Structure of the Lengguru fold-and-thrust belt, New Guinea Island: Consequence of rapid kinematic changes

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Holding Date 03 September 2008

Eastern Indonesia is a complex arrangement of microblocs jammed between Sundaland, Australia (AU) and the Pacific Ocean (PAC). The very oblique convergence between PAC and AU is partitioned and thus resulted in both the formation of strike-slip faults and mountain ranges (Central Range, CR and Lengguru Belt, LFTB). Surprisingly, geologic data show that shortening has ceased in both mountain ranges, geodetic data indicate pure wrenching along the CR and seismic/radar data show that the LFTB is cut by a major left-lateral strike-slip fault.
The LFTB is a 250km long, 150km wide NW-SE trending arcuate belt composed of Mesozoic sediments of the Australian Margin, Paleogene platform limestone and Neogene clastics stacked against a metamorphic buttress. We can distinguish:
1) a typical external zone, extending offshore. It is composed of in sequence long-wavelength folds (most of them thrusted) detached at 6-7km within Triassic and armed by the Paleogene Lst. Folding is Miocene to Pliocene, as recorded by syntectonic clastic deposits in few piggy-back basins onshore and in the foreland.
2) further east, the internal zone composed of shorter wavelength folds and thrust sheets over a shallower decollement at 4-5km in Mesozoic shales. They affect Cretaceous and Paleocene clastics and the deeper equivalents of the Paleogene carbonate platform. Metamorphism increases eastward (as observed on the field and analysed through Raman spectroscopy).
3) then ultimately, the high grade N-S elongated Wandamen Metamorphic Core (WMC, up to 2000m) between two opposite detachment faults, showing retromorphosed eclogites, amphibolites and metagrauwacke. A syn-exhumation stretching fabric of N-S lineations are present in the core, whereas high T accounted for migmatisation in places.
The main morpho-structures of the Belt were controlled by a NE-SW compression during Late Miocene to Early Pliocene against an ophiolitic or arc backstop. Thin-skinned thrusting of various thicknesses of Mesozoic and Tertiary sediments over a previously structured basement was followed by thick-skinned thrusting. The Late Pliocene-Quaternary deformation (still active seismically) is extensive with the exhumation of the WMC in the internal zone and with NE-SW collapses along high-angle normal faults (e.g. Triton Bay) cross-cutting the folds in the external zone.
The structure of the LFTB therefore results from events occurring over a very short time span; a NE-SW compression from Late Miocene to Early Pliocene and a Late Pliocene-Quaternary global extension in the whole range. This evolution of the belt can be linked to rapid changes in the accommodation of the AU and PAC oblique convergence.