ACTIVE DEFORMATION IN THE NEW GUINEA FOLD‐AND‐THRUST BELT: SEISMOLOGICAL EVIDENCE FOR STRIKE‐SLIP FAULTING AND BASEMENT‐INVOLVED THRUSTING

Abstract

The New Guinea fold‐and‐thrust belt forms a 1000‐km‐long mountain chain running the length of New Guinea, but its trend is oblique to the predicted convergence direction. A large component of left‐lateral shear across the mountain belt is expected, but geological evidence for such motion is ambiguous. Surface structures in the fold‐and‐thrust belt show mountain building by folding and thrusting, supposedly thin skinned, of pre‐Pliocene continental shelf sediments, although some evidence indicates that active faulting penetrates the basement. We examine the active deformation of this mountain belt by using teleseismic long‐period P and SH waveforms, short‐period P waveforms, and P wave first motions to determine reliable focal mechanisms and source depths for the 18 largest earthquakes in the New Guinea fold‐and‐thrust belt since 1964. Eight thrust events, with seismic moments (M0) between 1017 and 2 × 1018 N m, are distributed throughout the thrust belt at depths ranging from 0.5 to 45 km and have steeply dipping fault planes that strike parallel to surface structural trends. Six of the eight events are between 11 and 25 km deep. The steep dips of nodal planes, the depths of these earthquakes, and the known thicknesses of the sedimentary section show that thrust faulting is not exclusively thin skinned (i.e., confined above a shallow detachment surface) but penetrates the crystalline basement at high angles. Most earthquakes in the western half of the thrust belt, including the three largest earthquakes examined (M0 > 5 × 1018 N m), show strike‐slip faulting with the probable fault plane oriented east‐west, so that displacement is left lateral. The nine strike‐slip earthquakes account for almost 90% of the observed seismic moment in the period 1964–1985 and indicate a seismic slip rate of 5–25 mm/a. All but one of these strike‐slip earthquakes are shallower than the thrust earthquakes, suggesting that east‐northeast convergence across the fold‐and‐thrust belt is accommodated by a decoupled system of strike‐slip and dip‐slip faults. While the great height of the mountain belt is an obvious indication that crustal thickening is an important consequence of plate motion in New Guinea, seismological evidence suggests that translation by strike‐slip faulting may play a much greater role than previously recognized. By analogy, in older collisional orogens, whose motions can be inferred only from remnant structures and not from seismicity, strike‐slip faulting may have been equally important in accommodating convergence. We estimate from seismic moments of earthquakes and from the volume of the mountains that deformation of the New Guinea Highlands may account for 5–20% of the total convergence between the Pacific plate and Australia in New Guinea.