User:Lgkirst/Geology of the Antarctic Peninsula

The Antarctic Peninsula, roughly 650 miles south of South America is the most northerly portion of the continent of Antarctica. Essentially a continuation of the Andes Mountains subduction zone, the Antarctic Peninsula exhibits textbook subduction zone tectonic activity. The peninsula has experienced subduction for over 200 million years . The geologic and tectonic history of the peninsula spans millions of years. Throughout this span of time the plate configurations that essentially formed the Antarctic Peninsula shifted. These shifts changed the orientation of the peninsula itself, as well as the underlying volcanics associated with the subduction zone.

=Tectonic Evolution and Geology of the Antarctic Peninsula=

The geology of the Antarctic Peninsula has occurred within three different stages:
 * 1) Pre-subduction stage of marginal basin deposition, later seperated by the Gondwanian orogeny,Permian-Late Triassic
 * 2) The middle subduction phase characterized by the formation of the Antarctic Peninsula(inner) and South Shetland Island(outer) magmatic arcs, middle Jurassic-Miocene.
 * 3) The late subduction phase where the opening of the Bransfield Rift and back-arc basins occur. This is followed by contemporaneous terrestrial and submarine volcanic activity, Oligocene-present day.

Pre-Subduction History


The oldest rocks found on the Antarctic Peninsula are from this time. They are sedimentary rocks from the Trinity Peninsula Group(TPG). Mostly composed of siliciclastic turbidite deposits ~1200-3000m thick deposited in a marginal marine basin. Unfortunately their age is poorly known, but are most likely from the upper Permian and Triassic. The clastic material used to create these sediments was provided by the weathering, erosion, and subsequent transportation of Gondwanaland to the northeast. Source rocks assemblage was most likely a mixture of metamorphic, igneous, and sedimentary complexes.

Gondwanian Orogeny
At this time the TPG sediments were folded and slightly metamorphosed, particularly at the peninsula's northernmost point. Retroarc thrusting was also occurring at that time. Both events were most likely caused by incipient stage subduction of the south-east Pacific Plate under the Gondwana supercontinent. As a result, marginal basin clastics were scraped from their oceanic crustal base and subsequently antithetic piling-up of the TPG sediments occured. These were then thrusted over the continental margin of Gondwanaland. It is believed that the TPG rocks were uprooted from their original oceanic-type basement and are allochthonous with respect to underlying crystalline basement rock of the Antarctic Peninsula.

Inner Magmatic Arc
The creation of this magmatic arc was first, making it the older of the two. This arc comprises the mainland of the Antarctic Peninsula and has the higher topographic relief. The creation of the inner magmatic arc is characterized by terrestrial clastic deposition and the early stages of acidic volcanism and plutonism. The Mesozoic clastic sequence(Number 2-Figure 2) is comprised of the Mount Flora Formation(MFF). Roughly 270 meters thick, these sediments are predominantly plant-bearing coarse sedimentary breccias and conglomerates, with a limited abundance of interbedded sandstones and shales. These clastic beds overlay the TPG sediments, and are separated by angular unconformities. Overlying the MFF clastic sequence are the acid volcanics of the Kenny Glacier Formation(KGF). This volcanic sequence consists of rhyolite-dacite lavas, ignimbrites, tuffs, and agglomerates which are all together roughly 215 meters thick. The acidic dykes and sills which intrude the MFF and TPG sediments may have been attributed to the KGF stratovolcano. The acidic volcanism that created the KGF sequence is caused by plutonic intrusions during the Middle Jurassic-Early Cretaceous in the northern Antarctic Peninsula. . These plutonic intrusions could be attributed to the up-doming and rifting in the continental margin of Gondwanaland at the onset of oceanic slab subduction. . This subduction zone is described by Pivoting Subduction, which is due to differences in slab-pull forces acting on the system.

Outer Magmatic Arc
An apparent westward migration of the inner magmatic arc, the outer magmatic arc volcanic events created what we now call the South Shetland Islands. Similar to the inner magmatic arc, the outer is composed of typical subduction-related acidic volcanism. The outer magmatic arc, like the inner mainland arc is composed acidic volcanics. A study focused on the conditions for the generation of andesitic lavas on Alexander Island postulate that either the development of a slab-window due to the subduction of a spreading ridge or the breakup of the subducted slab beneath the fore-arc basin could be the source for the andesitic lava. The South Shetland Islands are dissected by two systems of younger and older strike-slip faults. The older system, parallel with the island arc, was active on King George Island during most of the Tertiary. This older system faults is characterized by right-lateral faults caused by the counterclockwise rotation of the Antarctic Continent with respect to the subduction zone. The younger system of faults displaced the older system, which were also a series strike-slip faults being transverse to the island arc.

Late Subduction Phase, opening of the Bransfield Rift
The late and most recent stage in the evolution of the Antarctic Peninsula subduction zone was the opening of the Bransfield Rift . The Bransfield Rift undergoes extensional tectonic forces forming a passive continental margin. . This rifting subsequently created the Bransfield back-arc basin during the Oligocene to present day . This basin separates the inner magmatic arc(mainland Antarctic Peninsula) from the outer magmatic arc(Shetland Islands). Submarine and terrestrial volcanic activity of alkaline and tholeiitic compositions is associated with this rifting event. The trenchward migration of the spreading center is attributed by the subduction of the Phoenix Plate under the Antarctic Plate . Slab rollback and South Shetland Trench oceanward retreat have led to extensional forces acting on the leading edge of the overriding plate . The Bransfield Strait is a result of the extension and is presumed to be less than or equal to 4 million years old . Magnetic anomalies created by the upwelling and formation of new crust are aligned along the axis of the Bransfield Rift. Models of these anomalies indicate that the newly formed oceanic crust i the Bransfield Strait is rought 1.3 million years old. Unfortunately due to the blanketing by newly formed sediments and extensive intrusions into the rift render modeling unreliable. Basalts dominate the submarine volcanics in the Bransfield Rift. Isolated occurrences of terrestrial volcanic activity is present, which are predominantly of alkaline to tholeiitic compositions.

=Relevant Links= Antarctic Peninsula

Subduction

Plate Tectonics

Andes Mountains

Rift

Fault (geology)

=References=