Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies

Abstract

We have compiled extensive gravity and bathymetry data for the whole Mariana Trough, which were collected during several Japanese scientific cruises over the last few years. This study aims to clarify the lateral distribution of the local differences in geochemical signatures, which have been observed locally in the Mariana Trough. Shipboard free‐air gravity anomaly data from eight Japan Agency for Marine‐Earth Science and Technology (JAMSTEC) cruises were compiled with those crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA) were calculated by subtracting the predictable gravity signal due to the seawater/crust and crust/mantle density boundaries. The crustal thickness variation along the spreading axis was estimated from the MBA. Different features in crustal thickness, its variation, and segment length for each segment, allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the Mariana Trough. Segment in region A (to the north of 20°35′N) shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources. A variety of crustal thickness values and of along‐axis crustal thickness variations in region B (between 15°38′N and 20°35′N) suggests two types of segments. One is similar to a slow spreading ridge segment that has a plume‐like mantle upwelling under the spreading axis, and the other is a magma‐starved segment. Region C (between 14°22′N and 15°38′N) is a less magmatic region (individual crustal thickness averages of 3.4–4.1 km). Region D (to the south of 14°22′N) has higher individual crustal thickness averages of 5.9–6.9 km, suggesting higher magmatic activity with a sheet‐like mantle upwelling under the spreading axis. Different features in the MBA for off‐axis areas suggest that these four regions have existed since the Mariana Trough started spreading. Moreover, comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust, which are locally distributed in both regions B and C, probably result from mantle source depleted of water and incompatible elements. This suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the Mariana Trough.