U-SERIES ISOTOPE MEASUREMENTS FOR UPLIFTED CORAL TERRACES FROM TIMOR-LESTE—IMPLICATIONS FOR ACTIVE FOLDING

Islands of the Timor region expose a detailed record of recent uplift associated with its transition from subduction (localized strain) to arc-continent collision (distributed strain). Extensive flights of coral terraces (>25 terraces to elevations of >600 m) document rapid recent uplift of mostly the collisional retrowedge. Variation in coral terrace development and stream asymmetry demonstrate differential tilting of several islands. We tested rival hypotheses for the cause and patterns of uplift by detailed GPS surveys and extensive U/Th age analyses.

We interpret variation of coral terrace ages along the 180km coastline of East Timor as a function of active growth of folds with wavelengths of several km, which creates geomorphic promontories. The numerical increase and warping of the terraces to the east and tilt to the southwest provides evidence for very young and relatively shallow deformation vs. lithospheric scale processes(i.e. slab detachment and delamination).

U-series age determinations for fossil corals along the northern coast of Timor-Leste are problematic in places due to excess 234 U, measured from unaltered clean samples (98 – 100% aragonite with low 232 Th concentrations). The excess uranium indicates open-system behavior where hydro-chemical influx and outflow have influenced the skeletal surface chemistry without altering the carbonate crystalline structure. New sample preparation methods involving careful petrographic evaluation and microdrilling of samples were used to obtain more reliable ages. We also applied a new numerical method for calculating model ages for coral samples with excess 234U.