P. Turner

Development of a continental forearc: A Cenozoic example from the Central Andes, northern Chile

In order to understand the response of a continental forearc to changes in subduction-zone geodynamics, we constructed a high-resolution chronostratigraphic cross section across the Central Andean forearc of northern Chile (21°–24°S). The tectono-stratigraphic development of the forearc differs from established models. No relationship was found between changes in rate of relative plate convergence and amount and style of deformation. Forearc response to continual compression since the Oligocene has been uplift and segmentation into discrete tectono-stratigraphic zones. From west to east, these zones are the extensional Coastal Cordillera, the extensional and/or transtensional Central depression, and the transpressional and/or compressional Precordillera-Preandean depression. Each area has recorded almost continuous sedimentation from Oligocene (?Eocene) time to the present day. Accommodation space has been generated by basin-margin uplift rather than active subsidence. We propose a model in which uplift of the leading edge of the South American plate is driven by subcrustal accretion of material removed at the trench by subduction erosion. Uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge. The tectono-stratigraphic complexity exhibited within the Central Andean forearc is likely to be representative of Cordilleran-type margins and would be difficult to recognize in an ancient continental forearc.

Sedimentological response of an alluvial system to Neogene thrust tectonics, Atacama Desert, northern Chile

The Llano de la Paciencia is a thrust sheet top basin in which the sedimentological and topographic evolution can be linked to thrust tip propagation. It is an elongate gravel plain which borders the Salar de Atacama, a major intermontane basin in the Andean forearc of northern Chile. The Llano is bounded to the west by the Cerros de Purilactis a Cretaceous–Paleocene sequence uplifted by the Frontal Domeyko Thrust. The eastern margin of the Llano is formed by the Cordillera de la Sal which was uplifted by a linked back thrust-frontal thrust system. The Salar de Atacama is thus divided into a number of discrete sub-basins: the Llano de la Paciencia, the Pampa Visachita and the western sub-Llano which from east-west are bounded by the Cordillera de la Sal, the northern imbricates and the ignimbrite back thrust. Two phases of sedimentological evolution can be distinguished within the Llano on the basis of Quaternary to Recent sediment dispersal patterns. Initially, Phase 1 alluvial fan lobes prograded eastwards into the main Salar de Atacama basin. Subsequently, uplift of the Cordillera de la Sal deflected drainage systems southwards parallel to the structural strike. These Phase 2 alluvial deposits drain into the Salar de Atacama at the lateral termination of the Cordillera de la Sal frontal thrust. In places where thrust tip ramps are emergent within the Llano, gullies have been incised into the drainage pathways. This has resulted in the reworking of the early Phase 1 gravels and progradation of the Phase 2 fan lobes. The evolution of the Llano de la Paciencia illustrates the close link between topography and syntectonic alluvial drainage patterns in an arid intermontane basin.

Extensional tectonics in convergent margin basins: An example from the Salar de Atacama, Chilean Andes

The Salar de Atacama basin of northern Chile preserves stratigraphic evidence for the evolution of the Andean cycle. It has evolved from a non-arc-related rift, through back-arc and inter-arc stages, to a Neogene fore-arc basin. Accumulation of the sedimentary succession was mainly due to extensional faulting. Important but short-duration contractional episodes do link to known fast-order plate-margin changes, but their stratigraphic effect appears to be restricted to uplift/erosion rather than creation of significant flexural subsidence. The Salar de Atacama basin originated as part of a regional rift system during Permian time and contains 2 km of Permo-Triassic continental detritus and volcanic rocks. The area remained above depositional base level throughout Late Triassic to Late Cretaceous time. Syn-rift continental red beds were deposited to a thickness of at least 2 km on a parallel rift segment in the Domeyko area to the west. Continued Triassic-Jurassic extension in the Domeyko basin resulted in a classic continental to marine transition, with deposition of a 2 km+ Jurassic mixed carbonate/clastic sequence. In latest Cretaceous-Eocene time, the Salar basin was an arc-related basin and accommo-dated some 4 km+ of continental detritus (Purilactis Group) due to back-arc extension, sediment being derived from the Domeyko Cordillera and arc rocks to the west. Late Eocene right-lateral strike-slip faulting and associated restraining bend uplift were driven by a high rate of oblique convergence between the Farallon and South American plates. This deformation complicated the stratigraphy of the Purilactis Group and inverted the western basin margin, which formed the dominant provenance area for a 2-km-thick Oligocene continental basin-fill component (Paciencia Group).The Oligocene basin was an extensional to trans-tensional basin. The Miocene-Holocene Salar basin is a continental fore-arc basin. This latest segment of the basin fill comprises pyroclastic and continental sedimentary rocks thrust over Quaternary gravels in many places. The Cordillera de la Sal is an intrabasinal uplift, initiated as a thin-skinned contractional feature. Thus, the superposed basin-fill components represent responses to distinctly different geodynamic settings.

