José Cembrano

THE LIQUINE OFQUI FAULT ZONE : A LONG-LIVED INTRA-ARC FAULT SYSTEM IN SOUTHERN CHILE

Abstract Plate reconstructions for the Cenozoic document relatively steady right-oblique subduction of the Farallon (Nazca) plate beneath the Chilean continental margin. The kinematic signature recorded along the intra-arc Liquine-Ofqui fault zone (LOFZ), a major feature of the southern Andes, may help to understand the way in which the Nazca-South America slip vector has been partitioned into strain and displacement along and across the continental margin. The LOFZ consists of two NNE-trending right-stepping straight lineaments, a strike-slip duplex at the right step, and curved features which splay off the straight lineaments toward the northwest. The LOFZ runs mostly through heterogeneously deformed Cenozoic plutonic rock of the North Patagonian Batholith and patchy metamorphic wall rock. Early Cenozoic volcano-sedimentary rocks and dyke swarms, found in close spatial association with the strike-slip duplex, are believed to have developed in strike-slip-related basins. Quaternary volcanoes are aligned parallel to the LOFZ. Both ductile and brittle kinematic indicators within centimeter-to-meter-wide high-strain zones, document late Cenozoic dextral shear deformation. Contrasting left-lateral deformation recorded on older and wider mylonitic zones, suggests that the LOFZ may be a long-lived shear zone that accommodated continental-scale deformation arising from the Farallon (Nazca)-South America plate convergence. A block rotation pattern, as indicated by paleomagnetic data, is consistent with the geometry and Cenozoic kinematics of the LOFZ.

Compressional- and transpressional-stress pattern for Pliocene and Quaternary brittle deformation in fore arc and intra-arc zones (Andes of Central and Southern Chile)

Kinematic analysis of fault slip data for stress determination was carried out on Late Miocene to Quaternary rocks from the fore arc and intra-arc regions of the Chilean Andes, between 33° and 46° south latitudes. Studies of Neogene and Quaternary infilling (the Central Depression), as well as plutonic rocks of the North Patagonian Batholith along the Liquine–Ofqui Fault Zone, have revealed various compressional and/or transpressional states of stress. In the Pliocene, the maximum compressional stress (σ1) was generally oriented east–west. During the Quaternary, the deformation was partitioned into two coeval distinctive states of stress. In the fore arc zone, the state of stress was compressional, with σ1 oriented in a N–S to NNE–SSW direction. In the intra-arc zone the state of stress was transpressional with σ1 striking NE–SW. Along the coast, in one site (37°30′S) the Quaternary strain deformation is extensional, with an E–W direction, which can be explained by a co-seismic crustal bending readjustment.

Late Cenozoic transpressional ductile deformation north of the Nazca–South America–Antarctica triple junction

Abstract The southern Andes plate boundary zone records a protracted history of bulk transpressional deformation during the Cenozoic, which has been causally related to either oblique subduction or ridge collision. However, few structural and chronological studies of regional deformation are available to support one hypothesis or the other. We address along- and across-strike variations in the nature and timing of plate boundary deformation to better understand the Cenozoic tectonics of the southern Andes. Two east–west structural transects were mapped at Puyuhuapi and Aysen, immediately north of the Nazca–South America–Antarctica triple junction. At Puyuhuapi (44°S), north–south striking, high-angle contractional and strike-slip ductile shear zones developed from plutons coexist with moderately dipping dextral-oblique shear zones in the wallrocks. In Aysen (45–46°), top to the southwest, oblique thrusting predominates to the west of the Cenozoic magmatic arc, whereas dextral strike-slip shear zones develop within it. New 40Ar–39Ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike-slip, oblique-slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen. Similar ages were obtained on both high strain pelitic schists with dextral strike-slip kinematics (4.4±0.3 Ma, laser on muscovite–biotite aggregates, Aysen transect, 45°S) and on mylonitic plutonic rocks with contractional deformation (3.8±0.2 to 4.2±0.2 Ma, fine-grained, recrystallized biotite, Puyuhuapi transect). Oblique-slip, dextral reverse kinematics of uncertain age is documented at the Canal Costa shear zone (45°S) and at the Queulat shear zone at 44°S. Published dates for the undeformed protholiths suggest both shear zones are likely Late Miocene or Pliocene, coeval with contractional and strike-slip shear zones farther north. Coeval strike-slip, oblique-slip and contractional deformation on ductile shear zones of the southern Andes suggest different degrees of along- and across-strike deformation partitioning of bulk transpressional deformation. The long-term dextral transpressional regime appears to be driven by oblique subduction. The short-term deformation is in turn controlled by ridge collision from 6 Ma to present day. This is indicated by most deformation ages and by a southward increase in the contractional component of deformation. Oblique-slip to contractional shear zones at both western and eastern margins of the Miocene belt of the Patagonian batholith define a large-scale pop-up structure by which deeper levels of the crust have been differentially exhumed since the Pliocene at a rate in excess of 1.7 mm/year.

