Geodynamic Significance of Regional Mélange Units at Divergent and Convergent Plate Margins Case Studies from the Scandinavian Caledonides and the North American Cordillera

Abstract

In plate tectonics scientists study the present and past motions of the lithospheric plates over the Earth’s surface. The reconstruction of the location and orientation of the continents and oceans in deep time is referred to as paleogeography. Paleogeographic reconstruction allow to test, learn about, and understand the long-living (many millions of years) processes that drive plate tectonics and that we only can observe snapshots of today. When continents break apart or collide most destruction and formation of rocks is taking place at the plate boundaries. It is along these plate boundaries where large-scale mixed and complex geological units, referred to as mélanges, are formed and witness important tectonic processes, which rarely can be studied by direct observation. J. Jakob studied the vestiges of two poorly-understood ancient mélanges: one in the Canadian Pacific Northwest on Southern Vancouver Island and another one in South Norway that stretches from Bergen to Otta. The mélange in South Norway, between Bergen and Otta, was initially formed during the breakup of a continent, probably as early as 610 Ma ago. The rocks of the mélange are strongly affected by a later continent-continent collision 420 Ma ago that gave rise to a since then long-vanished Himalayan-sized mountain chain, the Scandinavian Caledonides. The rocks on Vancouver Island recorded the prolonged tectonic activity and complex deformation of the rocks since their deposition at ca. 103 Ma and their final amalgamation to the North American Continent at 51 Ma. The results challenge some the long-standing plate tectonic models that were developed to explain the crustal architecture of the Pacific Northwest as well as of the Norwegian Caledonides.