The Great Eucrite intrusion of Ardnamurchan, Scotland : Reevaluating the ring-dike concept

Ring-dikes are cylindrical sheet intrusions that develop at a subvolcanic level due to ascent of magma along steep outward-dipping ring fractures. Magma ascent is triggered by central block subsidence, and fully formed ring-dikes are composed of a flat-lying sill-like roof as well as steeply outward-dipping walls on all sides. The Great Eucrite of the Ardnamurchan Paleocene igneous complex, NW Scotland, is a spectacular gabbro intrusion that has been cited as one of the classic examples of a ring-dike for the past 70 yr. We combine field observations, detailed structural measurements of primary magmatic features, and anisotropy of magnetic susceptibility data in a reinvestigation of this intrusive body. Magmatic layering and macroscopic planar crystal arrangements dip inward, and magnetic lineations plunge consistently toward the center of the intrusion, in contrast to what would be expected for a ring-dike. We propose that the Great Eucrite ring-dike is in fact a lopolithic intrusion with an overall funnel-shape geometry. This conclusion brings into question the presence of three individual foci of activity in Ardnamurchan, purported to have shifted throughout the development of the complex. It also has significant implications for the status and structural evolution of other igneous complexes of the British Paleocene igneous province, which contain layered mafic intrusions currently regarded as ring-dikes.

Stratigraphic and structural evolution of the Tertiary Cosmelli Basin and its relationship to the Chile triple junction

At the Taitao Peninsula, in southern Chile (46.5°S), an active mid-ocean ridge is being subducted under the South American continent. Continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid-Miocene. A series of Mid-Late Tertiary sedimentary basins lies inboard of the triple junction: the Cosmelli basin lies in the region which has experienced the earlier ridge subduction events. Within the Cosmelli basin fill, abrupt facies dislocations give clear evidence of marked base level changes. Four sequence boundaries are delineated, with an upward increase in degree of facies dislocation, which is believed to reflect increasing magnitudes of base level fall. The lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices, while the overlying, greater thickness of fluvial sediments is only gently tilted westwards. This geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited; these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life. This complex basin history, plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted.

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.

Evolution and geochemical signatures in a Neogene forearc evaporitic basin: the Salar Grande (Central Andes of Chile)

Abstract The Salar Grande is a 45-km-long (N–S axis), 4–5-km-wide, fault-bounded (pull-apart) Neogene forearc evaporitic basin, located in the Cordillera de la Costa of northern Chile whose sedimentary record is composed almost exclusively of a massive halite salt body. Underground waters coming from the east (now the Precordillera and Altiplano) were progressively enriched in solutes by interaction with volcanic rocks. These fluids drained into a lacustrine system located in the Depresion Central (in the Llamara–Quillagua area) and thick sequences of diatomites, carbonates, evaporites and clastic sediments were deposited. These lacustrine areas eventually dried out and became a salar with anhydrite and halite. During this stage (Late Miocene–Pliocene), evolved brines of the Llamara–Quillagua area migrated eastwards through structural paths, reaching the Salar Grande basin which acted as a final sink. A proof of this connection is the anhydrite level that links the western Llamara and southeastern Salar Grande areas. The accessory minerals and geochemical signatures in the rock salt are a record of the inputs arriving to the former lake and reflect variations in climate, and in tectonic and volcanic activity. The arrangement of halite textures along the sequence shows an evolution from a very shallow ephemeral saline lake (some cm–dm deep on average) to a salar where interstitial processes controlled by phreatic brines dominate. This evolution is coherent with a slight increase in the terrigenous content of the rock salt, and documents the increasing aridity of the climate as a response to the tectonic uplift of the Andean Ranges. In Pliocene times pore brines were definitively lost as a response to the downfall of the water table generated by the beginning of the exorheism due to the opening of the Loa River canyon towards the sea. The aim of this paper is to describe the Salar Grande through the study of its geological setting and its salt record. A detailed description of the salar will enable us to reach valuable conclusions about environmental conditions in the area during the Neogene, prior to the opening of the hydrologic system to the Pacific Ocean.

