Tectonic Evolution of the Cape Egmont Fault in the Maui PML - an Interpretation from the First Application of 3D Pre-stack Depth Migration in New Zealand

摘要

The Cape Egmont Fault has historically posed a complex interpretation problem in the Maui PML owing to the associated complex velocity structure rendering conventional 3D time migration inadequate. To solve this problem, 3D Pre-Stack Depth Migration (PSDM) has been applied to a portion of the Maui 3D survey. This type of processing has only become possible in the last few years due to the availability of the necessary computing power and the Maui project represents the first application of the technology in New Zealand. Furthermore, it represents one of the first 3D PSDM applications anywhere to a complex velocity problem associated with a major normal fault. Prior to 3D PSDM the Cape Egmont Fault was interpreted on 3D post-stack, time-migrated data as a complex zone of parallel faults, progressively stepping down to the southeast. Each fault plane was shown on interpreted sections as being listric, with regional interpretations often indicating the fault system soling out at depth. Owing to the complexity of the interpreted structure, the degree to which strike slip was involved in the formation of the Cape Egmont Fault at Maui was open to debate. After 3D PSDM the seismic data show a dramatic improvement in the continuity of reflections in both the footwall and the hanging wall blocks, and in places fault plane reflections can be reliably recognised. These data, which are displayed and interpreted in depth, show the Cape Egmont Fault at Maui to largely comprise a simple, planar surface, whose dip varies very little with depth. Movement on the fault plane is interpreted to have been almost entirely dip slip, though in the Pliocene to Recent the amount of extension increase northwards, consistent with rotational bulk strain model proposed by King and Thrasher (1996).