U51B-01
Preliminary Geophysic Results in the Calingasta Bolson, Province of San Juan, Argentina
The Calingasta bolson is situated between the Frontal Cordillera and the Andean Precordillera from 31° to 32° south latitude and 69° 20' to 69° 45' West longitude. Its average elevation is about 1600 m a.s.l. The seismic information indicates that the eastern margin of this tectonic depression was affected by convergence during the latest tertiary and the quaternary. The depositional mesozoic sequences in this bolson lie on a strong angular unconformity evidenced by a marked lithologic and structural change. These deposits triassic in age occur in remnants of continental half-grabens formed by rifting that gave rise to NNW-SSE trending basins with rapid subsidence (Kokogian et al., 1999). These rift basins are coeval to the triassic depocenters in the oil producing Cuyana basin . Contemporary marine sedimentation is restricted to the Chilean territory, that is, to the west of the Frontal Cordillera, where narrow troughs were opened to the NW as marine embayments. In the Calingasta bolson, the outcropping triassic deposits were laid mainly in fluvial and lacustrine environments and are exposed along the western flank of the Precordillera, as observed at the latitude of the locality of Hilario, among other localities. The bolson can be considered as a ramp basin formed between the Precodillera and the Frontal Cordillera, limited by antithetic overthrusts controlled by the inversion of the extensional faulting associated to the triassic fill of the basin (Rossello et al., 1996). A new gravimetric survey carried out in the Calingasta bolson area led to the generation of a recent gravimetric chart, which was processed with different modern analytical and interpretation techniques such as Analytic Signal, Tilt and its Gradient and Euler Deconvolution. The Bouguer anomaly chart was filtered with the upward continuation at 40 km. The negative Bouguer isoanomalies in the resulting residual anomaly chart describe the geometry of the Calingasta bolson. Two outstanding depocenters with concentric isoanomalies of -20 mGal were determined, one at the latitude of the Calingasta village and the other situated to the SW of the locality of Barreal. The depths arising from the solutions of the localized Euler deconvolution, in the depocenters, reach 5000 meters. The distribution of the solutions of the Euler Deconvolution, as the responses of the Analytic Signal and Tilt Gradient, indicate the presence of zones with NNW-SSE and NNE-SSW trends which segment the bolson and separates both depocenters. This trends are interpreted as produced by antithetic (or conjugated) faulting, which is in accordance to seismic results. References Kokogian, D. A.; Spalletti, L. A.; Morel, E.; Artabe, A.; Martínez, R. N.; Alcober, O.A.; Milana, J. P.; Zavattieri, A. M. and Papu, O.H., 1999. Los depósitos continentals triásicos. En: Geología Argentina, Instituto de Geología y Recursos Minerales, Anales 29 (15): 377-398. Buenos Aires. Rossello, E.A.; López-Gamundí, O.R. and Vaillard, C. L., 1996. Geometry of an Andean ramp basin: the Calingasta Valley, western Argentina. 30° International Geological Congress (Beijing), Abstract 2 ( 5-6- 45): 318.
U51B-02
Structural Style and Tectonic Evolution of the Domeyko Range, North Chilean Precordillera
The structure of the Domeyko Precordillera is dominated by a number of elongated N-S trending basement ridges. These ridges were exhumed by steep reverse N-S faults that deformed the Mesozoic-Cenozoic cover. The vergence of the fault system varies along the strike, conferring an apparent doubly-vergent pop-up geometry to the axial zone. New structural data show that these geometries resulted from the reactivation and inversion of both the Triassic and the Late Jurassic - Early Cretaceous extensional faults. Typical basement short-cut faults are present. Basement faults uplifted the Paleozoic rocks of the hanging wall and transferred part of the shortening to the Mesozoic - Cenozoic cover of the footwall. Both thick and thin-skinned fault systems have a common origin and root in a rear basement fault. Fieldwork provided little evidence of strike-slip movement in these N-S basement faults. Tertiary porphyry intrusion emplacement was strongly controlled by basement faults that facilitated ascension of magma and its intrusion into the sedimentary cover as sills in the hanging wall anticlines. The Late Eocene-Early Oligocene giant porphyry copper bodies (Chuquicamata, La Escondida, El Salvador) located in the Domeyko Range show an adakitic affinity. This affinity together with structural evidence indicates that porphyry emplacement occurred at the end of the basement-involved contractional stage and points to the existence of a flat-slab subducting beneath the Central Andes (22°-26° SL). The eastward migrating compressional regime in the upper plate from the Late Cretaceous onward could be attributed to the shallowing of this slab.