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 (

Trace Fossils and Paleoenvironments of a Late Silurian Marginal-Marine/Alluvial System: the Ringerike Group (Lower Old Red Sandstone), Oslo Region, Norway

Abstract The Late Silurian Ringerike Group of southern Norway is a lower Old Red Sandstone megasequence that marks the regressive culmination of Cambro–Silurian marine deposition in the Oslo Region. The basal Sundvollen Formation represents deposition in a number of sub-environments of a broad, muddy coastal-plain setting, and is succeeded by fluvial deposition of the Stubdal and Store Arøya Formations. The terminal formation of the Ringerike Group, the Holmestrand Formation, represents deposition in sub-environments of a sandy beach setting. Both the marginal-marine Sundvollen and Holmestrand formations contain a wide variety of trace fossils (Arenicolites, Cruziana, Didymaulichnus, Diplichnites gouldi, Diplocraterion, Gordia marina, Margaritichnus, “Merostomichnites,” Oniscoidichnus, Paleohelcura, Palmichnium stoermeri, ?Polarichnus garnierensis, Rusophycus, Siskemia bipediculus, Skolithos, Steinsfjordichnus brutoni, Taenidium) that are discussed and analyzed, comprising the first complete Late Silurian study from the Baltic area. The trace fossils, which occur in facies-controlled assemblages, are combined with sedimentologic evidence to perform a high-resolution paleoenvironmental analysis of the Ringerike Group. Multivariate cluster analysis of the bulk Ringerike ichnofauna with similar-aged ichnofaunas from other paleocontinents supports the hypothesis that localized environmental factors outweighed provincialism as the dominant control on the composition of arthropod-trackway-dominated trace-fossil assemblages during the Siluro–Devonian.

Climatic controls on alluvial-fan activity, Coastal Cordillera, northern Chile

Abstract A description of the distribution, drainage basin characteristics, surface morphology, depositional process and age of 64 alluvial fan systems from both flanks of the hyper-arid Coastal Cordillera of northern Chile between 22°15′S and 23°40′S is presented. The coastal fans on the western flank of the Coastal Cordillera are dominated by debris-flow deposits fed from steep catchments. Two drainage basin types are recognized: type A drainage basins are small (10–30 km2) and do not cut back beyond the main coastal watershed; and type B drainage basins are large (up to 400 km2) and cut inland beyond the coastal watershed. The western Central Depression fans on the eastern flank of the Coastal Cordillera are characterized by sheetflood deposition fed from relatively shallow catchments in small drainage basins (10–50 km2). The surface morphology, sedimentation rates, a luminescence date and regional cosmogenic radionucleide data suggest that these fans have been inactive for at least the last 230 000 years and probably for much of the Neogene. The principal control on fan activity in the study area is climate. The Coastal Cordillera forms an orographic barrier to recent El Niño-related precipitation events that are restricted to the western flank of the Coastal Cordillera. These events did not penetrate into the Central Depression as indicated by the inactive nature of the western Central Depression fans located 25 km east of the active coastal-fan catchments. This scenario is considered to have prevailed for much of the Neogene. Climate also controls rates of weathering on alluvial-fan surfaces. The coastal fog results in rapid salt weathering of clasts on coastal fans resulting in the production of fines, but does not penetrate into the Central Depression. Fault activity is important in controlling drainage basin size. The larger (type B) drainage basins are commonly focused on active faults that cut the coastal watershed, facilitating drainage basin expansion. Source-area lithology is not important in controlling depositional processes. Fans on both sides of the cordillera have the same basaltic andesite and granodiorite source lithologies, yet coastal fans are dominated by debris-flow and western Central Depression fans by sheetflood deposition. A combination of chemical weathering and stream power related to gradient are considered to account for the differences in process.