Shortly after the recognition of plate tectonics, Wilson, (1966) proposed his now famous cycle describing the creation and demise of ocean basins. The original four stages which include; 1) creation of an ocean basin by breaking apart a continent, 2) the two continental fragments drift apart due to the creation of new ocean seafloor and formation of ocean basins, 3) progressive closure of ocean basins by subduction of oceanic lithosphere, and finally 4) continent-continent collision from final closure of an ocean basin.  These four steps still form a fundamental basis for plate-tectonic discussions.  Detailed studies of continent-continent collision as well as numerical and analogue modeling experiments have revealed that Wilson’s phase 3, initiation of oceanic subduction, frequently starts by inversion of a mid-oceanic ridge or fracture zone on the ocean floor, leading to subduction of an ocean plate under another ocean plate and development of a volcanic arc on the overriding plate. These mid-ocean subduction systems presently occupy ~40% of the total length of the Earth’s 55,000 km of convergent margins. As subduction of ocean crust continues, a continental margin will arrive at the subduction zone and try to subduct below an oceanic plate.  What happens next?  We teach students that it is very difficult to subduct continental crust due to its thickness and buoyancy and that often a new subduction zone will develop on the ocean side of the island arc.  How much continental crust subducts before this happens?   The Island of Timor, immediately north of Australia is a natural laboratory to test how much continental crust can subduct.  Here, the continent of Australia is attempting to subduct underneath the Indonesian volcanic island archipelago.   Global positioning measurements  (GPS) tell us that 30% of the plate motion is happening on the subduction zone between the island of Timor and Australia.  The remaining 70% of motion is occurring north of Timor, perhaps on a new subduction zone. 

To address this first order question of how much continental crust has subducted we collected map data, including rock type, identification and location of structures (faults and folds) and orientations of rocks and structures over a significant portion of east and west Timor. A major outcome of our research is a new geologic map of Timor-Leste with an emphasis on the map pattern geometry of deformed Australian margin rocks. We have shared the geologic map and our findings with the Secretary of State for Natural Resources in Timor Leste.  The map pattern of rocks and structures allow us to predict what a cross section, or vertical slice though the crust should look like.  These cross sections show how faults identified at the surface project into the crust.  By estimating the orientation of faults in the subsurface, we can predict the amount of displacement on each fault. Our study shows that the upper portion of the Australia margin exposed on Timor has shortened 326-362 km.   This requires that 199-229 km of the buoyant Australian continental margin has been subducted into the mantle below the volcanic arc.   To address the question of when did the volcanic arc collide with Australia and initiate the subduction of Australian crust we used a series of minerals to determine the age and temperature that the rocks cooled.  The data show that significant portions of the island were rapidly cooling at 4-5 Ma indicating that the island was emergent at this time and actively eroding.  Very young cooling ages of 1-2 Ma indicate regions of the island that are actively being uplifted and eroding now.   Active uplift and erosion is also indicated by stream profiles that show active landslides, drainage capture and river reorganization.   Map...

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