Fracture lineaments, fault mesh formation and seismicity
Towards a seismotectonic model for Viti Levu, Fiji
Viti Levu, the main island of Fiji, is located in a seismically active area within the Fiji Platform, a remnant island arc that lies in a diffuse plate boundary zone between the Pacific and Australian tectonic plates in the SW Pacific. The upper crust of Viti Levu is dissected by numerous intersecting fault/lineament zones mapped from remote sensing imagery of the land surface (topography, radar and aerial photos) and basement (magnetic) and have been subject to rigorous statistical tests of reproducibility and verification with field mapped fault data. Lineaments on the various imagery correlate with faults mapped in the field, and show spatial continuity between and beyond mapped faults, thereby providing a fuller coverage of regional structural patterns than previously known. Some fault/lineaments zones extend beyond the coastline to the offshore area from the SE Viti Levu study area. Multibeam bathymetry and seismic reflection data show the fault zones occur along and exert control on the location of a number of submarine canyons on the SE slope of Viti Levu. Evidence for Late Quaternary fault activity is only rarely observed in onshore SE Viti Levu (e.g. by displaced shoreline features), and in seismic reflection profiles from offshore.
The principal fault sets in Viti Levu represent generations of regional tectonic faulting that pervade the Fiji Platform during and after the disruption of the proto Fijian arc in the Middle to Late Miocene (~15Ma). These fault sets combine to form a complex network of interlocking faults creating a fault mesh that divides the upper crust into a number of fault blocks ranging from ~2-30 km wide. It is inferred that the fault mesh evolved throughout the Neogene as a response to the anticlockwise rotation of the Fiji Platform through progressive development of different fault sets and intervening crustal block rotations. Regional tectonic deformation is presently accommodated in a distributed manner through the entire fault mesh. Low magnitude earthquakes (<M4) occur regularly and may represent ruptures along short linking segments of the fault mesh, while infrequent larger earthquakes (>M4) may result from complex rupture propagation through several linking fault segments of the mesh that lie close to optimum stress orientations. The interpreted model of distributed deformation through the fault mesh for the study area in SE Viti Levu is inferred to be characteristic of the style of active deformation that occurs throughout the entire Fiji Platform.
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