U52A-01
The Southern Central Andes at 25.5şSL: a Non-Balanced Crustal Cross-Section
The Andes is the type orogen for subduction contractional systems but many uncertainties still exist about the crustal structure and few complete cross-sections have been done and published. Estimates of shortening have been in the past years a matter of debate mainly as far as the deficit of tectonic shortening to account for the known crustal thickness is concerned. Estimates for this shortening have mainly relied on shortening calculations made on cross-sections of parts of the orogen. It is known that along strike motion of material do occur in most mountain belts and has to be taken into account in any precise orogenic mass balance. It has also recently argued that the shortening deficit in the Andes results from plane strain calculations not considering the out of plane section motion of material. However, 2D estimates of shortening on cross-section balancing are still a first valid approximation to a mass balance and construction of cross-sections across the entire Andean orogen is a priority. A complete cross-section of the Central Andes at 25.5° S has been constructed based mostly on surface geological data. This work has involved field mapping and interpretation of aerial and satellite images across all the main structural units at this Andean transect. Structural data have been acquired all along the cross-section. Interpretation of seismic reflection profiles has also integrated where available, mostly in the eastern part of the cross-section. Published geophysical data have used to constrain the deep crustal structure. Along this cross-section, crustal mass balance and shortening estimates along the non-thinned South America crust shows that shortening based on the flexural-slip restoration of the bottom of syn-compressive rocks (bottom of the Cenozoic in Argentina and of the Cretaceous in Chile) is of 108 km whereas the shortening based on the South America crust area restoration, considering an initial crustal thickness of 38 km, increases to 271 km. This denotes that contractional structures only explain 39,85% of the Andean crustal thickening along the presented section, and that the remaining Andean crustal thickening (60,15%) is related to other processes. It should be noted, that this thickening non explained by the observed contractional structures is equivalent to 163 km of non- thickened South America crust and not very different to the area of the low-velocity zone observed at middle crust levels. This 163 km are also similar to the calculated amount of forearc tectonically eroded by the subducted Nazca plate.
U52A-02 INVITED
U-Pb Geochronologic Evidence for the Evolution of the Gondwanan Margin of the North- Central Andes
The Neoproterozoic - Early Paleozoic evolution of the Gondwanan margin of the north-central Andes has been
investigated by a U-Pb zircon geochronology study in the Eastern Cordilleras of Peru and Ecuador, combining LA-
ICPMS detrital zircon analysis with dating of syn- and post-tectonic intrusives by TIMS and ion microprobe. The
majority of detrital zircon samples exhibit prominent peaks in the ranges 0.45 - 0.65 Ga and 0.9 - 1.3 Ga, with
minimal older detritus from the Amazonian craton. The detrital zircon data demonstrate that the basement to the
western Gondwanan margin was likely composed of a metamorphic belt of Grenvillian age, upon which an Early
Paleozoic magmatic belt was situated in a similar way to the Sierra Pampeanas and Famatina Terranes of
northern Argentina. These two orogenic belts are interpreted to be either buried underneath the present-day
Andean chain or adjacent foreland sediments.
Plutons associated with the Early Paleozoic subduction-related magmatic belt have been identified in the Eastern
Cordillera of Peru, and have been dated by U-Pb zircon TIMS and ion microprobe to 474 - 442 Ma. This is in
close agreement with the ages of subduction-related magmatism in the Arequipa - Antofalla Basement (e.g.
Loewy et al., 2004). This Early Paleozoic arc is clearly not linear as it jumps from a coastal location in the
Arequipa - Antofalla Basement to several hundred kilometers inland in the Eastern Cordillera further to the north.
This is interpreted as an embayment on the Proto-Andean margin at the time the arc was initiated; if this is the
case the northern termination of the Arequipa-Antofalla Basement in the vicinity of Lima is an Ordovician or older
feature.
The arc magmatism pre- and post dates phases of regional metamorphism in the Eastern Cordillera of Peru. U-
Pb zircon ion microprobe dating of zircon overgrowths in high-grade leucosomes demonstrates that the presence
of a metamorphic event at c. 478 Ma, and refutes the previously-assumed Neoproterozoic age for orogeny in the
Peruvian Eastern Cordillera. The presence of an Early - Middle Ordovician age magmatic and metamorphic belt
in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism
was continuous from Patagonia (Pankhurst et al., 2006) through northern Argentina and Chile to as far north as
Colombia and Venezuela, a distance of nearly seven thousand kilometers. The presence of an extremely long
Early - Middle Ordovician active margin on western Gondwana invites comparison with the Taconic - Grampian
orogenic cycle of the eastern Laurentia margin (which is of similar age and strike length) and supports models
which have these two active margins facing each other during the Ordovician.
