Adrian J. Hartley

Paleomagnetic evidence for rotation in the Precordillera of northern Chile: structural constraints and implications for the evolution of the Andean forearc

Abstract Palaeomagnetic results are reported for two late Mesozoic to early Cenozoic continental sedimentary sequences from the Precordillera (Cordillera de Domeyko) of northern Chile. Comparison of isolated components of magnetisation from the Late Cretaceous-Palaeocene Purilactis Formation and the unconformably overlying Oligo-Miocene Paciencia Group with reference poles for South America indicate that significant post-Palaeocene and post-Lower Miocene clockwise rotation has taken place. A structural study of the area has revealed the presence of a number of thrust sheets that were periodically active throughout the Cenozoic. Stratigraphie, structural and palaeomagnetic data indicate that thrust sheet movement took place following deposition of both the Purilactis Formation (?lower Eocene Incaic Orogeny) and the Paciencia Group (?lowest Miocene Peheunche Orogeny) and resulted in the folding, uplift and clockwise rotation of the two sequences. Differences in the amount of rotation between the two units reflect incremental movement of the thrust sheets, 12° of rotation took place prior to deposition of the Paciencia Group followed by 17° of post-Lower Miocene clockwise rotation. This study demonstrates that palaeomagnetically detected rotation in the Precordillera of northern Chile, 250 km inland of the Peru-Chile trench, is related to the periodic movement of an easterly propagating thin-skinned thrust front. Between 22°30′ and 23°30′S the north Chilean forearc contains areas of non-rotation, thin-skinned compressional related rotations and large, block fault rotations indicating that the simplistic models previously presented to account for forearc rotation in the Central Andes should be modified. In particular, evidence implies that rotation could be related to a number of tectonic events and may not simply be due to one deformation event.

Tectonic controls on the development of a semi-arid alluvial basin as reflected in the stratigraphy of the Purilactis Group (upper cretaceous-eocene), northern Chile

Abstract The Upper Cretaceous-Eocene Purilactis Group of the north Chilean Precordillera consists of over 4100 meters of continental strata deposited in a retro-arc extensional basin. Deposited in an arid/semi-arid climate with no marine influence, the group comprises alluvial fan (51%), playa (35%), aeolian (8%), and lacustrine (6%) facies associations locally interbedded with volcaniclastic material. The basin-fill has an overall coarsening-upward profile and shows an increase in proximal fan facies up section, indicating basinward (eastward) fan progradation. Within the coarsening-upward profile, fan and playa facies are organized into: 1) large-scale (50–700 m thick) coarsening- (CU) and fining- (FU) upward cycles extending tens of kilometers, in which CU cycles represent tectonically induced (allocyclic) fan progradation during periods of decreased accomodation space (FU cycles reflect vertical aggradation and fan retreat during periods of increased accomodation space); 2) medium-scale cycles (15–50 m thick) extending up to 9 km, also representing tectonically induced fan progradation and retreat but superimposed on the larger scale cycles; and 3) small-scale, predominantly FU cycles (up to 15 m thick) extending only a few hundred meters and reflecting minor, autocyclically induced changes in sedimentation. Purilactis Group sediments were derived from a westerly (footwall) source of: 1) Upper Triassic to Lower Cretaceous sediments and volcanics (back-arc basin-fill), and 2) an andesite-dominated Upper Cretaceous volcanic arc sequence, unroofing of which is indicated by a systematic provenance change in the upper 500 meters of the group from dominantly andesitic to granodioritic detritus. Localized development of volcaniclastics in the uppermost part of the group — together with evidence of arc unroofing — indicates that arc activity, although synchronous, did not contribute significantly to the overall Purilactis basin-fill. Basin subsidence may have been influenced by thermal contraction related to cooling of the Late Cretaceous arc and/or isostatic uplift following arc unroofing, processes likely to result in relatively localized extension. A larger scale cause of extension may have resulted from the relatively slow convergence rates along the Andean margin during Late Cretaceous to Eocene times (

Controls on the distribution of volcanism and intra-basaltic sediments in the Cambo–Rosebank region, West of Shetland

The NE Atlantic margin is one of the last frontier areas of hydrocarbon exploration within the UK Continental Shelf. In 2004, a major oil and gas discovery (Rosebank) was made within Paleocene–Eocene-age lavas in the Faroe–Shetland Basin. The Rosebank Field consists of intra-basaltic terrestrial to marginal-marine reservoir sequences, separated by basaltic lava flows and volcaniclastics. Despite the identification of a major intra-lava-incised drainage system running parallel to the SW–NE-trending Rosebank Field, the controls on the distribution of both the volcanics and the intra-basaltic sediments were previously unclear, in part due to the difficulties that volcanic sequences pose to seismic acquisition, processing and interpretation. This has led to uncertainty in defining the wider intra-basaltic play. However, the examination of the recently acquired FSB 2011–12 MultiClient GeoStreamer® Survey has facilitated an increased definition of the geological units within and below the volcanic succession, and a reinterpretation of the Late Paleocene–Early Eocene stratigraphy. Through integration of 3D seismic data and well analysis, we present a regional overview of the volcanics and intra-basaltic sediments in the Rosebank region of the Faroe–Shetland Basin. We find that the structural setting of the Rosebank Field, in addition to lava-flow morphology, strongly influences the distribution of the intra-basaltic play fairway within the Paleocene–Eocene-aged Flett Formation. Restriction of fluvial siliciclastic sediments adjacent to the Corona Ridge extends the theorized Rosebank play fairway to the area SW of the Rosebank Field. Our analysis indicates that understanding the underlying basin structure is integral to the success of intra-basaltic plays.