Gravity field of the Kuril-Kamchatka Arc and its relation to the thermal regime of the lithosphere

Abstract

Five gravity anomaly profiles across the Sea of Okhotsk and the Kuril island arc and trench and two profiles from Fiji to Tonga are presented and studied. The four profiles across the Kuril arc are similar and differ from the profile extending off southern Kamchatka. The contributions to gravity anomalies from the crust and mantle are investigated by means of continuous seismic refraction measurements along tracks coincident with the gravity profiles. The mantle gravity anomalies computed from various density models are positive over the island arc, trench, and adjacent Pacific basin and amount to at least +50 mGal. Gravity anomalies due to the mantle suggest the existence of a denser slab 50–60 km thick parallel to the seismic zone. The positive gravity effect of the slab appears to be balanced over the Okhotsk basin by the negative effect of the heated, less dense mantle. The local minimum of Bouguer anomalies occurring over the shoreward flank of the Kuril trench is rather insensitive to the density contrast adopted. The sediments measured seismically account for this Bouguer minimum. The gravity field over the Kuril and Tonga ridges supports the idea of reduced density under the volcanic chain. The local minimum of gravity anomalies associated with the northwestern boundary of the Okhotsk basin is probably related to the fault separating the Academy of Sciences rise from the basin. The ocean‐to‐continent trend of the free air anomalies in the Kuril area is unlike that in Tonga, which might be related to features of heat flow anomalies in the Kurils.

Gravity anomalies and main tectonic units of the southwest Pacific

“…In particular, the nonseismic Mussau Trench appears as an active trench with respect to gravity anomalies. As is shown elsewhere [Kogan, 1975], the gravity field near trenches is also significantly affected by density contrasts within the crust. Gravity anomalies due to the mantle can be estimated if the reliable seismic refraction data are available.…”

“…It is almost absent in Bouguer anomalies, and hence the topography is out of isostatic balance. Unfortunately, deepseated sources give rise to large gravity effects in the area, and we refer to further discussion in the paper by Kogan [1975]. A marked positive free air anomaly of +300 mGal over the Western Samoa Platform implies that here the islands present an additional load supported by the lithosphere.…”

“…We next consider the gravity profiles. We begin with the major part of Melanesia south and southeast of New Guinea (Figure 1 ) Both the profiles were discussed elsewhere [Kogan, 1975]. It is important to note that the free air anomaly progressively increases by about 70 mGal from Fiji toward the Tonga Ridge, while there is no associated trend in topography.…”

The thermal structure of subduction zone back arcs

“…Slow mantle velocities are also observed in surface wave studies [e.g., Kaila and Krishna , 1984; Ritzwoller and Levshin , 1998]. In addition, the Kamchatka back‐arc region is characterized by relatively low gravity, which has been modeled using a hot, low‐density mantle [ Kogan , 1975]. …”

Large-scale gravity profiles across subducted plates

“…Although our areas of study mainly involve continental regions, some oceanic belts are also found and allow the preceeding features to be compared with the oceanic data. Negative residual depth anomalies are typically found over the marginal seas (Sclater et al 1976;Karig, Anderson & Bidee 1978) and in particular over the eastern coast of Eurasia (Sclater, k t t e r & Dixon 1972;Kogan 1975), giving an illustration of the former feature.…”