding to the post-mid Cretaceous sediment deposits During the Mid- Jurassic the area was exposed to volcanism, domal regional uplift and erosion, followed by crustal thinning and normal faulting in the grabens. We use a numerical model considering 3D thermal evolution, flexural isostasy, erosion, sedimentation and compaction together with isopach data to simulate the geodynamic evolution of the area since the Mid-Jurassic. Our modelling studies show that the broad distribution of post Jurassic sediments cannot be explained by uniform stretching in the graben areas alone. Regional Mid-Jurassic thinning of the subcrustal lithosphere producing first uplift and erosion and later accommodation space for Cretaceous and Cenozoic sediments is also required. The uniform crustal thinning factor in the grabens amounts to a maximum of 1.14. The required subcrustal lithospheric thinning amounts to about 15 km. Our results are in accordance with observations from recent rift systems such as the Rhine Graben, Eastern Africa and the Baikal Rift, which show that crustal thinning is restricted to the graben areas while thinning of the subcrustal lithosphere (up to 100 km) and the associated domal surface uplift are more regionally distributed.