U51B-03
Numerical Study of the Influence of Crustal Rheological Layering and Thermal Perturbation on the Orogenesis of the Andean Fore-arc
Diverse mechanical processes involved in build-up and segmentation of the Andes have been addressed (i.e. fore-arc composition, shortening rates, back-arc strength, etc.). In this work we use the 2-D thermo-mechanical simulation code PARAVOZ to study the effects of rheological differences between the upper and lower crust in the fore-arc, on localisation of deformation. Results show that a relatively weak lower crust leads to wide and homogeneous thickening of the crust, associated to an increase of the western component of descendent crustal flow. A weak lower crust consequently achieves lower topography than the opposite case, and a better transmission of stress and deformation to the west of the growing topography. Furthermore, testing the thermal perturbation corresponding to an active-arc, shows an important control over the down-west flow of the lower crust, at isotherms 400-500'C around 40 km depth. High temperatures rise the brittle-ductile transitions, favor decoupling of the upper and lower crust, consequently diffusing deformation and smoothing the topography. Rheology and temperature control the thickness and width of the competent layers, therefore determining the Eastward or Westward vergence of modeled fault structures. Finally, we propose that a plateau-like deformation might be controlled by a weak lower crust, independent of the thermal weakening produced by arc-magmatism, while a stronger lower crust will produce a more Puna-like mountain building.
U51B-04
Synorogenic Extensional Tectonics in the Forearc, Arc and Southwest Altiplano of Southern Peru
There is increasing evidence that paradigms, as in many fields of science, deeply influence interpretations and even observations of the actual geology of the Andes, to the point that some same areas have be mapped in dramatically different ways by geologists who favored distinct models. The belief that the Central Andes originated by tectonic shortening has commonly biased cartography in this orogen, for instance by forcing high-angle or poorly-exposed faults to be mapped as reverse faults and thrusts. Extensional structures have often been overlooked, because they were thought to be irrelevant in the investigation of orogenic issues. However, observations and models from a variety of undoubtedly extensional settings in Europe and Africa have recently shown that some structural geometries previously thought to be typical of contractional processes, as in the Central Andes, in fact also occur in extensional contexts, in particular where normal faults were initiated as flexure-forming blind faults. Traditional mapping in the Central Andes has therefore to be re-evaluated. Identification and correction of such biases result in major revisions of structural mapping in southwestern Peru. The forearc, arc, and SW Altiplano of southern Peru in fact appear to have been dominated by extension and transcurrence since ~30 Ma, in contrast with the NE Altiplano, Eastern Cordillera, and sub-Andean belt, where shortening has been indeed significant. These two contrasting orogenic domains are separated by the SFUACC fault system, which corresponds to a major lithospheric boundary. Basins SW of the SFUACC formed in extension and along transcurrent faults. At least one low-angle extensional detachment, placing near-vertical Miocene conglomerates over a Cretaceous unit, occurs just west of Lake Titicaca. Other detachments occur in the forearc. Significant transcurrent faulting, including transpressional deformation, developed along specific structures over southern Peru. SW of the SFUACC, undisputable reverse faults are rare, but are common along the lower slope of the Pacific Andean escarpment, suggesting incipient oceanward gravitational collapse of the Western Cordillera. We find that extension has accompanied the Andean orogeny SW of the SFUACC, and therefore question the currently dominant paradigm.
U51B-05
Numerical models of stress and strain distribution at a Chilean-type subduction zone
The Chilean subduction margin presents a large-scale constancy as it spreads along 3000 km from north to south, linked to large scale relatively constant conditions between the subducting Nazca plate and the south american continent. In greater detail, the variable height and width of the Andean mountains or the existance of a Central Depression in the fore-arc indicate the role of rheological differences at depth. A 2D numerical approach calculates the distribution of the stress field and deformation in a model 2000 km wide per 200km deep, in which a basal traction is applied to a flexed oceanic lithosphere, simulating slab-pull over a time-span of 2 Myrs. The influence of the strength of the subduction channel and that of the overriding continent are tested, with the use of elastic-viscous-brittle temperature dependent rheology. Loading of the model is accompanied by the development of localised shear strain from the subduction zone through the continent up to the free surface. A strong subduction interface and a strong continental crust favor the development of a subsiding depression in the forearc, as the crust is dragged downwards coupled to the descending plate. Such a scenario could be valid for the Atacama basin (24°S). The role of the continental mantle is discussed, since high friction along a subduction channel suggests resistant normal forces acting from the continental side at depths corresponding to recorded earthquakes. When such a strong continental mantle wedge is accounted for, it can act as an indenter to the subducting plate, making it deviate and flatten. Finally the models illustrate how potential fluids may use channels of localised shear in the continental mantle to migrate upwards.
