Animated tectonic reconstruction of the Southern Pacific and alkaline volcanism at its convergent margins since Eocene times

Abstract

An animated reconstruction shows South Pacific plate kinematics, in the reference frame of West Antarctica, between 55 Ma and the presentday. The ocean floor in the region formed due to seafloor spreading between the Antarctic, Pacific, Phoenix and Nazca plates (a plate formed by fragmentation of the Farallon plate early in Oligocene times). The Pacific-Antarctic Ridge remained fairly stable throughout this time, migrating relatively northwestwards, by various mechanisms, behind the rapidly-moving Pacific plate. The Nazca and Phoenix plates also moved quickly, but relatively towards the east or southeast, and were subducted in these directions beneath the South American and Antarctic plates. Segments of spreading centres forming at the trailing edges of the Nazca and Phoenix plates periodically collided with these subduction zones, resulting in the total destruction of the Nazca-Phoenix spreading centre and the partial destruction of the Nazca-Antarctica spreading centre (the Chile Ridge) and Antarctic-Phoenix Ridge, which ceased to operate shortly before its northeasternmost three segments could collide with the Antarctic margin. Following collision of segments of the Chile Ridge, parts of the Antarctic plate underwent subduction at the Chile Trench. After these collisions, slab windows should have formed beneath both the South American and Antarctic convergent margins, and the animation shows occurrences of alkaline volcanism that have been, or can newly be, related to them. Further occurrences of alkali basalts, at the margins of the Powell Basin and, more speculatively, James Ross Island, can be related to the formation of a slab window beneath them following the collision of segments of the South America-Antarctica spreading centre in the northwest Weddell Sea.

Tectonic reconstructions for paleobathymetry in Drake Passage

“…The oldest, at 49-48 Ma, suggests that the basin may have started to open by continental extension in Eocene times. The youngest have been interpreted in terms of a slab window opening beneath the basin following ridge-crest-trench collisions in the northwest Weddell Sea (Eagles et al, 2009a). Prominent high-amplitude normal-polarity magnetic anomalies traverse the South Orkney Microcontinent and opposing Antarctic Peninsula from east to west.…”

“…Shag Rocks bank thus remained juxtaposed with South Georgia and, initially, with northern Davis Bank across the transform fault. This time may also have seen subduction of parts of a Phoenix-Nazca ridge crest south of Cape Horn, opening a slab window that has been related to the occurrence of alkali basalt volcanism at Packsaddle Island (Breitsprecher and Thorkelsen, 2009;Eagles et al, 2009a;Puig et al, 1984). By removing an along-ridge larval dispersal route between the Pacific Ocean and Scotia Sea, such a collision may also have contributed to the 25.9-13.4 Ma divergence in Kiwaid crab biogeography, which is otherwise attributed to the onset of the Antarctic Circumpolar Current (Roterman et al, 2013).…”

Structure and evolution of Cenozoic arc magmatism on the Antarctic Peninsula: a high resolution aeromagnetic perspective

“…Evidence that slab windows formed along the AP comes from geochemical analysis of back arc volcanoes and intrusions (Hole & Larter 1993;McCarron & Smellie 1998) and plate modelling (Eagles et al 2009a). Additionally, low-temperature thermochronology reveals uplift and exhumation at 10-15 Ma linked with development of a slab-window beneath the Adelaide Island/Bisco Island region (Guenthner et al 2010).…”

Depósitos burdigalianos de la Formación Santa Cruz en Sierra Baguales, Cuenca Austral (Magallanes): Edad, ambiente de deposición y vertebrados fósiles.

“…Additionally, this interpretation is supported by the synchronous acceleration of exhumation rates across the Patagonian Andes (Thomson et al, 2001), estimated to have occurred between 17-14.24 Ma by Blisniuk et al (2006), and between 22-18 Ma by Fosdick et al (2013) in the retro-foreland region. The dynamic Oligocene-Miocene tectonic configuration of the Patagonian Andes generated by: 1. changes in the subduction velocity, convergence rate, and obliquity of the Nazca and South American plates (Pardo-Casas and Molnar, 1987); and 2. an increase in compression and shortening related to the collision of an unstable oceanic spreading center (quadruple junction) at 18-17 Ma (Breitsprecher and Thorkelson, 2009;Eagles et al, 2009), followed by the subduction and northward migration of the Chile Ridge (Cande and Leslie, 1986;Lagabrielle et al, 2004), seem to be the primary factors triggering the uplift, concomitant widening and foreland propagation of the fold-and-thrust belt during Early Miocene times (Ramos and Ghiglione, 2008). The resulting effects of these remarkable geological processes on the past configuration of the ecosystems and biogeographic patterns of the austral biota are the subjects of our present and future investigations in Aysén and Magallanes.…”