Scaling of fault damage zones with displacement and the implications for fault growth processes

[1] Knowledge of the spatial extent of damage surrounding fault zones is important for understanding crustal fluid flow and also for understanding the physical processes and mechanics by which fault zones develop with slip. There are few data available on the scaling of the fault damage zone with fault displacement, and of those that exist, deriving scaling relationships is hampered by comparing faults that run through different lithologies, have formed at different crustal depths or tectonic regimes (e.g., normal versus strike‐slip movement). We describe new data on the microfracture damage zone width from small displacement fault zones within the Atacama fault zone in northern Chile that formed at ∼6 km depth within a dioritic protolith. The microfracture damage zone is shown by an alteration halo surrounding the faults in which the density of the microfractures is much greater than background levels in the undeformed protolith. The data show that damage zone width increases with fault displacement and there appears to be a zero intercept to this relationship, meaning that at zero displacement, there is no microfracture damage zone. This is supported by field observations at fault tips that show a tapering out of fault damage zones. These data, combined with data from the literature, indicate that this same relationship might hold for much larger displacement faults. There is also a distinct asymmetry to the fracture damage. Several processes for the development of the observed scaling are discussed. The widely accepted theory of a process zone predicts that fault damage zone width increases with fault length and thus should always be largest at a propagating fault tip where displacement is lowest. This prediction is opposite to that seen in the current data set, leading to suggestion that other processes, such as damage zone growth with increasing displacement due to geometric irregularities or coseismic damage formation might better explain the spatial extent of damage surrounding even low‐displacement faults.

On the structure and mechanical properties of large strike-slip faults

Abstract Elucidation of the internal structure of fault zones is paramount for understanding their mechanical, seismological and hydraulic properties. In order to observe representative brittle fault zone structures, it is preferable that the fault be passively exhumed from seismogenic depths and the exposure must be in arid or semi-arid environments where the fragile rocks are not subject to extensive weathering. Field observations of two such faults are used to constrain their likely mechanical properties. One fault is the Carboneras fault in southeastern Spain, where the predominant country rocks are phyllosilicate-rich lithologies, and the other is part of the Atacama fault system in northern Chile, where faults pass through crystalline rocks of acidic to intermediate composition. The Carboneras fault is a left lateral fault with several tens of kilometres offset exhumed from approximately 4 km depth, and displays multiple strands of clay-bearing fault gouge, each several metres wide, that contain variably fractured lenses of protolithic mica schists. The strain is evenly distributed across the gouge layers, in accordance with the measured laboratory mechanical behaviour which shows predominantly strain hardening characteristics. The overall width of the fault zone is several hundred metres. Additionally, there are blocks of dolomitic material that are contained within the fault zones that show extremely localized deformation in the form of faults several centimetres wide. These are typically arranged at an angle of c. 20° to the overall fault plane. These differing types of fault rock products allow for the possibility of ‘mixed mode’ seismicity, with fault creep occurring along the strands of velocity strengthening clay-rich gouge, punctuated by small seismic events that nucleate on the velocity weakening localized faults within the dolomite blocks. The Caleta Coloso fault in northern Chile has a left-lateral offset of at least 5 km and was exhumed from 5–10 km depth. The fault core is represented by a 200–300 m wide zone of hydrothermally altered protocataclasite and ultracataclasite. This is surrounded by a zone of micro and macro-fractures on the order of 150 m thick. The fault core shows a heterogeneous distribution of strain, with alternate layers of ultracataclasite and lower strain material. The strain-weakening behaviour of crystalline rocks might be expected to produce highly localized zones of deformation, and thus the wide core zone must be a result of additional process such as precipitation strengthening or geometric irregularities along the fault plane.

