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We use digital elevation models ana satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the Central Cordillera of Patagonia between 46[degrees]S and 48[degrees]S, in a region overlying subducted segments of the Chile Spreading Ridge. This region shows a number of peculiar features, not expected in a normal subduction-related belt located away from a buried spreading center. First, it is characterized by overall short-wavelength topography and highly contrasted reliefs, typical of deeply incised young mountain belts, but surprisingly, also by an average very low elevation of ~900 m. Second, the Cordillera lacks central high reliefs; instead, there is a central, depressed domain with average low slope gradients including the deepest point of the study area (300 m below sea level in the bottom of Lake General Carrera/Buenos Aires). This depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips. To the east, the central Patagonian Cordillera is now separated from the flat plains of the Pampean domain (representing its former piedmont) and several elevated basaltic plateaus (mesetas) by 160[degrees]N-180[degrees]N trending morphotectonic corridors and the western border scarps of the mesetas. These scarps correspond to neotectonic features controlled by recent vertical faults, mostly active after 3 Ma. Remnants of a former peneplain, the Avellanos surface, are widespread within the entire Cordilleran region. From geological evidence, this peneplain formed between 15 and 4 Ma and was connected originally with the mesetas domain and now is dissected by these corridors. It is shown that a negative inversion related to extensional and strike-slip faulting occurred in the frontal region of the Cordillera at 3 Ma synchronously with the emplacement of magmatic products. Timing of the extensional deformation in central Patagonia is concomitant with the migration at depth of different segments of the South Chile Ridge, which entered the trench at 6 and 3 Ma, respectively, and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions. Finally, migration of segments of the Chile Spreading Ridge since 6 Ma triggered regional uplift of the Cordillera followed by limited extension in relation with increased thermal weakening of the crust. doi: 10.1029/2009TC002453

[1] The importance of west verging structures at the western flank of the Andes, parallel to the subduction zone, appears currently minimized. This hampers our understanding of the Andes-Altiplano, one of the most significant mountain belts on Earth. We analyze a key tectonic section of the Andes at latitude 33.5[degrees]S, where the belt is in an early stage of its evolution, with the aim of resolving the primary architecture of the orogen. We focus on the active fault propagation-fold system in the Andean cover behind the San Ramon Fault, which is critical for the seismic hazard in the city of Santiago and crucial to decipher the structure of the West Andean Thrust (WAT). The San Ramon Fault is a thrust ramp at the front of a basal detachment with average slip rate of ~0.4 mm/yr. Young scarps at various scales imply plausible seismic events up to [M.sub.w] 7.4. The WAT steps down eastward from the San Ramon Fault, crossing 12 km of Andean cover to root beneath the Frontal Cordillera basement anticline, a range ~5 km high and >700 km long. We propose a first-order tectonic model of the Andes involving an embryonic intracontinental subduction consistent with geological and geophysical observations. The stage of primary westward vergence with dominance of the WAT at 33.5[degrees]S is evolving into a doubly vergent configuration. A growth model for the WAT-Altiplano similar to the Himalaya-Tibet is deduced. We suggest that the intracontinental subduction at the WAT is a mechanical substitute of a collision zone, rendering the Andean orogeny paradigm obsolete. doi: 10.1029/2008TC002427

