Alfredo Lahsen

Upper Cenozoic volcanism and tectonism in the Andes of northern Chile

Abstract Recent geological and radiometric information of the Upper Cenozoic volcanic sequences of northern Chile, in conjunction with previous evidences, point out the complex evolution of the Central Andes volcanism. Volcanic activity developed along the Andean Range during the last 23 m.y., appears to have been temporally and genetically related to tectonic movements that shaped the major morphostructural features of this region. Since the Middle Miocene (12 m.y.) there has been a constant overlap, both in time and space, between lavas and ignimbrites. However, ignimbrite eruptions seem to predominate up to the Upper Miocene, while lava eruptions are dominant from the Pliocene to the Quaternary. Small-scale chronological and volumetric variations of the volcanic activity in the studied areas, make difficult the correlations with volcanic episodes through long distances. Units older than Pliocene have been folded and thrusted as a result of compressive movements that occurred ca. 7-4 m.y. ago. This deformation is mostly remarkable at the borders of major faulted blocks. These blocks were structured along pre-existing fractures caused by tectonic movements during the Lower Miocene. Pliocene and Quaternary block movements developed mainly through N-S fault systems. These faults, together with NW-SE and NE-SW systems, control the spatial distribution of most volcanic vents in this region. Locally, the N-S faults may correspond to reactivations of previous fault zones. This allowed successive effusions of volcanic material, at least during the last 17 m.y.

Ages and geochemistry of Mesozoic-Eocene back-arc volcanic rocks in the Aysén region of the Patagonian Andes, Chile

Edades y geoquimica de las rocas volcanicas del trasarco del Mesozoico-Eoceno en la region de Aysen de los Andes patagonicos, Chile. Diez y ocho nuevas edades radiometricas (catorce 40Ar-39Ar, cuatro K-Ar) junto con las ya publicadas confirman la existencia de tres eventos volcanicos (previamente definidos por relaciones estratigraficas) en la Patagonia chilena (Region de Aysen) durante el intervalo Mesozoico-Eoceno: Jurasico Medio-Cretacico temprano (160-130 Ma), Cretacico (114-75 Ma) y Eoceno (55-46 Ma). Sobre la base de las caracteristicas geoquimicas e isotopicas de Sr y Nd de las rocas volcanicas estudiadas, se pueden reconocer dos dominios magmaticos Mesozoico-Eoceno: Dominio Magmatico Norte (DMN) y Dominio Magmatico Sur (DMS). Los basaltos y rocas intermedias del DMN tienen afinidades alcalinas y valores isotopicos de Sr-Nd deprimidos a moderadamente deprimidos similares a aquellos derivados de una fuente dominada por material astenosferico. Las rocas volcanicas maficas del DMS tienen un caracter subalcalino y caracteristicas isotopicas de Sr-Nd mas enriquecidas comparables a aquellas derivadas de una fuente litosferica. Las rocas volcanicas felsicas del DMS tienen razones iniciales 87Sr/86Sr mas altas y valores de eNd mas bajos que las rocas volcanicas felsicas de DMN, sugiriendo una mayor contribucion cortical en sus fuentes magmaticas. Las distinciones geoquimicas e isotopicas entre el DMN y el DMS podrian estar influenciadas por la presencia de rocas metamorficas paleozoicas como basamento de las rocas volcanicas del DMS. Por otra parte, la distincion entre los basaltos del DMN y DMS podria corresponder a diferencias en la magnitud de la extension, siendo el DMN donde la extension habria sido mayor y, consecuentemente, la litosfera mas delgada

