The limits of 3 distinct upper Palaeozoic to lower Mesozoic tectono-stratigraphic zones of the ancestral Pacific margin of Gondwana in southern South America are documented. These are: a fore-arc region consisting in part of ‘oceanic’ components; a magmatic arc that is coincident with Carboniferous to Triassic continental sedimentation; and a back-arc region comprising the upper Palaeozoic epicratonic sequences of the ‘Samfrau Geosyncline’. These zones, traceable 2300 km from 29–56 ° S (Cape Horn), document the semi-continuous subduction of the ancestral Pacific floor from the Middle Devonian to the Triassic. Between 29 ° S and 37 ° S the palaeo-subduction zone was roughly coincident with the present Pacific coast line. Between 37 ° S and Cape Horn, the subduction zone migrated progressively westward as an ‘Alaskan’ style accretionary prism grew to a width of 250 km. While in the northern sector, Jurassic are intrusions are coincident with the upper Palaeozoic are intrusions, in the S they intruded the upper Palaeozoic to lower Mesozoic accretionary prism 200 km W of their Palaeozoic equivalents. No post middle Palaeozoic ‘sutures’ have been identified in southern South America. Hence, there is no evidence of late Palaeozoic, Mesozoic, or Cenozoic accretion of discrete microplates.
The Cenozoic magmatic and tectonic histories of the southern Andes adjacent to the Chile Triple Junction (CTJ) reflect the evolution and migration of the triple junction during the last 20 m.y. in a number of fundamental ways. Pliocene near-trench magmatism, including the intrusion of calcalkaline stocks within the forearc basement and the creation of the Taitao Ophiolite within the leading edge of the forearc, relates directly to ridge subduction processes. Distribution and kinematics of faulting, observed geologically and through earthquake seismology, reflect the plate margin kinematics and are also strongly influenced by the thermal, topographic, and kinematic heterogeneities related to the Chile Triple Junction. The most important structure in this respect is the 1000-km-long, trench-parallel, Liquifie Ofqui Fault (LOF). This extends north from the CTJ and has a predominantly dextral displacement. Complex faulting and related subsidence within the Golfo de Penas basin have continued throughout the late Cenozoic and appear to define a " pull-apart" basin within the South American forearc at the southern trailing edge of the LOF. The history of the Cenozoic uplift of the Andean divide appears to be dramatically different north and south of the triple junction. The backarc fold-and-thrust belt of southernmost Patagonia appears to end at the present latitude of the triple junction. Passage of slab windows corresponding to ridge segments subducted during the late Cenozoic corresponds spatially with arc gaps and centers of backarc alkalic flood basalts.
Ocean Drilling Program Leg 141 drilling recovered an extensive suite of Pliocene to Pleistocene forearc basin deposits within the Chile margin near latitude 46°S, in the vicinity of the Chile spreading ridge-trench collision. The outer margin setting is dominated by terrigenous siliciclastic sediment input from the Andean volcanic arc, Paleozoic to Mesozoic crustal sources, and the forearc, in slightly varying proportions. The overall controls on sedimentation are complex; of major importance are fluctuations in glaciation, sea-level changes, volcanism, and tectonism (i.e., thrust faulting and uplift due to subduction accretion or subsidence due to subduction erosion).The sediments encountered are predominantly structureless muds, massive to graded sand, sandstones, and some gravels and conglomerates that were deposited from slope failures, turbidity currents, and suspension processes associated with hemipelagic fallout in basin plain/trench to stacked slope apron environments. During the late Pliocene to Pleistocene, fluctuations of the ice sheet also influenced the outer margin sedimentation; during the glacial maxima in the Pliocene and early Pleistocene, larger quantities of terrigenous sediment were transported to the trench slope by turbidity currents and related gravity flows. Tectonically induced accretion led to overall shallowing-up successions and the formation of small slope basins with a slight coarsening-upward general character in the upper portions of proximal sites. Onshore, uplifted and eroded Paleozoic metasedimentary rocks and deeper crustal complexes shed sediments of dissected arc provenance into the forearc region. Forearc sediment composition indicates a waning of the arc volcanism during the Pliocene-Pleistocene and an emplacement of subaqueous near-trench volcanism during Pliocene along the Taitao Fracture Zone, owing to the progressive spreading-ridge subduction along the Chile margin. Basinal tectonism occurred in the form of ridge-subduction-related subsidence and associated tectonic erosion. The hydrothermal alteration of part of the Chilean accretionary wedge sediments was most likely created by high heat flow from the subducted spreading ridge.
Leg 141 of the Ocean Drilling Program recovered predominantly clays and silty clays from four sites drilled into the inner trench wall and slope in the vicinity of the Chile Margin Triple Junction. Shipboard analysis of the cores indicated significant variations in the physical and chemical properties but limited variation of grain size and mineralogy. Quantitative grain-size analysis has now been completed for a suite of nearly 600 samples obtained directly from the shipboard physical-properties sampling intervals. The analyses provide insights into facies within the finer-grained units deposited on the Chile margin slope as well as into the origin of the variability of the physical properties. The distinctive patterns of sorting and mean grain-size parameters found at each site suggest differential mixing of fine-grained facies. These clay and silty clay facies may include contourites, suspension fallout, distal turbidites, and nepheloid suspensates. Comparisons of downhole variations in the grain-size parameters with the porosity data suggest textural factors have partly controlled porosity loss during burial. The model developed for a tectonically imbricated section at Site 860 is consistent with the cyclic pattern evident in the grain-size data set. Differences seen between holes in the porosity loss curves probably are not caused by grain-size variations, but more likely the result of variations in lithologies on the scale of the depositional basin, as influenced by diagenesis and the tectonic setting.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.