Gloria Arancibia

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.

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.

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.

Constructing forearc architecture over megathrust seismic cycles: Geological snapshots from the Maule earthquake region, Chile

We present new field structural data from the Chilean Coastal Cordillera located above the northern and central parts of the interplate contact ruptured by the A.D. 2010 M w 8.8 Maule earthquake. The northern study area contains the northwest-striking Pichilemu normal fault, an intraplate structure reactivated after the megathrust event by crustal earthquakes up to M w 7.0. The structural style of this region is dominated by kilometer-scale normal faults that have been active at least throughout the Quaternary. The orientations of these main faults define three structural systems: (1) northeast- and (2) northwest-striking margin-oblique faults, and (3) north- to north-northeast–striking margin-parallel faults. From north to south, these three systems vary in their predominant occurrence, starting with bimodal orientations of groups 1 and 2, followed by predominantly single north to north-northeast orientations of group 3. Reverse faults coexist in time and space with the normal structures, but are scarce and display variable, apparently random orientations. The shallow crustal normal faults, including the Pichilemu fault, show a persistent kinematic history probably spanning thousands of subduction seismic cycles. Though historically smaller in magnitude than those of the triggered normal faults, interseismic forearc thrust events were recorded above the rupture area prior to the Maule earthquake. The Quaternary reverse faults identified in our study regions may be preserving interseismic, slow-strain-rate, permanent deformation signature in the structural grain. Analogous observations along the A.D. 2011 Tohoku earthquake rupture in Japan imply that such a link between the short- and long-term deformation patterns of the forearc is not exclusive of the Maule earthquake region.

Precipitación por despresurización y ebullición en vetas hospedadas en un dúplex de rumbo extensional: evidencias microestructurales y microtermométricas

La interaccion entre transporte de fluidos y deformacion cortical es relativamente bien conocida teoricamente, sin embargo, escasamente documentada en ejemplos reales. El examen estructural, microestructural y microtermometrico de redes de fracturas-vetas asociadas, en tiempo y espacio, al duplex de rumbo extensional Caleta Coloso (125-118 Ma) en el Sistema de Falla Atacama,en el norte de Chile, entrega informacion clave sobre las condiciones termodinamicas (presion, temperatura y composicion) de los fluidos responsables de la precipitacion mineral, senalando evidencias de los mecanismos que la gatillaron. El analisis microestructural indica que los cristales de cuarzo, calcita y epidota crecieron en espacios abiertos, llenos de fluido y a presion subhidrostatica. Las inclusiones fluidas primarias en cuarzo y calcita arrojaron temperaturas de homogenizacion entre 102 y 253°C, con un amplio rango de salinidades (3,5-23,7 % en peso de NaCl equivalente). Las presiones obtenidas varian entre 0,4 y 7,4 MPa, valores muy bajos para el contexto geologico de las vetas, el cual indica una profundidad de generacion de al menos 3 km. Estos valores de presion documentan una drastica caida local de la presion de fluido que pudo haber generado ebullicion y precipitacion mineral en la corteza superior en sitios dilatacionales del Duplex Caleta Coloso.

Deformation and magma transport in a crystallizing plutonic complex, Coastal Batholith, central Chile

The Carboniferous–early Permian Santo Domingo complex in coastal Chile (33.5°S) preserves magmatic structures that allowed us to partially reconstruct and compare the deformation histories of two intrusive units within a mid-upper crustal zoned pluton. The oldest history is preserved in the Punta de Tralca tonalite, where microgranitoid enclaves record the emplacement and partial assimilation of mostly mafic magma into an intermediate host. Enclaves record early foliation development by a mechanical sorting and alignment of minerals during hypersolidus flow in melt-rich magma currents, followed by diffusion creep and sliding along melt-coated crystals. Structures in a weaker, tonalitic matrix record compaction, flattening, and near-solidus deformation as porous flow, aided by brittle deformation, drained residual melts. These processes produced penetrative S > L fabrics (i.e., planar more dominant that linear fabric) in an increasingly viscous, crystal-rich mush and promoted folding, fracturing, shearing, and crystal-plastic deformation as the mush approached its solidus. The deformation disrupted igneous layering and helped mobilize and concentrate melt-rich aggregates, forming diffuse patches and dikes that intruded previously deformed enclaves and matrix and aided pluton differentiation. A different deformation history is recorded by the Estero Cordoba dike, which intruded and interacted comagmatically with the Punta de Tralca tonalite. The dike records how magma flow near stiff boundaries resulted in velocity gradients that drove deformation during magma replenishment. This deformation reset inherited enclave fabrics, increased ductile stretching and winnowing, and formed linear (L > S) fabrics. This example illustrates how different styles of deformation assisted magma movement through a mid-upper crustal magma chamber and highlights the diverse origins and significance of structures generated by deformation in magmas of variable crystal-melt ratios.

Tectonic significance and hydrothermal fluid migration within a strike-slip duplex fault-vein network: an example from the Atacama Fault System.

El Duplex Caleta Coloso es una estructura de rumbo desarrollada durante la deformacion fragil del Sistema de Falla de Atacama (SFA) en el Cretacico Temprano. En su interior hay un sistema de vetas hidrotermales que documentan la naturaleza de la relacion entre el transporte de fluidos y el desarrollo del duplex. El sistema de vetas de orientacion dominante NW se localiza en la roca de caja, adyacentes a las zonas de falla. Segun su mineralogia dominante hay vetas de clorita, epidota-cuarzo y calcita-limonita, y segun las relaciones de corte o su estructura interna, se definieron vetas tempranas (clorita), intermedias (epi dota-cuarzo) y tardias ( calcita-limonitas). Algunas vetas muestran cristales perpendiculares u oblicuos a sus paredes (vetas de extension y de extension oblicua) o fibras minerales orientadas paralelas a las estrias de las fallas (vetas-fallas). Estas ultimas tienen indicadores cinematicos compatibles con las fallas, evidenciando que fueron sincinematicas con el desarrollo del duplex. Segun su microestructura, ellas se habrian formado en fracturas abiertas llenas de fluidos, bajo condiciones de presion inferior a la hidrostatica, lo cual indicaria que la precipitacion mineral ocurrio por caidas abruptas de la presion en una corteza somera (<4 km), probablemente asistidas por el mecanismo de bomba de succion. Se entregan cinco edades 40Arf39 Ar y K-Ar, entre 119 y 141 Ma. Considerando el contexto geologico y el error analitico asociado a la naturaleza de los minerales datados, estos valores son interpretados como edades aproximadas del desarrollo de las vetas, que son concordantes con los antecedentes geocronologicos existentes.

Geochronology of the Lower Cretaceous volcanism from the Coastal Range (29°20'-30°S), Chile

Ar/ 39 Ar age data (laser and furnace step heating) on plagioclase from Lower Cretaceous volcanic sequences from the Arqueros Formation in two sections of the Coastal Range at the latitude of La Serena (≈29°S) have been obtained. Due to the partial alteration of plagioclase crystals, disturbed age spectra in the furnace experiments have been observed, whereas laser heating determinations involving a much smaller quantity of grains carefully selected, could display plateau ages corresponding to pure plagioclase, as demonstrated by a constant 37