Precise relocations of earthquakes and seismotectonics of the New Hebrides Island Arc

Abstract

Earthquakes for the period 1962–1973 are relocated by the method of joint hypocenter determination in order to resolve better the configuration and structure of the inclined seismic zone in the New Hebrides island arc. Twelve new focal mechanism solutions are reported and, together with previously published solutions, are integrated with the new information on the spatial distribution of hypocenters. At intermediate depths the seismic zone has a uniformly steep dip of about 70° and exhibits no resolvable contortions or disruptions along at least 700 km of the subduction zone. The thickness of the zone, about 20 km, may be in part due to seismically active fault zones which cut across a portion of the descending lithosphere. Features associated with the anomalous central region of the arc, where the d'Entrecasteaux fracture zone is being subducted and where the islands of Santo and Malekula appear to be in positions normally occupied by the oceanic trench, include the following: (1) an inclined zone of shallow earthquakes with very much smaller dip than is found elsewhere in the arc, (2) a pronounced gap in seismic activity at depths between about 50 and 120 km, (3) evidence for features in the upper or overthrust plate with trends transverse to the arc and parallel to the east‐west trends of the topographic feature being subducted, including nodal planes of shallow focal mechanism solutions, and (4) two features which appear to coincide with the downdip projection of the northern scarp of the d'Entrecasteaux fracture zone, including a well‐defined boundary between two adjacent zones of plate slippage along the main plate boundary and a faultlike feature in the intermediate depth seismic zone which also has a strike parallel to the fracture zone. Appendix table and entire article are available on microfiche.Order from American Geophysical Union, 1909 K Street, N.W.,Washington, D. C., 20006. Document J78‐006; $1.00. Payment mustaccompany order.

Temporal Magmatic Evolution of the Aoba Basin, Central New Hebrides Island Arc: Pb, Sr, and Nd Isotopic Evidence for the Coexistence of Two Mantle Components beneath the Arc

“…Seismologic sections of the NHIA at several locations along the arc (Pascal et al, 1978;Goula and Pascal, 1979;Louat et al, 1988) show that in the southern part of the arc, where only "normal" mantle is contributing to the arc magmatism, the subducted slab is continuous and the top of the slab is only -130 km below the arc volcanic islands of Tanna and Anatom (Louat et al, 1988) and 80 km below Matthew island (Monzier et al, unpubl. data).…”

Magmatic Evolution of the North Aoba Intra-Arc Basin: Sites 832 and 833

“…Since early Late Miocene, the oceanic slab has been underthrusting from west to east during the time that the NFB has been opening (e.g., Auzende et al, 1988). Seismological data are consistent with the existence of the presently steeply east-dipping oceanic slab (Pascal et al, 1978). However, to explain clusters of deep earthquakes beneath the NFB, east of the central part of the New Hebrides Island Arc, Hamburger and Isacks (1987) suggested the continued existence beneath this region of a remanent (detached?)…”

Introduction

“…The physiographic trench is clogged by the DEZ, which is composed of ridges and basins (Daniel, 1978;Collot et al, 1985;Fisher, 1986;Fisher et al, 1986). The oblique collision of the DEZ with the central New Hebrides Island Arc (Pascal et al, 1978;Isacks et al, 1981) has produced a complex structural pattern. Differential uplift of the arc platform has further complicated the structure through the formation of horsts and grabens, which influenced the sedimentary depositional history of the central New Hebrides Island Arc (Carney and Macfarlane, 1982;Johnson and Greene, 1988;.…”

“…3). The trench correlates with a fairly continuous east-dipping Benioff zone (Karig and Mammerickx, 1972;Pascal et al, 1978;Louat et al, 1988;Macfarlane et al, 1988). The physiographic trench is clogged by the DEZ, which is composed of ridges and basins (Daniel, 1978;Collot et al, 1985;Fisher, 1986;Fisher et al, 1986).…”

“…Many different explanations have been proposed to describe the origin of the New Hebrides Island Arc (e.g., Chase, 1971;Dickinson, 1973;Pascal et al, 1978;Falvey, 1975;Coleman and Packham, 1976;Ravenne et al, 1977;Carney and Macfarlane, 1977, 1979Katz, 1988;Macfarlane et al, 1988). However, two major hypotheses are presently favored.…”