Extensional and Compressional Systems Posters
Presiding: N Dawers, Tulane University; M Bibee, Scripps Institution of Oceanography, Geosciences Research Division
T43B-01 1330h
Modeling Low-Angle Subduction Initiation by Ductile Deformation
The concept that new subduction systems are initiated repeatedly arises from the absence of oceanic crust older than ~200 Ma. Passive margins are considered a favorable site for subduction nucleation due to the existence of Andean type convergence margins. However, the force constellation and mechanisms that enable the development of a subduction system from a passive margin remain unclear. The difficulty arises when comparing the sum of the driving forces to the sum of the resisting forces. Such a calculation suggests that subduction cannot initiate spontaneously at passive margins, despite geological observations that indicate otherwise. To further investigate the subduction onset problem, we conduct scaled analogue experiments under the enhanced gravity of a centrifuge, and no other external force. We aim to study the conditions and processes by which the instability emerging from lateral density differences between the oceanic and continental lithospheres may lead to initiation of a low-angle subduction system. Furthermore, we analytically study the evolution of the interface between two juxtaposed high viscosity fluids, with lateral density contrast, simulating passive margin configuration. Our analogue experiments show that low-angle subduction may develop by ductile deformation and rotation of the ocean-continent interface (OCI), driven by lateral density variations. Hence we conclude that frictional resistance between the plates need not be overcome throughout the whole lithospheric profile, contrary to previous scenarios of subduction initiation. Our experiments also demonstrate that the force induced by the negative buoyancy of the oceanic plate with respect to the asthenosphere, which is widely considered a main force in driving incipient subduction, is in some cases irrelevant to subduction nucleation. The analytical formulation shows that the evolution of the OCI is a function of √ t, where t is the age of the oceanic plate, and the velocity field is expected to take the form of a convection cell. The results of both the analogue models and the analytical formulation are compared to south-east Australia passive margin, and show excellent fitting to its geometry and stress distribution.
T43B-02 1330h
Is the Subduction Actually a Mantle Wedge Upduction? Clues and Interpretations From the Mediterranean Arcs
Recent reviews of large scale seismic tomographies show that the prolongation of the rigid/colder lithospheric slab up to the core mantle boundary is not clearly recognizable. Often the tomographic image of these high-velocity bodies become horizontal - at the deep of the 640 km discontinuity - and often it continues to bend coming back toward the upper mantle. Because the strict similarity of some Italian and Mediterranean tectonic situations to the East Asia tectonics - arcs, trenches, Wadati-Benioff zones, volcanic and seismic activities, and the above mentioned horizontal bending of the alleged lithospheric slab -, many clues are examined in search of new interpretations of the Mediterranean geological and observational evidence, with the aim to find solutions that are exportable to the problems of the circumpacific arc-trench zones. The inspection of facts coming from surface geology, magmatism, geochemistry, different method tomographies, etc., is at variance with the alleged Africa-Eurasia convergence. The clues for rifting prevail on those for compression, and many tectonic situations previously interpreted as due to plates' collision, are associated or mixed to rifting evidence. The high velocity bodies characterizing the Wadati-Benioff zone connect gently with large extents of anomalous high velocity mantle trapped in the transition zone. Then the proposal is put forward that uprising - or upduction - of mantle material wedges - driven by isostasy - between two separating lithospheric plates could be a new work hypothesis. Because on an expanding Earth the Mediterranean region has ever had a little latitudinal extension, it is possible in this view, to identify as Mediterranean phases of opening also the Paleo Tethys and Neo Tethys currently alleged `closures', which have added to the Proterozoic nuclei the Variscan and Alpine terranes respectively. These phases and their orogens have to be considered as extensional phases, and the added terranes of African provenance (e.g. the Adriatic fragment) should be regarded as fragments left behind by the going away continental Africa. The recent Sumatra large earthquake has provided an important geophysical data in favour of the views expressed in this contribution.
T43B-03 1330h
GPS Constraints on Lesser Antilles Forearc Motion and Rigid Caribbean Plate
We are using a decade of Global Positioning System data to address two tectonic problems of the Caribbean (CA) plate; 1) Whether a forearc sliver exists along the Lesser Antilles forearc and if so what is its dynamics and location, and 2) Whether the Caribbean plate is deforming internally. We approach this problem by developing GPS-derived velocity vectors at sites within the CA plate and its boundaries and comparing them to four decades of earthquake data. In a number of subduction zones, misfits between slip vectors and predicted convergence azimuths from Euler vectors suggest the presence of a forearc sliver, where trench-parallel motion is accommodated along a strike-slip fault system. Such a situation may be occurring at the eastern boundary of the CA plate along the Lesser Antilles (LA) forearc, where the North America (NA) plate subducts obliquely. Comparing slip vectors of shallow (0-60 km) thrust events to the predicted motions of GPS-based Euler vectors show a systematic northerly misfit, suggesting a trench-parallel component of motion taken up by the forearc sliver. This possibility can be tested with GPS data from the forearc. In addition, we use new GPS data to constrain the internal rigidity of the plate. Previous GPS work yielded a possible upper bound on internal deformation of 4-6 mm/yr. With an expansion in the data set on critically located stations in the CA plate (SANA, ROJO, CRO1 and AVES), we have computed new sets of Euler vector pairs for the CA-NA and CA-South America plate pairs.
