LINKING MéLANGE TYPES AND OCCURRENCES WITH ACTIVE MéLANGE-FORMING PROCESS IN TIMOR AND TAIWAN

Active arc-continent collisions provide a unique environment to investigate mélange development processes because mélange exposures can be traced along orogenic strike to active mélange forming settings. Tectonic, sedimentary and diapiric processes are commonly all found in these settings. However, the fundamental association of mélanges with plate boundary segment development indicates that they are primarily of tectonic origin, and connected in some way to a subduction channel. For example, in the active arc-continent collisions of Timor and Taiwan different types of mélange are found that connect directly to evolving plate boundary segments in the front, back, and slicing through the orogenic wedge that are ultimately tapping a subduction channel.

In the Timor region of the active Banda arc-continent collision mélange is found primarily along the base of the Banda Terrane, which is a large crystalline nappe of forearc basement emplaced over a duplex of mud-rich Australian continental margin rocks. The Banda Terrane formed the roof of the initial subduction channel. A well-known mélange comparison is the Oman mélange at the base of the Oman ophiolite. The Banda mélange is stratified with more subduction-related (serpentinitic and pelagic) compositions near the top and collision-related materials (dismembered thrust sheets of continental margin material) at its base. The high degree of mixing, of blocks of a wide variety of types, its structural setting and connection with active mélange forming processes documents its primary origin as part of the original subduction channel.

Two other settings of mélange development are notable in the Banda arc-continent collision. On Savu Island and along the north coast of Timor mélange associated with retrowedge development is well exposed and includes young volcanic arc material not found in mélange at the base of the Banda Terrane. Also, mélange extruded by a line of large mud diapirs marks the boundary of a major wrench fault slicing through the orogenic wedge west of Timor and is exhumed on the islands of Rote and Semau. Both of these settings have smaller blocks, but display similar high degrees of block dispersion and mixing indicating that ultimately these conduits for mélange mobility are connected to the subduction/collision channel.

Taiwan provides two similar contrasting examples in the Kenting and Lichi mélanges, which differ in their structural setting, but are fundamentally similar in composition and structure. The Kenting Mélange developed within the frontal accretionary prism of the Hengchun Peninsula and separates two different accretionary packages along a major shear zone that was one time part of the subduction channel before it was clogged by shelf facies continental margin material. It contains matrix and blocks of mostly chaotically-sheared late Miocene continent-derived clastic successions, seamount volcanics and oceanic crust-mantle materials. Like the Banda mélange, the Kenting mélange was generated by decollement propagation as part of a clogged subduction channel, but materials derived from the South China Sea floor differ from typical Tethyan-type mélanges like the Banda mélange.

In comparison, the Lichi Mélange represents chaotically-sheared forearc material associated with development of the retrowedge of the Taiwan collision. Like the mélange associated with the Savu Thrust system in the Banda Arc, it consists of chaotic forearc sediments and some accreted continental margin units with a variety blocks including andesite and tuff from the Luzon arc, sepentinite blocks, gabbro and glassy basalt of SSZ origin. The Lichi Mélange records a two-stage development: an initial arc-continent collision stage (~3 Ma) where the Lichi Mélange developed within 3.5-3.7 Ma forearc strata that were back-thrust as part of the retrowedge; and a second stage of mélange development that occurred during advanced arc accretion (last 1.5 Ma) as the forearc ridge and basin is thrust westward over the Lichi Mélange along a low-angle decollement.

Ultramafic rocks and gabbro are found in each of these mélanges and are most likely derived from the upper plate deep in the subduction channel and made their way to the surface through return flow (Poiseuille flow) via various shear zone conduits. It is highly unlikely that the harzburgite blocks and SSZ volcanics are derived from the mantle or MORB-type crust of the lower plate. Poiseuille flow in the subduction channel would systematically sample and incorporate material from the upper part of the lower plate (seamount volcanics, pillow basalt and overlying sedimentary cover of MORB and OIB composition) and the lower part of the upper plate (serpentine, gabbro, and ultramafic rocks). These types of relations are characteristic of most mélanges.