Crustal structure of a short length transform fault in the central Mariana Trough
Abstract
The crustal structure of the Pagan fracture zone, a short length transform fault system in the central Mariana Trough, was determined from a detailed seismic refraction study by using ocean bottom seismometers. The Mariana Trough is a back arc basin with a spreading center opening at a half rate of less than 2 cm/yr. Interpretation of both the seismic travel times and amplitudes suggest that the crust within the transform valley is 1-1.5 km thinner than oceanic crust generated at slow spreading centers away from transform faults but is thicker than fracture zone crust in longer transform offset systems. The crustal differences can be accounted for solely by a thickening, relative to longer transforms, of the lower crust by as much as 3.0 km. Other major differences between the Pagan short transform system and transform systems of greater length are (1) the transition from Pagan transform crust to oceanic crust is only 2.0-4.0 km wide, in comparison with a reported 5-10 km for longer transforms, (2) the 50-mgal free air gravity anomaly over the Pagan transform valley is half that of longer transforms, and (3) whereas bathymetric ridges parallel to longer transform systems are underlain by high-density material, the entire valley of the Pagan transform system is underlain by high-density material. These differences can be explained by the different thermal properties of the third wall at the spreading center/transform intersections, and its affect on crustal accretion. As the third wall becomes colder, the crust becomes thinner, the free air gravity anomaly is larger, the transition from transform to oceanic crust is wider, and the bathymetric relief across the valley is greater. No significant difference was observed between crustal structure in the transform zone in comparison with the fracture zone portion of the Pagan transform system. Use of amplitude modeling with extended WKBJ synthetic seismograms, valid in two-dimensional laterally inhomogeneous media, made it possible to constrain the structural limits of the transform valley crust more closely than would have been possible by using only travel times.
- Publication:
-
Geophysical Monograph Series
- Pub Date:
- 1983
- DOI:
- 10.1029/GM027p0236
- Bibcode:
- 1983GMS....27..236S