T43B-04 1330h
Testing simple models of brittle normal faulting: slip rate, spacing, and segmentation
Fault growth and evolution is a complex process, however any predictable pattern will yield important information for assessing seismic hazard and clues to what controls fault behavior. Models of slip rate variation along strike, spacing of active faults, and scaling of segment length are investigated using data from faults located within the parabola of seismicity around the Yellowstone hotspot. Based on displacement-length relations and segment size, Cowie and Roberts used fault geometry to estimate along-strike slip rate variation in their 2001 paper (JSG,23,1901-1915). Following their model, along-strike slip rate profiles were calculated for three active normal faults: the Beaverhead, Lemhi, and Lost River faults. Though the method yields estimated slip rates, the results roughly mirror along-strike variation in total displacement, because the three faults are similar in size and age. The profiles indicate that the Beaverhead is underdisplaced, i.e. having a low slip rate relative to its length. This suggests that segment linkage occurred later in the development of the Beaverhead than in the others. Cowie and Roberts also proposed a model for fault spacing based on initial fault length and spacing, and maximum length and spacing of fully developed fault systems. Fault spacing is important in determining incidence and magnitude of fault movement. If the distance between faults is too small, strain becomes localized along one while the other exhibits a decrease in seismicity until no activity occurs. In practice it is impossible to know if the distance between the largest faults represents maximum fault spacing, because the fault population is still active and evolving; thus, it is difficult to test or implement the method. A relationship was found among faults within the study area, where spacing of adjacent active faults is proportional to the sums of their lengths. It was also observed that average segment length increases with increasing total fault length. The implications of segment length scaling with fault length are that larger faults should have larger earthquakes and surface ruptures, assuming segment length represents characteristic rupture size, and that processes controlling fault length also control segment length. No clear relationship between segment size and seismogenic layer thickness is observed.
T43B-05 1330h
Discrete Particle Model for Deformation of Assemblies of Steel Balls Under Extension: Application to Sandbox Models
Experiments on sandboxes provide a useful tool for understanding the deformation of both sedimentary sequences and the continental crust. Boerner and Sclater (1992) have shown that the deformation under extension of assemblies of steel balls can reproduce many of the basic features of these experiments. We use a discrete particle model (DPM) to simulate previously performed physical experiments, which consisted of extending close-packed assemblies of steel balls placed on a rubber sheet attached to moveable boundaries. Steel balls are modeled with spherical elements in the DPM where normal and tangential forces generated at contact points are modeled with springs and friction, respectively. The simulations allow for a quantitative examination of the displacement and rotation of each steel ball as well as provide estimates of the contact forces between them. We perform a suite of simulations to explore the behavior of the model due to variations in the material properties of the balls and rate of extension of the assembly. Qualitative visual analysis shows that the simulations reproduce the qualitative features from the physical experiments such as dilation, rotation, and fault formation. Our success in matching most of the major features of the experiment indicates that the DPM may have a real future in improving our understanding of the deformation of both sedimentary sequences and the continental crust.
T43B-06 1330h
Spatial Evolution of Neogene Normal Faults, Northern Owens Valley: Constraints on Oblique-slip Partioning Within the Eastern California Shear Zone.
Simple geometric constraints can be used to predict fault interaction at depth. Such interaction within crustal scale fault populations plays an important role in the tectonic evolution of extensional tectonic settings. Here we use a theoretical relationship between fault dip, horizontal fault spacing, and depth to the base of the seismogenic zone to explain the late Cenozoic temporal and spatial evolution of faulting within the Eastern California shear zone, including the northern extent of Owens Valley, California. Our results show that during its evolution, the east-dipping Sierra Nevada frontal fault in northern Owens Valley became inactive due to intersection with the larger west-dipping range-bounding fault of the White Mountains. The horizontal spacing of 10 km between these two conjugate faults is such that they intersect within the brittle seismic layer resulting in the locking of this segment of Sierra Nevada frontal fault. Continued accumulation of normal displacement along the White Mountains fault zone has since resulted in the present-day half-graben basement structure of northern Owens Valley. This down-dropping along the eastern margin of the valley imposes a flexural tension across the surface of the Coyote Warp, which can be considered a large relay zone between the Sierra Nevada frontal fault and the Round Valley fault further west. It is suggested that this tension is responsible for the formation of west-dipping antithetic normal faults that are distributed locally around the Coyote Warp. This extensional fault geometry has imposed a kinematic restraint on the development and distribution of right-lateral shear within this part of the Eastern California shear zone, including northeastward transfer of right-lateral slip from the Owens Valley fault to the White Mountains fault.