[1] The Neogene evolution of the Ene and southern Ucayali basins of the Subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles. The lower thrust wedge is formed by deep-seated decollements within the basement related to thick-skinned foreland structures inherited from an Early Carboniferous thrust system. Seismic reflection data show that this Paleozoic compressional system has been eroded and unconformably covered by Late Carboniferous clastic sediments. It generated an irregular Paleozoic sedimentary architecture controlling the Neogene thrust propagation. The upper thin-skinned thrust wedge developed within this Paleozoic sedimentary series and constitutes the Subandean zone. Cross-section balancing shows an along-strike homogenous horizontal shortening of ~56 km (~30%) across the Ene--southern Ucayali thrust system. This amount of shortening was vertically partitioned onto the two stacked thrust wedges. The N-S thickness variations of the Paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge. The southern thickening of the Paleozoic series generated major decollements and the shortening excess is of 7 km (16%) in comparison to the north. Consequently, the northern lack of shortening onto the upper thrust wedge was transferred to the Early Carboniferous compressional structures of the lower thrust wedge. We suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp: the Tambo transfer zone. This geometrical analysis of the Ene--southern Ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region. Citation: Espurt, N., S. Brusset, P. Baby, W. Hermoza, R. Bolanos, D. Uyen, and J. Deramond (2008), Paleozoic structural controls on shortening transfer in the Subandean foreland thrust system, Ene and southern Ucayali basins, Peru, Tectonics, 27, TC3009, doi:10.1029/2007TC002238.

[1] Latitudinal changes in topography, climate, and thrust belt geometry in the central Andes have led to conflicting hypotheses that climate or tectonics exert a first-order control on orogen evolution. The relative roles of climate and tectonics in the evolution of the Andean orogen are difficult to quantify because of a lack of detailed observations for both the long-term deformation and erosion history of the Andean foldthrust belt. We contribute to the resolution of this problem by presenting a sequentially restored, balanced cross section based on new mapping across the northern Bolivia portion of the thrust belt (1517[degrees]S). The timing and magnitude of exhumation across the cross section are determined by synthesizing 10 new and ~70 previously published mineral cooling ages. Once balanced and restored, the section was sequentially forward modeled using stratigraphic and cooling age constraints. Results indicate the Eastern Cordillera (EC) records the highest magnitudes of shortening (123 km or 55%). The Interandean zone (IA) has shortened 48 km or 30%. In both the EC and IA individual thrust sheets are tightly folded and have minor offsets of 1-5 km. The Subandes (SA) has multiple levels of detachments allowing for thrust sheets with relatively large offsets (6-17 km). Total shortening in the SA is 66 km or 40%. Total magnitude of shortening for the entire fold-thrust belt in this region is 276 km (40%). New apatite and zircon fission track cooling ages in conjunction with published ages indicate two phases of rapid exhumation; an earlier phase from ~40 to 25 Ma in the EC and one prior to ~25 Ma in the IA, followed by distributed exhumation of the entire fold-thrust belt from ~15-0 Ma. Combined exhumation estimates from the balanced cross section and thermochronology suggest ~911 km of exhumation in the EC, ~5-9 km in the IA, and ~3-4 km in the SA. Long-term shortening rates are 7 mm/a for the EC and IA and 4-8 mm/a for the SA. The SA shortening rates are based on a ~15-0 Ma or 8-0 Ma deformation window. By linking cooling ages to location and magnitude of shortening, we suggest an ~10-17 Ma pause or a dramatic deceleration in the rate of deformation and propagation of the fold-thrust belt between 25 and ~15 or 8 Ma. Citation: McQuarrie, N.,J.B. Barnes, and T.A. Ehlers (2008), Geometric, kinematic, and erosional history of the central Andean Plateau, Bolivia (15-17[degrees]S), Tectonics, 27, TC3007, doi:10.1029/2006TC002054.

[1] Quantifying the timing, magnitude, and rates of exhumation and deformation across the central Andes is a prerequisite for understanding the history of plateau rise. We present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire (~500 km) Andean fold-thrust belt at ~19.5[degrees]S in Bolivia. Exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data, regional stratigraphy, geothermal gradients, and mass deficits inferred from a balanced section. Results suggest the following: (1) Initial exhumation of the Eastern Cordillera (EC) fore-thrust and back-thrust belts began in the late Eocene to early Oligocene (2736 Ma) and continued in a distributed manner in the late Oligocene to early Miocene (19-25 Ma). Interandean zone (IA) exhumation began 19-22 Ma, followed by a third pulse of exhumation (11-16 Ma) in the EC back-thrust belt and initial cooling in the westernmost Subandes (SA) 8-20 Ma. Finally, exhumation propagated eastward across the SA during the late Mio-Pliocene (2-8 Ma). (2) Exhumation magnitudes are spatially variable and range from maximums of

The tectonic inheritance, crustal architecture and contrasting structural styles in the Venezuela Andes were investigated using surface and subsurface data images. The results suggested that the Venezuela Andes comprised a Neogene transpressional feature. The tectonic inheritance of older structures were proposed to explain the contrasting structural styles along the strike of the North Andean flank and the middle of the northern, central and southern segments of the orogen. The results further suggested that the observed asymmetry was due to preexisting Paleozoic thrusts.

