Structural Analysis of the Hohonu Thrust Fault and Relationships with the Neogene Tectonic Evolution of the Grey Valley Region. (2014)
AuthorsSintenie, Janshow all
The main aims of this study are to; (1) constrain the kinematics and tectonic evolution of the Hohonu Fault; (2) understand the Neogene tectonic history of the Greymouth area and its context in terms of regional plate motions; and (3) determine if Late Cretaceous extensional structures in the study area are influencing deformation and Neogene tectonic development. Two major extensional tectonic episodes have significantly affected the crustal architecture of the study area prior to Neogene compression. The first being NNE-SSW directed extension as new Zealand broke away from Gondwana between c. 115 and c. 82 Ma. This was accompanied by granitoid intrusion, core complex formation, and was culminated with rifting and opening of the Tasman Sea. The second being WNW-ESE directed extension that was related to the propagation of the Challenger Rift System through New Zealand between c. 45 and c. 23 Ma. This phase was characterised by the development of NNE-SSW trending normal faulted basins. Both of these tectonic episodes have left strong structural discontinuities in the crust. During the phase of Neogene compression since c. 23 Ma, the change from WNW-ESE extension to compression in the same direction resulted in inversion of many of the basins that had formed in the Paleogene to early Neogene stage of extension. The compressional reactivation of the inherited NNE-SSW normal faults has been illustrated by many authors, and therefore their presence as a seismic hazard is relatively clear. The possible reactivation of the easterly trending Late Cretaceous structures in the current stress field is less well understood, and therefore represent an unquantified seismic hazard. This study uses a diverse range of data including a large amount from petroleum exploration (open source) as well as recently released (2013) aeromagnetic data, and kinematic data from fieldwork done during this study. A principal tool used to synthesise the data is the 3D structural geology software Move. This software was used to map structures across the study area as well as create two balanced geological cross sections which were restored to significant Neogene horizons. Findings from this study include a new hypothesis that suggests the existence of a c. 25 km wide, WNW-ESE trending Late Cretaceous extensional structure between the Hohonu Range and the Takutai Half Graben offshore Greymouth. The presence of this Cretaceous extensional structure appears to have significantly effected Neogene tectonic development across the Grey Valley Trough and Hohonu Range. In the Grey Valley Trough, the Cretaceous extension zone is coincident with an area where Neogene subsidence is c. 2km greater than the largest amounts of subsidence occurring to the north or south. On the southwest margin, and to the north of the Hohonu Range, the inferred presence of conjugate strike slip faults indicates the reactivation of the Cretaceous extensional structures in the current stress field and may present a seismic hazard risk. The concept of the Greymouth area developing as craton-ward migrating foreland system during the Late Miocene is also suggested.