T43B-07 1330h
Structural Evolution of the Incipient Okavango Rift Zone, NW Botswana
Studies of the East African Rift System (EARS) and other continental rifts have significantly improved our understanding of rifting processes; however, we particularly lack studies of the embryonic stages of rift creation. The Okavango Rift Zone (ORZ), NW Botswana is one of few places worldwide where one can study the early stages of continental extension prior to the accumulation of significant amounts of sediments, volcanism, and multiphase deformation that obscure the investigation of these early time processes in more evolved continental rift zones. In this study, gravity and aeromagnetic data have been used to examine the initiation and development of the nascent ORZ. The Okavango basin in NW Botswana is located at the southern tip of the southwestern branch of the EARS. The rift is hosted within the Proterozoic fold and thrust belt of the Ghanzi-Chobe formation. Our objectives include (1) assessing the role of pre-existing structures on the development of rift faults and basin architecture, (2) Examining fault linkage patterns and boarder fault development, and (3) determining the shallow subsurface basin geometry. Aeromagnetic data from the ORZ suggest two main structural trends: 1) northeast-southwest (030- 070o) and 2) northwest - southeast (290 - 320o). The 030- 070o structures occur within the rift zone and throughout the surrounding basement. They form the main bounding fault system of this incipient rift. The NE - SW orientations of rift faults mirror the fold axes and foliation of the basement rocks, suggesting that the basement fabric played an important role in localizing the development of faults within the stress regime present during the initiation of this rift. Additionally, the greatest throw (~400- ~700 m) occurs along the Kunyere (NW dipping) and Tsau faults (SE dipping), defining a full graben as observed on gravity models. This differs from the half-graben model typical of most continental rift zones. Thus, it appears the basin geometry was strongly influenced by the position of these pre-existing faults. Evidence of fault linkage is seen along some of the faults. Linked segments of faults are well defined and some are > 200 km long. We suggest from this result that fault linkage and propagation occurred very early and prior to significant basin development. We conclude that basement fabric seems to be a controlling factor at least in the early stages of basin architecture and structural evolution of ORZ.
T43B-08 1330h
Processes in Environmental Depositional Systems and Deformation in Sedimentary Basins: Goals for Exoloration in Mexico
Among the recent needs to establish new goals in the mexican energy industry to increase the petroleum reserves, has been necessary to recapitulate on some academic an operative concepts and definitions applied to the Petroliferous Basins Exploration; first of all, in order to understand the Petroleum System in given tectonophysical framework. The tectonophysical environment experienced by the petroliferous basin in the southwestern Gulf of Mexico, merely in the Campeche Sound and adjacent terrestrial regions (Figure 1); has been the result of interaction among the tectonic plates, the Coco's Plate with impingement and subduction beneath the Northamerican Plate and the Yucatan Microplate and even in very deep connection with the oceanic crust of southwesternmost portion of the Gulf of Mexico and the one of the Caribbean sea beneath the gulf of Belize-Honduras. The tectonosedimentary effects in the Campeche Bay starting with the skeleton formed for the Cenozoic Era, kept simultaneous conditions in depositions and deformations because of strain, stress and collapse fields, acted through this Era up to the present day, as observed in the surface Aguayo et al, 1999 and Sandoval, 2000. The involved portions of the crust and its boundaries have also been performing the relative sinking of the mere southwestern centre of the Gulf of Mexico, and the rising of the southeastern lands of Mexico. In the middle contiguity are found the productive Tertiary basins of: Comalcalco, Macuspana, Salina del Itsmo, Campeche-Champoton and other in deep waters; all of them, in an arrangement of basins among distensive faulted blocks in echelon, falling down to the deep centre of the Gulf Sandoval, op cit. With this scenario and that ones of other basins, a recapitulation on concepts and definitions, has been made on the regional natural processes of the environmental depositional systems and on the basins analysis in the tectonophysical framework, in order to reflect on the depositional and formational rather than deformational natural processes, and under a new paradigm; that include: "gas", "Tertiary", "sands", "shallow" and "structural formation"; once that the hydrocarbon reservoir systems is known. The paradigm also results in benefit for the hydrocarbon exploration of any basin, even, when it is previously known. In this review the concepts and the identification of the nature of the regional processes have been reconsidered as can be seen in the disciplines of the Enviromental Depositional Systems and the Basin Analysis for hydrocarbons in the framework of the Tectonophysics. It has been the marine-terrestrial scenario in southwestern Gulf of Mexico after a regional integrated study with the Advanced Geomorphometry and Geophysics. Likewise in this review, the deformational phenomena has been included, such as salt or mud solifluxion associated in some way with the basamental or tectonostructural movements. All of those which are apparently chaotic in the local aspect, have shown, at least since the Tertiary, an order in the regional aspect. In this terms all depositional and deformational-formational phenomena in a regional scale, obey to more than one of the four natural depositional-formational processes Sandoval, 2002 which are ubicuitus and permanent in relative terms.
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