GEOLOGICAL SOCIETY OF THE PHILIPPINES
| DEEP STRUCTURAL CONTROLS ON THE GEOLOGY OF SABAH FROM GRAVITY MEASUREMENTS |
John Milsom and Robert Holt
Dzazali bin Ayub
The Malaysian state of Sabah, which occupies the northernmost part of the island Borneo, is geologically complex. In the west the metamorphic Crocker Ranges culminate in the granitic mass of Mt Kinabalu, the highest mountain in southeast Asia. In the centre and east two belts of ophiolitic rocks, one trending N-S, the other E-W, have uncertain relationships to Miocene sedimentary basins which have attracted attention because of their characteristic, but unusual, circular to ellipsoidal shapes in plan. The nature of the basement of Sabah is unknown but has often been considered to be oceanic.
Understanding of the structural controls on the geology of Sabah has been impeded by the still thick rain forest cover which has made mapping difficult to impossible in many areas. Geophysical data are also generally sparse, but a regional gravity survey of the entire area has now been completed. Deep Bouguer gravity lows are associated with the isostatic compensation of the Crocker Ranges and with the Miocene basins, and gravity highs mark the locations of the ophiolite belts. It is clear that there is currently no linkage between the two belts and the gravity patterns place severe constraints on the possibilities of past connections. The eastern ophiolite is interpreted as an onshore continuation of the Sulu arc, and in Sabah produces a gravity high which is virtually co-extensive with the broad Darvel Bay inlet. This prominent physiographic depression clearly owes its existence to subsidence, which local seismicity suggests is still continuing, driven by the excess mass of the high density rocks.
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