Post-Caledonian extension during orogenic collapse and Mesozoic rifting in the West Norway-northern North Sea region was accommodated by the formation and repeated reactivation of ductile shear zones and brittle faults. Offshore, the Late Palaeozoic-Mesozoic rift history is relatively well known; extension occurred mainly during two rift phases in the Permo-Triassic (Phase 1) and Mid-Late Jurassic (Phase 2). Normal faults in the northern North Sea, e.g., on the Horda Platform, in the East Shetland Basin and in the Viking Graben, were initiated or reactivated during both rift phases. Onshore, on the other hand, information on periods of tectonic activity is sparse as faults in crystalline basement rocks are difficult to date. KAr dating of illite that grows synkinematically in fine-grained fault rocks (gouge) can greatly help to determine the time of fault activity, and we apply the method to nine faults from the Bergen area. The K-Ar ages are complemented with X-ray diffraction analyses to determine the mineralogy, illite crystallinity and polytype composition of the samples. Based on these new data, four periods of onshore fault activity could be defined: (1) the earliest growth of fault-related illite in the Late Devonian-Early Carboniferous (>340 Ma) marks the waning stages of orogenic collapse; (2) widespread latest Carboniferous-Mid Permian (305-270 Ma) fault activity is interpreted as the onset of Phase 1 rifting, contemporaneous with rift-related volcanism in the central North Sea and Oslo Rift; (3) a Late Triassic-Early Jurassic (215-180 Ma) period of onshore fault activity postdates Phase 1 rifting and predates Phase 2 rifting and is currently poorly documented in offshore areas; and (4) Early Cretaceous (120-110 Ma) fault reactivation can be linked either to late Phase 2 North Sea rifting or to North Atlantic rifting.

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K-Ar dating on synkinematically formed illite in low-grade metamorphic pelites was used to set new time constraints on the development of the Patagonian retroarc fold-andthrust belt caused by the subduction of the Antarctic plate beneath the South American plate. The combined use of illite crystallinity (Kübler index), polytype quantification, and K-Ar dating of illite fractions (<0.2, <2, and 2-6 μm) is a powerful tool in constraining the missing time gap between the emergent foldand-thrust belt formation and early Miocene uplift of the Magallanes Basin. Four distinct periods of illite growth were identified based on their K-Ar ages and degree of regional metamorphism: (1) early Cenomanian (98 Ma) illite crystallization, (2) widespread early Campanian (ca. 80 Ma) long-lasting burial illitization processes under anchizonal metamorphic conditions, (3) a significant period of illite formation in the early Paleocene (ca. 60 Ma), and (4) a late stage of illite growth in the early Eocene (55-46 Ma) under epizonal conditions. Based on K-Ar fine fraction ages and Kübler index values, we propose to subdivide the internal domain along the Río Nutria and Río Rincon thrust into a thrust zone with maximum rock uplift in the west and a foreland-vergent imbricate thrust zone in the east. The earliest indication for the emergent fold-and-thrust belt formation in the hinterland is documented in a metapelitic clast (sample 14-9, <2 μm) within the Upper Cretaceous Cerro Toro conglomerate, which yielded a K-Ar cooling age of 98.3 ± 1.2 Ma and an epizonal Kübler index value of 0.24 Δ°2Ɵ. After a certain period of geological quiescence, an interval of major thrusting and rock uplift occurred between ca. 60 and 46 Ma. The west-dipping Río Nutria and Río Rincon thrusts record the onset of fold-andthrust belt activity, which can be placed close to 60 Ma. In the western part of the internal domain, thrusts remained active, and associated structural uplift is recorded until 46 Ma by K-Ar illite cooling ages. Flexural subsidence driven by thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and a rapid increase in sedimentation rate along the monoclinal belt. Our results prove a synchronous onset of thickskinned thrusting between 49°S and 55°S and suggest constant convergence rates of the Patagonian arc against the adjacent South American continental margin. Time-averaged exhumation rates along the Río Rincon anticline (54.6-22 Ma) and along the Río Nutria thrust (46.5-22 Ma) suggest rather low exhumation rates of 1.6-1.2 mm/yr (maximum). The low exhumation rates link back to constantly low subduction rates, resulting in a period of geological quiescence between 46.5 and 22 Ma.

The [MnO|SiO2,Al2O3,FeO,MgO] balanced ratio (i.e. the isometric log-ratio of the MnO concentration relative to the concentration of SiO2, Al2O3, FeO and MgO) of chlorite and of whole-rock composition is an effective discriminant between Mesozoic stratigraphic formations in the Magallanes Basin (Chile). The MnO content in chlorite is only controlled by the host rock chemistry and is dependent on the geological environment. The MnO content in chlorite remains unchanged at low-grade metamorphic conditions. Single-grain chlorite analysis (n = 1042, electron microprobe) and whole-rock analysis (n = 40, X-ray fluorescence) were used to discriminate stratigraphic formations and to decipher differences in the depositional environment in the Magallanes Basin. The samples are from one Upper Jurassic and three Cretaceous sedimentary units that were affected either by low-grade regional metamorphism or by Miocene contact metamorphism. The highest [MnO|SiO2,Al2O3,FeO,MgO] values are recorded in the upper Zapata Formation. The Punta Barrosa, Cerro Toro and Tobífera Formations show slightly lower [MnO|SiO2,Al2O3,FeO,MgO] values. Elevated [MnO|SiO2,Al2O3,FeO,MgO] values at the transition between Zapata and Punta Barrosa Formations record an oxygenated shallow marine environment that can be linked to the closure of the Rocas Verdes Basin and the onset of fold-and-thrust belt formation. Decreasing [MnO|SiO2,Al2O3,FeO,MgO] values from the Punta Barrosa towards the Cerro Toro Formation indicate gradually increasing water depths during the Upper Cretaceous that correlate well with the global sea level.

This study focuses on the northernmost part of the Mesoproterozoic Espinhaço Supergroup that crops out in the Chapada Diamantina. The fine-fraction K/Ar dating obtained on slightly metamorphosed sediments of the siliciclastic Espinhaço Supergroup shows a polyphase deformation history that corresponds to the Brasiliano (Pan-African) orogenic cycle. The isotopic results are interpreted to indicate three age domains coincident with three structurally different domains. Constrained by the Kübler Index ('illite crystallinity') and illite polytypism, the thermal conditions generated during the tectonic activity show a gradual trend from the craton margins to the interior from epizonal to diagenetic. The northern Chapada Diamantina is situated in the foreland of the Riacho do Pontal belt and comprises the sediments of the Espinhaço Supergroup northeast of the Irecê basin. The K/Ar ages for < 2 µm illite fractions range between 645 and 621 Ma [mean 637±9 Ma (2σ)] and for < 0.2 µm fraction range between 625 and 603 Ma [mean 614±9 Ma (2σ)]. Samples from the central Chapada Diamantina east of the Irecê basin are not affected by a Brasiliano deformation event and therefore, the N-S-trending structures are assumed to be older. The deformation of the southern Chapada Diamantina was established in conjunction with the formation of the Araçuai orogenesis and the inversion and reactivation of the Paramirim impactogen. The last stage of deformation in this area is recorded by the K/Ar fine-fraction dating between 470 and 460 Ma.

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