Structural evolution of arc-continent collisions: what can we learn from geology and numerical modeling ?

We present 2D numerical thermo-mechanical experiments of a subduction leading to the collision of a volcanic arc with (1) a magma-poor hyperextended margin, (2) a highly volcanic margin, (3) a hyperextended margin that has experienced various phases of magmatism. In recent years, data on passive margins show that on any continental margin the structure and composition of the continent ocean transition (COT) varies greatly. The COT at magma-poor margins is characterized < 120 km zone of hyperextended crust ( 250 km wide COTs with ~15 km thick continental crust underplated by high velocity bodies interpreted as magmatic. We explore the effect of the structure and the strength of the continental margin and the intensity and localization of erosion on mountain building on the dynamic evolution of arc-continent collisions. Observations and the result of modeling experiments lead us to define, (1) the “orogenic wedge” as an accretionary wedge that includes a sedimentary pile and an accreted mixed crustal/sedimentary wedge resulting from the underplating of the thinned continental margin to the core of the mountain belt, (2) the “thrust belt” as the structures resulting from the collision of the arc and accretionary wedges to the thick continental crust of the margin, (3) “arc collapse” as the shortening process of the arc and forearc complex into a strong backstop. In the numerical experiments we distinguish the following phases of deformation that may or may not coexist during the evolution of the collision. (1) Arc obduction and sedimentary wedge accretion, (2) Arc collision and orogenic wedge accretion, (3) Arc collapse and thrust belt formation, (4) Arc collapse and slab delamination that leads to subduction reversal. At first order the strength of the OCT controls the evolution of the orogenic wedge. When the material accreting in the wedge is weak (quartzite + sediments) the subduction interface dip is shallow and the wedge is wide. When the material is strong (anorthite + sediments) the dip is steep and the wedge is narrow. Erosion and sedimentation allow for the exhumation of material from greater depth in the orogenic wedge. We find that there exist of wide range of behavior for arc-continent collisions depending on the nature of the continental margin involved in the collision. In the classical case of Taiwan, the preferred evolution is that of the collision of an arc and a wide magmatic underplated margin: (1) Initiation by arc obduction and sedimentary accretion, (2) orogenic wedge accretion by crustal underplating, (3) arc collapse and initiation of the thrust belt, (4) thrust belt formation after the final collapse of the arc. The final structure is that of a steep collapsed arc structure playing the role of a backstop, a thick orogenic wedge exhuming the deeper crust of the continental margin, a fold and thrust belt structure initiated in the thick crust of the continental margin and the sedimentary accretionary wedge.