U51B-06
What do Great Subduction Earthquakes tell us About Continental Deformation of the Upper Plate in the Central Andes Forearc? Insights From Seismotectonics, Continental Deformation and Coulomb Modelisation Along Southern Peru Margin
Subduction of the Nazca plate beneath the Peruvian margin has produced numerous megathrust earthquakes during the last century and still constitutes mature seismic gaps in some places such as in between Ilo (Peru) and Arica (Chile). The rupture zones of the 1604, 1784 and 1868 southern Peru events were partially reactivated by the Arequipa 2001 (Mw = 8.5) seismic event, whose rupture zone was about 350km-long and stopped its propagation towards the south on Ilo Peninsula. Just after the occurrence of 2001 event, some reactivation of continental fault systems are identified and monitored thanks to the Peruvian seismic network and describe continental deformation processes occurring perpendicularly to the trench or parallel to the trench, traducing the continental plate response to major subduction earthquakes and some partitioning of the deformation. The Chololo and associated ( perpendicular to the trench) fault systems define some 80-km-long margin crustal blocks and the major one coincides with the 2001 earthquake southern limit of the rupture zone as it propagated to the south. These blocks are made from Late Jurassic and Cretaceous plutonic rocks from the Coastal Batholith; these are outcropping in some places and are evidenced by the aeromagnetic mapping elsewhere around the area. Northward along the subduction zone, another boundary between two rupture zones of major subduction earthquake was reactivated recently, perpendicularly to the trench, by the seismic crisis of October 2006, M=6.4, near Lima, right at the southern end of the rupture zone of the 1974 event (Mw=8.1).Those boundaries corresponding to discontinuities (lithospheric fault systems) in the upper plate, trending nearly perpendicular to the trench, act as earthquake barriers during rupture of large seismic events. Additionally occurred on 20 of November 2006 another seismic event (Mw=5.6 Neic, Ml=5.3) in Tacna region, showing a reverse focal mechanism compatible with the trend of the Sama Calientes Fault system (parallel to the trench) and a crustal depth of about 20km. Such a magnitud and crustal depth in the area correlates perfectly with the Quaternary geomorphic evidences of tectonic activity along the Sama-Calientes thrust fault in the forearc in Southern Peru. Some questions are raised by the occurrence of such continental seismicity, just after a major subduction event, as none has been registered in the area since more than 40 years. Continental fault systems constitute a key to the understanding of the forearc deformation in the Arica Elbow, where the Andes obliquity with respect to the Nazca plate convergence direction. Also these results suggest that continental deformation should give us clues to define the pattern of segmentation of the subduction zone by studying seismotectonics and its relation to the segmentation of the upper continental plate.
U51B-07 INVITED
Eustacy Feedback Coupling to Seismogenic Behavior of the Northern Peru Subduction Zone
We present a combined analysis of geological and geophysical data collected both onshore and offshore along the northwestern Peru forearc area (3°30'-7°30'S), from the coastal plain to the trench axis. Onshore, geomorphic analysis places constraints on the relative importance of eustatic versus tectonic factors in preserving and modifying the uplifted coastal landforms along the coastal plain. Breaking-wave morphologic markers were dated using the in situ-produced 10Be cosmonuclide. The data document a tectonic segmentation, allowing us to differentiate two areas as regard their evolution through time: the northern Cabo Blanco and the southern Paita-Illesca segments. For the past 200 kyr, both segments uplifted at high rates of 10 to 20 mm.yr-1 through tectonic pulses coeval with the eustatic deglacial sea level rises of isotope stage 1 and warm isotope substage 5e, respectively. Offshore, industry-acquired reflection seismic lines combined with EM12 bathymetric data allow us to investigate the tectonic regime and deformation of the continental margin and shelf. Major dipping-seaward detachments control the long-term subsidence of this area. These main tectonic features define a tectonic segmentation. No clear tectonic correlation in time exists between the onshore and the continental margin segmentations, or in space either. The long-term subsidence of the offshore, indicative of subduction- erosion working at depth requires low coupling along the subduction channel at depth. The distribution of permanent deformation along the northern Peru forearc area includes long-term uplift along the coastal plain and long-term subsidence along the continental margin. The neutral line is located within the 10 km seaward from the Present coastline. An extensive sequence of raised marine cliffs and associated notches evidences that the most recent uplift step (20-23 ka to Present) along the Cabo Blanco segment is related to a sequence of major earthquakes. We infer that eustacy exerts important feedback coupling to seismogenic behavior of the North Peru subduction zone. We speculate that during sea level fall, pore-fluid pressure diminishes along the subduction channel inducing a possible seaward migration of the locked zone (i.e. migration of the updip limit) reaching a maximum by the end of the eustatic low stand. During eustatic sea level rise pore-fluid pressure increases along the subduction channel. This in turn is capable of weakening the previously locked zone along the plate interface beginning an earthquake sequence. Earth's orbital variations are a potential external cause that may control the physical processes at work along plate interface. Reference: Bourgois, J., F. Bigot-Cormier, D. Bourles, R. Braucher, O. Dauteuil, C. Witt, F. Michaud, 2007, Tectonic record of strain buildup and abrupt co-seismic stress release across the northwestern Peru coastal plain, shelf, and continental slope during the past 200 kyr, J. Geophys. Res., in press.