U-Pb zircon ion microprobe dating of zircon overgrowths in migmatites yields ages of c. 312 Ma, and represent a
previously unreported high-grade Gondwanide event which has affected the Peruvian segment of the Proto-
Andean margin. The pattern of crustal growth in the north-central Andes implies that it was dominated by a series
of progressive crustal accretion events, which results in a series of age domains that young away from an old
Amazonian core.
http:www.tcd.ie/Geology/Staff/chewd/publications.php
U52A-03
Oroclinal Bending and Mountain Uplift in the Central Andes
The large paleomagnetic database now available for the Central Andes permits a good understanding of the overall spatial and temporal variations of rotations. Mesozoic to Early Paleogene rocks along the forearc of northern Chile (23°-28°S) record significant clockwise rotations (>25°) [Arriagada et al., 2006, Tectonics, doi:10.1029/2005TC001923]. Along the forearc of southern Peru, counterclockwise rotations recorded within flat lying red-beds (Moquegua Formation) increase from about -30° at 17.5°S to - 45° at15.5°S and decrease through time from the late Eocene to the late Oligocene-early Miocene [Roperch et al., 2006, Tectonics, doi:10.1029/2005TC001882]. Recently published thermo-chronological studies show evidence for strong exhumation within Bolivian Eastern Cordillera and the Puna plateau starting in the Eocene while structural studies indicate that the majority of crustal shortening in the Eastern Cordillera occurred during the Eocene-Oligocene, although the final stages of deformation may have continued through the Early Miocene. Rotations in the Peruvian and north Chilean forearc thus occurred at the same time than deformation and exhumation/uplift within the Eastern Cordillera. In contrast Neogene forearc rocks in southern Peru and northern Chile do not show evidences of rotation but low magnitude (10°) counterclockwise rotations are usually found in mid to late Miocene rocks from the northern Altiplano. These Neogene rotations are concomitant with shortening in the Sub-Andean zone and sinistral strike-slip faulting along the eastern edge of the northern Altiplano. We interpret the rotation pattern along the southern Peru and north Chile forearc as a result of strong late Eocene- late Oligocene oroclinal bending of the Central Andes associated with shortening gradients along the Eastern Cordillera associated both with the Abancay deflection and the Arica bend. The amount and spatial distribution of pre-Neogene shortening needed to account for oroclinal bending is difficult to estimate as the rotations may be partly driven by transpression along strike slip shear zones. The large rotations strongly highlight the importance of the pre-Neogene tectonic history in the evolution of the Central Andes.
U52A-04 INVITED
Surface Uplift History of the Central Andes: Implications for the Growth of Orogenic Plateaus
Sedimentation, paleoelevation, and incision histories provide important constraints on the timing and magnitude of regional surface uplift of mountain belts that point to specific processes that led to surface uplift. The sedimentary record and stable isotopic compositions of carbonates are used to reconstruct the late Miocene subsidence history, paleoenvironment, and paleoelevation of the northern Altiplano basin. Multiple paleoelevation proxies, including paleoleaf physiognomy, δ18O paleoaltimetry, and Δ47 paleothermometry, suggest that the Altiplano rose by 2.5±0.5 km to 3.5±0.5 km to its current elevation between ~10 and 7 Ma. Geomorphic evidence from widespread, low-relief paleosurfaces on both the eastern and western flanks of the Andes also shows that the onset of rapid incision of paleosurfaces occurred between ~10 and 6.5 Ma over the entire width of the mountain belt and over at least 5° latitude. Stream profile analysis of the drainage systems that incise these paleosurfaces has been inferred to reflect ~1 to 2 km of surface uplift of the flanks of the Andes. Combining geomorphic evidence with paleoelevation constraints, the paleotopographic evolution of the Andes is reconstructed over the late Miocene. Late Miocene regional surface uplift requires the removal of mantle lithosphere as the dominant geodynamic mechanism for raising the plateau during this time. However, crustal thickening and redistribution of crust by erosion/sedimentation and/or lower crustal flow set the limit of surface uplift. Regional surface uplift of the Andean plateau in the late Miocene predicts a decrease in the horizontal deviatoric stress in the plateau that is consistent with observations of upper crustal shortening, sedimentation rates, and magmatism in the plateau. Shortening ceased across the plateau between 10 and 7 Ma, coincident with widespread ignimbrite eruptions and an abrupt decrease in sedimentation rates. The combination of geodynamic processes that appear to have occurred in the Andes in late Miocene time, including removal of high density lower lithosphere and redistribution of crust by erosion/sedimentation and flow of low density middle-lower crust, are likely mechanisms for building broad, flat, high elevation plateaus in convergent tectonic settings.