Crustal deformation effects on the chemical evolution of geothermal systems: the intra-arc Liquiñe–Ofqui fault system, Southern Andes

A better understanding of the chemical evolution of fluids in geothermal and hydrothermal systems requires data-based knowledge regarding the interplay between active tectonics and fluid flow. The Southern Andes volcanic zone is one of the best natural laboratories to address this issue because of the occurrence of numerous geothermal areas, recent seismic activity generated by regional fault systems, and intense volcanic activity. Geothermal systems have been understudied in this area, and limited scientific information exists about the role of local kinematic conditions on fluid flow and mineralization during the development and evolution of geothermal reservoirs. In this study, we provide data for a 1:200,000 scale geological and structural map of the Villarrica–Chihuio area as a setting in which to perform a structural analysis of active geothermal areas. This structural analysis, combined with geochemical modelling of hot spring data, allows the identification of two magmatic-tectonic-geothermal domains based on fault systems, volcanic activity, and lithologies. The Liquiñe–Ofqui fault system (LOFS) domain encompasses geothermal areas located either along the master or subsidiary faults. These are favourably orientated for shear and extension, respectively. In the LOFS domain, the geochemistry of hot spring discharges is controlled by interaction with the crystalline basement, and is characterized by low B/Cl conservative element ratios and high pH. In marked contrast, the arc-oblique long-lived fault systems (ALFS) domain includes geothermal occurrences located on the flanks of volcanoes forming WNW-trending alignments; these systems are built over faults that promote the development of crustal magma reservoirs. Unlike the first domain, the fluid chemistry of these geothermal discharges is strongly controlled by volcanic host rocks, and is typified by lower pH and higher B/Cl ratios. Reaction path modelling supports our model: chemical evolution of geothermal fluids in the Villarrica–Chihuio area is strongly dependent on structurally controlled mechanisms of heat transfer. Within this framework, heat transfer by conduction is responsible for the LOFS domain, whereas magmatically enhanced advective transport dominates heat flow in the ALFS domain. Although more studies are needed to constrain the complex interplay between tectonics and fluid flow, results from this study provide new insights towards efficient exploration strategies of geothermal resources in Southern Chile.

Influence of sex difference and hormones on elastine and collagen in the aorta of chickens.

Collagen and elastine of the aorta were estimated in normal chickens of both sexes in cockerels, gonadectomized or treated with estradiol, and in hens treated with testosterone. The results showed that collagen and elastine were significantly higher in males than in females. Gonadectomy in males decreased significantly the content of collagen and elastine, so that the values became similar to those observed in females. The treatment of males with estradiol lowered the collagen and elastine content to values similar to those observed in females and in gonadectomized males. The treatment of females with testosterone increased significantly the collagen and elastine content of the aorta to levels similar to those observed in males.

