Age, origin and tectonic controls on rapid Pleistocene exhumation of the Sibela Mountains, Bacan, Indonesia

The Sibela Mountains of Bacan island in eastern Indonesia contain one of the Earth’s youngest metamorphic complexes, now exposed at elevations up to 2000 m. Exhumed basement consists of Permo-Triassic (c. 249-257 Ma) granitoids and metamorphic rocks. Mica ⁴⁰Ar/³⁹Ar and apatite (U-Th-Sm)/He data from these rocks indicate that they were rapidly exhumed in the Pleistocene (c. 0.7 Ma) accompanied by partial melting. The rapid exhumation observed on land was associated with significant subsidence in adjacent basins offshore that reach depths up to 2.4 km. Neogene metamorphic core complexes and other metamorphic complexes are well-known from eastern Indonesia, and they usually record much higher exhumation rates than those reported from older classic metamorphic core complexes found in other parts of the world and require a different formation mechanism. Unlike classic metamorphic core complexes that are characterized by low-angle detachment faults, the Bacan metamorphic rocks were exhumed on steep bounding normal faults forming a rectilinear block pattern. A similar exhumation mechanism can be observed on the island of Sulawesi. We suggest such complexes be termed metamorphic block complexes (MBC). The Bacan MBC is exceptionally young and like the other east Indonesian complexes was rapidly exhumed during subduction rollback.