Chilean geothermal resources and their possible utilization

Abstract Most of the hot spring areas in Chile are located along the Andean Cordillera, associated with Quaternary volcanism. The volcanic—geothermal activity is mainly controlled by the subduction processes of the Nazca and Antarctic oceanic plates under the South America continental plate, and occurs at three well-defined zones of the Chilean Andes: the northern zone (17°30′–28°S), the central—south zone (33φ–46°S) and the southern-most or Austral zone (48°–56°S). Some tested high temperature geothermal fields, and geological and geochemical surveys of many other hot spring areas, evidence a great potential of geothermal resources in this country. Both electrical and non-electrical applications of this potential are considered in this paper. Taking into account the potentially available geothermal resources, the development of natural resources, the geographic and social—economic conditions existing in the different regions of Chile, it is concluded that power generation, desalination of geothermal waters, recovery of chemicals from evaporite deposits and brines and sulfur-refining are the main possible applications of geothermal energy in northern Chile; in central—south Chile geothermal energy is suitable for agribusiness such as greenhouses, aquaculture and animal husbandry.

Copper geochemistry in salt from evaporite soils, Coastal Range of the Atacama Desert, northern Chile: an exploration tool for blind Cu deposits

The evaporite soil (1.5 to 15 m thick) in the Coastal Range of northern Chile is a serious obstacle to mineral exploration. We conducted a Cu geochemical study in salt beds (mainly gypsum and anhydrite) from this soil. The sampling was distributed over mineralized and barren rocks, in salts of evaporitic soils from hills, slopes, gullies, plains and alluvium-filled valleys. The samples of salt from hills and slopes reflect very well the presence of blind ore deposits, and the magnitude of the Cu anomalies (<200 to >1000 ppm Cu) is independent of the depth at which the mineralization is located. Similar geochemical response has been found in salt from evaporitic soils that cover gullies, plains and alluvium-filled valleys. However, in the last case the magnitude of the Cu anomalies (<20 to >200 ppm Cu) depends on the thickness of the gravels and the depth of leached bedrock that covers the blind deposits. Although the Cu anomalies are spiky, their contrast with the background Cu population is remarkable. Sampling of salt from evaporitic soils appears to be a valuable tool for exploration of buried Cu deposits in the exceedingly arid Coastal Range of the Atacama Desert.

Evolución geológico-geomorfológica cuaternaria del tramo superior del valle del río Laja

La historia geologica del Cuaternario en el tramo superior del valle del rio Laja corresponde a una compleja interrelacion entre volcanismo, procesos de remocion en masa y sedimentarios aluvio-fluviales. El valle fue modelado inicialmente por la accion glaciaria del Pleistoceno inferior en rocas de las formaciones terciarias Cura-Mallin y Trapa-Trapa, ademas de rocas plutonicas, sobre las cuales se disponen en discordancia cuatro grandes unidades cuaternarias; el cono poligenico de Quilleco, una secuencia de rocas volcanicas pleistocenas, el volcan Antuco y el deposito de avalancha volcanica de Antuco. Los depositos del cono poligenico de Quilleco representan a facies mixtas volcano-sedimentarias intermedias y distales de los estratovolcanes que originaron las secuencia de rocas volcanicas pleistocenas con las cuales engranan lateralmente. El volcan Antuco corresponde a un estratovolcan mixto y compuesto, de composicion basaltica y andesitico-basaltica, cuya actividad se inicio ca. 130.000 a A.P. La primera etapa de su desarrollo (Antuco 1) culmino 9.700p 600 a A.P. con el colapso gravitacional lateral del edificio, que origino la gran avalancha volcanica de Antuco, cuyos materiales represaron el desague natural del lago del Laja y de sus quebradas afluentes. El colapso gravitacional fue el resultado de una actividad eruptiva freatomagmatica de tipo Bandai-San la que produjo, casi simultaneamente, flujos piroclasticos turbulentos, de tipo de oleadas de base humeda, compuestos de cenizas basalticas negras cuyos depositos primarios se han denominado Arenas Negras de Trupan-Laja. El volcan actual (Antuco 2) incluye un cono principal de lavas y escorias y la emision de, al menos, tres flujos piroclasticos importantes de poco espesor, localmente separados por depositos de corrientes de barro y coluvios. Posteriormente, debido a la ruptura del represamiento del lago del Laja, las cenizas negras fueron removidas hasta la Depresion Central, donde formaron un gran abanico aluvial de aproximadamente 50 x 60 km2.