Aspects of a suture zone - the Mariner Glacier Area, Antarctica

This volume contains the results of joint work of German and Italian geologists in North Victoria Land, Antarctica, carried out during several expeditions. The Mariner Glacier area is part of the Early Paleozoic Ross orogen which is exposed in the present Transantarctic Mountains over a length of at least 3000 km. In this particular area, a Paleozoic continent/ocean suture is exposed providing evidence for the plate tectonic setting at the Gondwana margin in the Ordovician. It is common understanding that there exist two major types of orogens: one formed by continent/continent collision after the closure of an ocean at the end of a Wilson cycle; the other one formed by subduction accretion at an active continental margin. There are many examples for the first type like the Alps, Himalayas or Caledonides, but usually but one example for the second type, the Cordilleras of America. Therefore it is of great interest that the Paleozoic Ross Orogen has many features in common with the Cenozoic Andes, i. e. a well developed magmatic arc as well as relics of an adjacent oceanic plate and the lack of a second continental plate. The suture in North Victoria Land separates an inboard granitic arc, hosted in low-pressure, high-temperature metamorphic rocks of the cratonic Wilson „Terrane“, from two outboard low-grade terranes of mainly oceanic character (Bowers and Robertson Bay Terranes). Intrusive and metamorphic ages in the Wilson „Terrane“ are at of around 500 Ma, the Bowers and Robertson Bay Terranes contain Cambrian to early Ordovician fossils. The suture is characterized by * a small belt of medium-pressure metamorphic rocks and high pressure eclogites; * the amputation of the continental margin as demonstrated by partly sheared arc granites now close to the suture; * a system of thrusts carrying the three major tectonic units outwards, the inboard units on top of the outboard ones; * lenses of ultramafic rocks in the form of cumulates, layered gabbros, serpentinites and eclogites lined up along the main suture fault. All these features provide evidence for a cratonward-dipping subduction system. The study of this suture zone has provided evidence that an ocean/continent boundary existed along the Paleozoic active margin of Gondwana similar to the present-day one along the Andes. If one considers other orogens of intermediate age along this active margin of Gondwana and the later continental fragments of Australia, Antarctica, and South America it appears that there is evidence for a continuous, though episodic, subduction from the early Paleozoic to the present, with shifting positions along the margin. The onshore geology thus indicates the persistence of a Paleo-Pacific ocean adjacent to Gondwana far beyond the time directly derived from the marine magnetic anomalies.