Zircon and apatite fission track (FT) thermochronology was applied to investigate the history of cooling and denudation of the Southern Andes between 41°and 42°15¢S in relation to the late Cenozoic activity of the Liquin˜e-Ofqui fault zone (LOFZ) and the northward migration of the Chile Triple Junction (CTJ). Fifty-six zircon and 51 apatite FT ages, plus 37 apatite confined track-length distributions were obtained mainly from plutonic rocks of the North Patagonian Batholith (NPB) in the main Andean Cordillera. Apatite FT ages and track lengths indicate a stage of rapid cooling at $5-3 Ma along both sides of the LOFZ, whereas older Miocene ages with monotonous cooling histories were obtained further away from the fault. Zircon FT ages range from Cretaceous to Pliocene, with marked differences observed along and across the LOFZ. Three different types of temperature-time histories characterise the post-magmatic cooling of the NPB in the region: deep intrusions with moderate and steady cooling rates, intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling, and rapidly cooled and exhumed shallow intrusions, the latter with younger ages towards the fault zone. The most prominent denudation episode along the LOFZ is late Miocene to Pliocene, coeval with plate tectonic reconstructions for the arrival and subduction of the Chile Rise beneath the Taitao Peninsula.

The southwestern part of the Lower Saxony Basin (LSB) is characterized by gravity and magnetic anomalies and by an extremely high thermal maturity of organic matter. This was for many years attributed to a Late Cretaceous intrusion, but actually deep burial is being debated. The complex thermal history of the area has been studied by fission track analysis. Zircon data provide evidence for widespread (hydro)thermal activity during the Permian and Upper Jurassic/Lower Cretaceous. Apatite ages indicate a major cooling event in the mid Cretaceous ($89-72 Ma) reflecting the time of inversion of the LSB. During the Cretaceous, the cooling of the basin centre was rapid compared to the basin margins. Apatite fission track ages from borehole samples which are recently within the upper part of the APAZ indicate a young heating of the sedimentary sequences until present.

ABSTRACT. Structural and microstructural observations combined with apatite and zircon fi ssion-track thermochronology within two sectors of the Main Andean Range in the Los Lagos Region of Chile reveal an episodic history of intrusion and deformation in the North Patagonian Batholith (NPB). A dextral displacement of ~30 km along the Liquiñe-Ofqui fault zone (LOFZ) is inferred from the correlation of corresponding Cretaceous intrusions of the NPB across the fault zone at Reloncaví. Relative uplift of the western block in the late Miocene is indicated by apatite and zircon fi ssiontrack cooling histories. Microstructures in samples from Miocene and Cretaceous plutons along the fault zone generally indicate deformation at temperatures below ~300°C, with the exception of some samples from Cretaceous intrusions showing deformation at higher temperatures. In the Hornopirén area, signifi cant relative uplift of the eastern block is indicated by 1. the different styles of deformation observed across the fault zone, 2. kinematic analysis of a shear zone in tonalite and 3. geobarometry of contact metamorphic mineral assemblages. For the plutonic rocks in the Hornopirén area, extremely rapid cooling is indicated by thermochronometry, suggesting the activity of hydrothermal systems or thermal effects of late shallow intrusions.

The Coastal Ranges in the western part of the Chiloé Archipelago represent an emerged forearc high at the subduction front of south-central Chile. Prior to the Cenozoic framework of the subducting Farallon and Nazca plates beneath the South American plate, the history of the metamorphic basement in the Coastal Ranges involves episodes of subduction and/or accretion of oceanic and ensialic material along the proto-Pacifi c margin of Gondwana. Along different segments of the Main Chiloé Island the thermal record of the metamorphic basement rocks includes regional metamorphism, exhumation and fi nally magmatism. Detrital zircons conventional U-Pb ages from a pelitic schist of the Central Segment of the Main Chiloé Island constrain the maximum possible sedimentation age to the Carboniferous (310 Ma). Other concordant zircon fractions (360, 390 and 412 Ma) suggest Ordovician primary sources. The sedimentary provenance of zircons could correspond to continental magmatic rocks of similar ages presently exposed in adjacent areas such as Nahuelbuta Mountains and Principal Cordillera and, additionally, from distal areas in the North-Patagonian and Deseado massifs. Widespread metamorphism in greenschist facies is well represented in the basement unit of the Main Chiloé Island. K-Ar and 40 Ar/ 39 Ar cooling ages in white mica range between 245 to 220 Ma, interpreted as near the peak of metamorphism during Middle to early Late Triassic times. Zircon and apatite fi ssion-track (FT) central ages in metamorphic rocks range from Late Jurassic (ca. 156 Ma) to Eocene (ca. 38 Ma) and are accompanied by relatively large dispersions. The modelling of the zircon FT single-grain age distributions depicts younger deconvoluted Late Cretaceous peak (ca. 80 Ma) and the apatite modelling shows two populations, an older Late Cretaceous peak (ca. 64-91 Ma) and a younger Eocene peak (ca. 38-53 Ma). The zircon and apatite Late Cretaceous peaks together with the presence of a marine sedimentary succession of presumably later Late Cretaceous age found at the forearc slope of the Main Chiloé Island, suggest exhumation of the metamorphic unit during the early Late Cretaceous. Magmatic zircons from a granodioritic body (Metalqui Pluton) emplaced in pelitic schists within the Central Segment of the Main Chiloé Island, a differentially uplifted block in relation to the northern and southern segments, gave an Eocene U-Pb crystallization age (39.6±0.3 Ma). Upper Eocene (ca. 37 Ma) biotite-bearing porphyric dacitic sills and dikes (Gamboa Dacite) also occur within the Central Segment. Zircon and apatite FT concordant ages (ca. 36 Ma) indicated rapid cooling for these subvolcanic rocks during Eocene times. According to this data, Eocene apatite central and modelling single-grain FT ages detected in the metamorphic rocks probably represent thermal resetting by shallow magmatism, coeval with the Gamboa Dacite and possibly the Metalqui Pluton.