The structure and kinematic evolution of the eastern Cordillera Oriental and Interandean Zone are analyzed. Their association to the Subandean Ranges based on three balanced cross sections in the Bolivian-Argentinian border region are investigated. Gravimetric data suggesting thin lower Paleozoic strata in the footwall of the Interandean thrust indicate that the Interandean thrust's main basement ramp follows an early Paleozoic synsedimentary normal fault.

[1] In this paper we present the results of the analysis of borehole breakouts from 115 wells drilled within Neuquen Basin in the Andean retroarc between 34[degrees] and 39[degrees]S (Argentina), The first-order present-day stress orientation in the Andean retroarc is expected to be mainly controlled by the plate boundary forces (azimuth 80[degrees]) and the topographic forces (E-W). The obtained maximum horizontal stress ([SH.sub.max]) has a preferred trend with a resultant direction of azimuth 88.7[degrees] and a 95% confidence interval of 13.3[degrees] consistent with the expected trend. The horizontal stress trajectory map achieved for this region shows that the [SH.sub.max] along the study area is not completely uniform. To the north of Colorado River, the [SH.sub.max] shows an ESE tendency interpreted as significant influenced by the topographic forces. To the south of Colorado River, [SH.sub.max] has an ENE trend similar to the expected based on plate boundary forces. To the southeast of the region, a NE direction was found, probably showing a basement structural control in the stress field geometry. The stress orientations obtained for the whole region show that plate boundary forces, drag basal, and topographic forces are strongly controlling the stress direction distribution. Citation: Guzman, C., E. Cristallini, and G. Bottesi (2007), Contemporary stress orientations in the Andean retroarc between 34[degrees]S and 39[degrees]S from borehole breakout analysis, Tectonics, 26, TC3016, doi: 10.1029/2006TC001958.

Research establishing data pertaining to the structural evolution and the geometry of the Andean backthrust belt of Bolivia is presented. Particular attention is given to the geological characteristics of the Central Andean plateau and the basement thrusts.

[1] The Santa Barbara System (SBS) of northern Argentina is a 400 km long segment of the Subandean foreland thrust belt. It is characterized by predominantly west verging, relatively high-angle thrust faults. Many of these faults are reactivated normal faults from one branch of a complex Cretaceous to Paleogene rift system. Heteroaxial folding in parts of the SBS is probably due to a slight component of range-parallel dextral strike-slip motion acting on preexisting faults striking north and NE during inversion. Regional balanced cross sections along two transects across the northern SBS indicate that the major faults flatten into a detachment in the basement at about 10 km depth. Neogene E-W contraction in the SBS is of the order of 21-26 km. Rift extension is not very well constrained but was probably less than 10 km. The structural style of the SBS differs from the thin-skinned Subandean thrust belt to the north and from the large-wavelength Sierras Pampeanas basement uplifts to the south. The changes between the different styles are sharp and coincide with the northern and southern boundaries of the rift in the foreland, suggesting that crustal or lithospheric heterogeneities exert an overriding control on foreland structural style. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8015 Structural Geology: Local crustal structure; 8102 Tectonophysics: Continental contractional orogenic belts; 9360 Information Related to Geographic Region: South America; KEYWORDS: central Andes, Subandean zone, Salta rift, fault reactivation, flat subduction. Citation: Kley, J., and C. R. Monaldi, Tectonic inversion in the Santa Barbara System of the central Andean foreland thrust belt, northwestern Argentina.