Deep structure of an arc‐continent collision: Earthquake relocation and inversion for upper mantle P and S wave velocities beneath Papua New Guinea

Abstract

We examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction, by analyzing the seismicity and upper‐mantle velocities in the New Guinea arc‐continent collision. Earthquakes in the Papua New Guinea (PNG) region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities, using P and S wave arrival times recorded by seismographs in PNG. A total of 957 well‐recorded earthquakes were chosen for use in three‐dimensional velocity inversions from an initial catalog of 12,960 events, based on the stability of initial hypocenter relocations. Constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters, and to exploit a priori knowledge of the geometry of velocity anomalies. Spherical geometry was used to accurately calculate long ray paths (up to 1500 km in length). Most of the ∼700 best located hypocenters from the PNG data set are in the upper mantle beneath the Finisterre‐Huon (FH) ranges (the newly accreted island arc terrane), beneath the Papuan Peninsula, and in the New Britain seismic zone (an oceanic subduction system). These hypocenters show a well‐defined seismic zone dipping vertically or steeply to the north beneath the northern FH ranges from 125‐ to 250‐km depth, continuous along strike with the New Britain seismic zone to the east (which shows earthquakes to 600 km depth). The steeply dipping intermediate‐depth seismicity flattens near 100 km depth beneath the FH ranges and forms a subhorizontal seismic zone beneath the FH ranges. By contrast, seismicity continues upward to the surface beneath New Britain where the arc has not yet collided with New Guinea. The 100‐km‐deep flat zone does not continue south of the FH ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane, and no clear evidence for a south‐dipping seismic zone could be found. The relationship of the flat part of the slab 100 km beneath the FH ranges and surface faulting is difficult to discern; the apparent surface suture bounding the south side of the FH ranges is 100 km directly above the southern termination of intermediate‐depth seismicity. Nowhere in PNG could clear evidence for arc polarity reversal be found from seismicity. All of the well‐located subcrustal earthquakes beneath the Papuan peninsula lie in a narrow horizontal line between 125‐ and 175‐km depth that follows the center of the peninsula, and are distinctly separated from the FH seismic zone to the north. These events do not require a southward subducting plate beneath the Papuan peninsula. An alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere. Information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals. Allowing for only crustal heterogeneity reduces travel time residual variances by 40% from a simple one‐dimensional structure, while allowing for additional mantle heterogeneity results in ≤ 10% further reduction for the various parameterization s attempted here. Inversions with several different block geometries show low velocities at 35‐ to 171‐km km depth beneath the Bismarck Sea back‐arc basin and high velocities just south of the intermediate depth FH seismic zone. The pattern of lateral heterogeneity supports the inference from seismicity of north‐dipping slabs beneath both northeast New Guinea and New Britain.