Magma type and tectonic position of the Una-Una Island, Indonesia

Abstract

On April 29, 1960, 19h. 32m. 13s. G.M.T. an earthquake took place originating SE of the Una-Una volcano, Celebes. In July 1960 an expedition visited the island to record the continuous aftershocks, its volcanology, and petrology. Due to the lack of a vertical component in the seismograph the exact direction of the epicenter could not be determined with certainty, but correlation between the seismograms in Djakarta and those recorded in the island, reveals two values for the epicenter, viz. 00° 21′ S - 121° 38′ E and 00° 15′ S -121° 40′ E with SE-NW direction. The absence of pP waves on the seismograms of Djakarta, Lembang and Medan stations suggests that these are shallow earthquakes. The survey for a new topographic map of the island of Una-Una reveals nine solfatara fields, one fumarole and six secondary, probably phreatic, eruption holes. The recent temperatures of the solfatara fields are somewhat higher than those before the earthquake. The collected samples are biotite andesite vitrophyres, biotite hypersthene andesites, and microdiorites. The difference between the Una-Una rocks, the andesites from other orogenic areas and trachytes from the hinterland volcanoes in Indonesia is clearly demonstrated on a Niggli-Becke projection diagram. The Una-Una rocks show transition between trachytes and andesites. Thep value of the rocks according to Rittmann’s method places them in the medium alkaline series. The petrology and chemistry of the volcanics, geological data from Una-Una and Togean islands, and the bathymetric map of the region suggest that this volcano lies outside the orogenic belt and that it occupies the intersection of two basement fissures of NE-SW and SE-NW directions. Summit observations of the volcano show similar directions for the volcanic fissures. The close relationship between tectonic earthquakes and volcanism was also demonstrated during the activity in 1898 in which the eruption was preceded by tectonic earthquakes. This, plus the presence of several epicenters SE of Una-Una indicate that both volcanic and seismic features may be associated with weak zones c. q. faults. The extinct volcanism in the Togean ridge can be explained by thickening of the earth crust caused by strong and continuous tectonic stress which is active since Tertiary time. The same tectonic condition was also responsible for the en echelon arrangement of the islands and the convexity of the Togean arc toward the hinterland.