The Nature of the Banda Arc–Continent Collision in the Timor Region

Arc continent collision is one of the most fundamental tectonic processes for the formation of new land, and the preservation on land of fragments of oceanic lithosphere (ophiolites). However, the tectonic evolution of arc continent collision is commonly over-simplified in everything from introductory textbooks to complex tectonic models. Major unresolved issues include the response of the forearc during collision, controlling factors of deep continental subduction, possible slab delamination and the role of structural inheritance. Many of these issues have been addressed by detailed studies of arc continent collisions in a variety of tectonic settings and stages of development. These studies reveal many common features, such as (1) arcuate orogens surrounding young, supra-subduction zone ocean basins, (2) high pressure metamorphism, (3) thrust sheets of forearc basement structurally overlying mélange in the hinterland of a continental fold and thrust belt, (4) reversal of sedimentation into a flexural trough, (5) contamination and modification of arc volcanism (6) arc accretion and suture zone development, and (7) uplift and exhumation of the orogenic wedge. These common features attest to similar syn-collisional processes associated with plate kinematics that controls the personality of arc continent collisions. I refer to these features as the tectonic nurture of the collision versus its tectonic nature, which is associated with pre-collisional or inherited features. For example, the arc continent collision of Taiwan displays only some of the features listed above. Upper plate nappes and high-pressure metamorphic terranes are notably lacking in Taiwan. This contrast is mostly attributable to the young nature of the Asian continental margin versus the old and cold nature of most continental margins colliding with arcs, such as the Tethyan continental margins and those involving the northern Australian margin. Does the increased positive buoyancy and reduced strength of warm continental margins resist subduction more than cold continental margins? Is a continental margin with high heat flow, as in Taiwan, more likely to thrust over the forearc versus under it to some extent as in most arc continent collisions? Deciphering the relative contribution of these inherited features versus plate kinematics in arc continent collisions is one of the most important aspects to reconstructing how continental crust is formed and shaped its nurture versus its nature. Active collisions are key to addressing these issues. They provide both the tectonic nature and nurture of the collision at a variety of temporal and spatial scales. If the continental margin is oblique to the plate boundary then it can be analyzed in the fourth dimension of time where various phases of collision are manifest along orogenic strike. Taiwan is the most intensely studied example, but the Banda arc continent collision of the Timor region also provides a classic example of an active, oblique arc continent collision that differs in some very important aspects from Taiwan. The plate kinematics or nurture of each is very similar, but they are very different in nature. The Timor region in many respects is more typical of arc continent collisions in general (Searle and Stevens 1984), yet it is not nearly as well constrained by geological research as Taiwan and because of this is used as a “one collision fits all” analog for almost any tectonic scenario. Only recently has improved R. Harris Brigham Young University, Provo, UT, USA e mail: [email protected]