Transpresión dextral y partición de la deformación en la Zona de Falla Liquiñe-Ofqui, Aisén, Chile (44-45ºS)

El segmento del Batolito Norpatagonico comprendido entre los 44 y 45oS esta afectado por una intensa deformacion fragil y ductil localizada en zonas de cizalle, asociada espacialmente a la Zona de Falla Liquine-Ofqui. Esta zona de falla esta representada regionalmente, por un lineamiento de azimut nor-noreste (Canal de Puyuguapi), que, segun estudios anteriores, forma parte de un duplex de rumbo a escala cortical en la Cordillera Patagonica. La deformacion ductil (Mioceno Superior-Plioceno) esta documentada por zonas de milonita tabulares de ancho centimetrico a metrico, con foliacion subvertical y lineacion de estiramiento noreste, mientras que la deformacion transtensional dextral se manifiesta a lo largo de zonas de cizalle este-oeste a noreste. La deformacion fragil (post-Mioceno Superior), representada por fallas decametricas con estrias y buenos indicadores cinematicos, tiene una distribucion areal mas amplia y se sobreimpone a la fabrica ductil. Su cinematica es compatible con un regimen dextral transpresional y transtensional a lo largo de zonas de falla de direcciones nor-noreste y este-noreste, respectivamente. El analisis cinematico de movimientos contemporaneos de rumbo, en el manteo y oblicuos de la deformacion fragil-ductil en los 44-45oS, sugiere algun grado de particion de esta en un regimen global de transpresion dextral en la Zona de Falla Liquine-Ofqui.

Coeval compressional deformation and volcanism in the central Andes, case studies from northern Chile (23°S–24°S)

Received 22 May 2009; revised 6 August 2009; accepted 24 August 2009; published 1 December 2009. [1] In this contribution we examine the relationship between active compression and construction of Pleistocene volcanoes in the present-day magmatic arc of the central Andes (23S–24S). Deformation producedseveralN–Sstriking, � 40kmlongsubparallel ridges. These ridges formed by folding of Pliocene ignimbrites and upper Pliocene and Pleistocene lavas; they are asymmetrical in profile and have a gentle back limb and steeper frontal limb. Andesitic monogenetic volcanoes show a close spatial relationship with the ridges; some volcanoes are on the hinge zone, whereas others lay on the sides of the ridges. We interpret this spatial pattern as a result of magma storage and migration along a system of subhorizontal reservoirs and reverse faults. Magma reservoirs probably formed along flat portions of reverse faults between ramp structures that serve as episodic magma transport

High magma oxidation state and bulk crustal shortening: key factors in the genesis of Andean porphyry copper deposits, central Chile (31-34°S)

RESUMEN Alto estado de oxidacion magmatico y acortamiento regional: factores claves en la generacion de los porfidos cupriferos de Chile central (31-34°S). El segmento andino comprendido entre los 31 y 34oS documenta una evolucion tectono-magmatica que involucra la generacion de tres porfidos cupriferos de clase mundial: Los Pelambres, Rio Blanco-Los Bronces y El Teniente. La genesis de estos tres depositos gigantes habria ocurrido como la culminacion de un ciclo de mineralizacion que actuo progresivamente de norte a sur, en estrecha asociacion con el emplazamiento de granitoides calcoalcalinos, fuertemente oxidados (razon Fe2O3/FeO = entre 1 y 3). Estos granitoides fueron emplazados en un ambiente de acortamiento regional y se fraccionaron a lo largo de zonas de falla sub-verticales activas, oblicuas al margen continental. Se propone que la actividad de estas zonas de cizalle habria jugado un rol clave en la exsolucion de los fluidos mineralizadores. El elevado estado de oxidacion de los magmas, junto con la ausencia de un aumento significativo en las razones iniciales 87Sr/86Sr, respaldados por nuevos datos geoquimicos y por datos previamente publicados, podria ser el resultado de un aumento de componentes de corteza oceanica alterada en la fuente mantifera de los magmas, bajo el arco. Se propone que este incremento de componentes de corteza oceanica en los magmas del Mioceno Superior estaria ralacionado con la progresiva subduccion, de norte a sur, de la dorsal de Juan Fernandez