FOREARC EXTENSION IN THE HARGY CALDERA AREA AS A RESPONSE TO OBLIQUE SUBDUCTION ALONG THE NEW BRITAIN TRENCH, PAPUA NEW GUINEA

Oblique subduction of the Solomon Sea Plate under the South Bismarck Plate along the New Britain Trench results in left lateral strike slip faulting across the Bismarck Volcanic Arc oblique to the direction of the trench and the subduction. First motion diagrams and plots of aftershocks from a magnitude 7.1 earthquake May 10, 1985 in Central New Britain imply a fault with strike of 100º E, dip 80º S (+/- 5º) and reinforce the left lateral movement (Mori, McKee and Letz, 1986). By plotting aftershocks and interpreting aerial photos, the proposed Siwanpuna Fault (new name) extends from (UTM zone 56) 263324.84 m E, 9388630.27 m S to 320721.21 m E, 9380647.84 m S.

A recent expedition to the remote Hargy Volcano (active fumarolic activity) and Hargy Caldera area 25 km north of the Siwanpuna Fault, found that the eastern border of the Hargy Caldera exhibits volcaniclastic conglomerates, mixed volcanic- carbonate conglomerates and discontinuous lenses of volcaniclastic sediments atop platform carbonates (Yalam Limestone). These Miocene Platform Carbonates occur locally as bedrock from the level of Hargy Lake to the top of the ridge east of the lake. This ridge serves as part of the “rim” of the caldera though it is generally over 200 meters higher than the south and west rims which are made of basalts and vesicular flow material. Bathymetric data and observation of faulted blocks make a normal fault with strike 45º E, dipping 30º SW (+/- 5°), likely along the eastern edge of the lake. This proposed Vovo Fault (new name) extends from (UTM zone 56) 290589.87 m E, 9404775.97 m S to 298764.44 m E, 9410079.25 m S. East of the Hargy Caldera, the Nakanai Mountains, a hyper-karst plateau made of Yalam Limestone, has been uplifted over 1500 m from its depositional environment and dips approximately 5° S (Lindley, 2006). Between the eastern rim of the Hargy Caldera and the Nakanai exists an intermediate shelf 4-6 km wide and 1000 m lower than the Nakanai Plateau. It is likely that a second west dipping normal fault exists at the eastern edge of that shelf, parallel to the Vovo Fault. Both this fault and the Vovo Fault are parallel with the New Britain Trench, and extension in this forearc area is consistent with the tectonic arrangement of the Siwanpuna and Vovo faults.