U52A-05
Compression vs. Extension in the Central Neuquen Basin (35°-37°S), Argentina
The present day geometry of the Neuquen basin located in the southernmost part of the Central Andes is inherited from different compressive pulses during the Late Cretaceous-Late Miocene time interval. However, the structural evolution of the central Neuquen basin (35°-37°S) during the Late Cenozoic is still a matter of debate. Some authors argue for compression since the Late Miocene onwards whereas others favor extension during the Quaternary. Here we present evidences of compressive, extensive and strike-slip structures of Quaternary age. Several observations show that thrusting and folding is still active during the Quaternary in the external part of the Neuquen basin: alluvial fans are deformed and Quaternary alluviums are uplifted respectively in the footwall and the hanging wall of the Tromen thrust; Quaternary terraces are folded with progressive unconformities along the external limb of the Pampa de Tril anticline; regressive erosion develops in the eastern part of the basin along the east limb of the Chihuidos anticline. We have also documented numerous normal faults of various ages. Syn- sedimentary normal faults developed during the Late Jurassic-Early Cretaceous close and parallel to the eastern border of the Loncopue through (western part of the Neuquen basin). The Mesozoic sediments are cut by some N160°E-trending hydrothermally altered normal faults that may have initiated in association with volcanism. This latter ~E-W extension may be contemporary with the formation of the Loncopue through during the Oligocene. In the central part of the Neuquen basin, we have observed normal faults affecting Quaternary terraces. Along the east limb of the Tromen volcano, normal faults postdate the Tromen thrust. In the northeastern part of the study area, a major N-S right-lateral strike-slip fault affects the Holocene lavas of the Payun volcano. The coexistence of both recent compressive and extensive tectonic structures seems to be contradictory at the scale of the Neuquen basin. However, this apparent contradiction may be explained by the development of relay zones at the stepovers of strike-slip faults trending N-S. A strike-slip tectonic regime characterized by a maximum horizontal principal stress axis trending NE-SW agrees well with the initiation of recent normal faults striking in the same direction. We propose that the central Neuquen basin corresponds to a particular area forming an extensional relay zone between (1) to the southwest, the N-S Liquine-Ofqui right-lateral strike fault that vanishes at the latitude of the study area and (2) to the northeast, a N-S right-lateral strike-slip fault cutting the Holocene lavas. Compression may also develop locally at stepovers along strike-slip fault segments. The central Neuquen basin is therefore controlled by strain partitioning in the frame of the oblique convergence between the Nazca and the South America plates.
U52A-06
Thrusting Evolution in the Southern Cordillera Oriental (Northern Argentine Andes)
Growth strata and unconformities observed in the Neogene-Quaternary synorogenic sediments in the southern Cordillera Oriental of northern Argentine Andes demonstrate that deformation migrated eastwards from the Puna to the Santa Bárbara system, as it is accepted and well documented. However, the newly described growth sequences and unconformities allow us to unravel a more precise timing of the structures and determining the partitioning of the deformation through time into the thrust and fold wedge. Thrust evolution was mostly controlled by the reactivation of earlier extensional faults of the Salta Rift Basin. The geometry of the inverted structures combined with the general forward migration of the deformation, resulted in different thrust sequences. The foreland dipping imbricate stack of the western Cordillera Oriental developed forwards during Middle-Upper Miocene, giving rise to a break back thrusting sequence. Thrust propagation rate increased at Upper Miocene- Lower Pliocene times when the eastern part of the Cordillera Oriental developed. At that time a widespread deformation and synchronous thrusting occurred in the Cordillera Oriental. Afterwards, deformation was mostly restricted in the eastern part of the Cordillera Oriental. Finally, during the Quaternary deformation involved a wider zone, including all the Cordillera Oriental and the Santa Barbara System, leading to the reactivation of previously developed thrusts.