Chalid Idham Abdullah

The evolution of Sumba Island (Indonesia) revisited in the light of new data on the geochronology and geochemistry of the magmatic rocks

Abstract The island of Sumba, presently located in the southern row of islands of the Eastern Nusa Tenggara province of Eastern Indonesia, has a unique position, being part of the Sunda-Banda magmatic arc and subduction system. It represents a continental crustal fragment located at the boundary between the Sunda oceanic subduction system and the Australian arc–continent collision system, separating the Savu Basin from the Lombok Basin. New data on magmatic rocks collected from Sumba are presented in this paper, including bulk rock major and trace element chemistry, petrography and whole rock and mineral 40K–40Ar ages. Three distinct calc–alkaline magmatic episodes have been recorded during Cretaceous–Paleogene, all of them characterized by similar rock assemblages (i.e. pyroclastic rocks, basaltic–andesitic lava flows and granodioritic intrusions). They are: (i) the Santonian–Campanian episode (86–77 Ma) represented by volcanic and plutonic rock exposures in the Masu Complex in Eastern Sumba; (ii) the Maastrichtian–Thanetian episode (71–56 Ma) represented by the volcanic and plutonic units of Sendikari Bay, Tengairi Bay and the Tanadaro Complex in Central Sumba; and (iii) the Lutetian–Rupelian episode (42–31 Ma) of which the products are exposed at Lamboya and Jawila in the western part of Sumba. No Neogene magmatic activity has been recorded.

Magmatism in western Indonesia, the trapping of the Sumba Block and the gateways to the east of Sundaland

Abstract The western Sulawesi magmatic belt and the Sunda–Banda arc define the eastern and south-eastern margins of Sundaland, which is part of the relatively stable Eurasian plate. The eastern margin is bounded by the Makassar Strait which separates western Sulawesi from Kalimantan. The initial opening of the Makassar Strait took place in the late Cretaceous–Early Tertiary leading to the opening of the Pacific–Indian Ocean gateway during Neogene time. Recent studies indicate similarities in the late Cretaceous–Paleogene stratigraphic sequence and calc-alkali magmatism between Sumba, south Sulawesi and south-east Kalimantan, suggesting a Sundaland origin for all of these areas. The southward migration of Sumba to the present frontal arc position of the Sunda–Banda arc has occurred since Late Cretaceous–Paleocene time.

Crustal Structure Along Sunda-Banda Arc Transition Zone from Teleseismic Receiver Functions

We analyzed receiver function of teleseismic events recorded at twelve Indonesian-GEOFON (IA-GE) broadband stations using nonlinear Neighbourhood Algorithm (NA) inversion and H-k stacking methods to estimate crustal thickness, Vp/Vs ratios and S-wave velocity structure along Sunda-Banda arc transition zone. We observed crustal thickness of 34–37 km in Timor Island, which is consistent with the previous works. The thick crust (> 30 km) is also found beneath Sumba and Flores Islands, which might be related to the arc-continent collision causing the thickened crust. In Timor and Sumba Islands, we observed high Vp/Vs ratio (> 1.84) with low velocity zone that might be associated with the presence of mafic and ultramafic materials and fluid filled fracture zone. The high Vp/Vs ratio observed at Sumbawa and Flores volcanic Islands might be an indication of partial melt related to the upwelling of hot asthenosphere material through the subducted slab.

Study on 2-D Crustal Shear Wave Splitting Tomography along The Sunda-Banda Arc Transition Zone

The Sunda-Banda Arc transition zone is an active region characterized by a change in tectonic regime from subduction of Indo-Australia oceanic lithosphere along the eastern part of Sunda Arc to collision of the Australian continental crust with islands arc in the western part of the Banda Arc. This complicated tectonic setting causes this area is an ideal place to study the crustal deformation along the plate boundary. The density contrast between the Australian continental crust and Indo-Australia oceanic crust in the transition zone may cause large stresses around the boundary between them. These plate boundary forces may control the distribution pattern of the deformation in the subduction to collision transition zone. The geometry of this deformation can be investigated using shear wave splitting (seismic anisotropy) study. We conduct shear wave splitting measurements from local earthquakes recorded at 17 broadband seismic stations around the Sunda-Banda arc transition zone. The 2D delay time tomography is then applied to determine the first order approximation of lateral varying anisotropic layers due to the local effect of geological structures. We observe strong anisotropy regions which coincide with the geological features as possible causes of anisotropy in the Sunda-Banda Arc transition zone. For instance, the high anisotropy zone found in Timor Island can be related to the alignment of metamorphic and igneous rocks, whereas the high anisotropy area around Sumba Island might correspond to the interaction of Sumba basement with the Australian margin increasing the frictional strength at the plate boundary.