Local earthquake tomography of Central America : structural variations and fluid transport in the Nicaragua-Costa Rica subduction zone

The Central American convergent margin is characterized by pronounced lateral changes from north to south such as a decreasing dip of the slab, a decreasing magma production and a shift in the volcanic front. To investigate this transition in terms of seismicity and tectonics, a joint on- and offshore local earthquake tomography and P-wave anisotropy studies were performed in central Costa Rica and in S Nicaragua/N Costa Rica respectively. In central Costa Rica, seismic travel time data sets of three on- and off-shore seismic networks were combined for a simultaneous inversion of hypocenter locations, 3-D structure of P-wave velocity and Vp/Vs ratio. The tomographic inversion was performed using about 2000 high quality events. The seismicity and slab geometry as well as Vp and Vp/Vs show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the NW, a seamount province in the center and the subducting Coscos Ridge in the SE of the investigation area. Three prominent features can be identified in the Vp and Vp/Vs tomograms: a high velocity zone with a perturbation of 4-10 % representing the subducting slab, a low velocity zone (10-20 %) in the forearc probably caused by deformation, fluid release and hydration, and a low-velocity zone below the volcanic arc related to upwelling fluids and magma. Unlike previously suggested, the dip of the subducting slab does not decrease to the south. Instead, an average steepening of the plate interface from 30° to 45° is observed from north to south and a transition from a plane to a stair-shape plate interface. This is connected with a change in the deformation style of the overriding plate where roughly planar, partly conjugated, clusters of seismicity of regionally varying dip are observed. It could be shown that the Costa Rica Deformation Belt represents a deep crustal transition zone extending from the surface down to 40 km depth. This transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be connected with the changing structure of the incoming plate as well. In S Nicaragua/N Costa Rica, the same inversion procedure was performed using 860 events. The analysis shows low S-wave velocities (~4 km/s), high Vp/Vs ratios (~ 2.0) and an aseismic gap in the upper mantle along the Sandino Basin. These findings are intrepereted as an indication of mantle wedge hydration. The existence of a hydrated forearc upper-mantle wedge in southern Nicaragua and the absence of it in northern Costa Rica is important to understand the variations in the tectonic structures along the margin and provides an improved view of the deep dehydration process in subduction zones. The sharp transition between the Nicaraguan and northern Costa Rican margins is explained by the dominating extensional forces in the southern Nicaraguan overriding plate. In addition to the velocity inversion, a P-wave anisotropy study was performed to have a better understanding in the mantle dynamics and tectonics of the Earth's interior. P-wave anisotropy results show two main structures: 1) Trench-perpendicular seismically fast directions in the incoming plate which can be explained either by the initial mineral orientation at the mid-oceanic ridge or by the deformation parallel to the subduction direction. 2) Trench-parallel seismically fast directions and abrupt rotations to trench-parallel anisotropy in the forearc which support the mantle escape towards to northwest. These patterns of seismic anisotropy may be caused by the olivine fabric transition from A-type to B-type or three dimensional flow associated with along-strike variations in slab geometry.