Geological structure of the forearc basin in central Hawke’s Bay, eastern North Island

Abstract

Central Hawke’s Bay lies within an extensive forearc basin in eastern North Island that developed during the Late Miocene to Pleistocene. The onshore structural elements of Hawke’s Bay can be classified into four structural domains, each reflecting differing styles and scales of deformation. These domains are from west to east, the axial range domain, the range front con¬tractional domain, the central forearc basin domain, and the eastern contractional domain. Some degree of the oblique-interaction of the Australia and Pacific plates on the subduction thrust is inferred to be partitioned across the four structural domains and to be expressed dominantly as oblique-(dextral) slip on faults bordering the axial ranges, and as short¬ening on reverse faults and folds in more eastern parts of the forearc. The axial range domain involves the eastern parts of the North Island axial ranges where there is marked oblique-slip displacement on major faults. Some dextral offest is accommodated in the range front contractional domain, although dip-slip displacement is more significant. The central forearc basin domain is comparatively undeformed with only minor reverse faulting and (fault-force driven) folding. By comparison, the ad¬jacent eastern contractional domain, which comprises an accretionary wedge, is characterised by imbricate reverse and thrust faulting and associated folding. A small degree of dextral-slip is also accommodated in this domain. The uppermost parts of the inboard margin of the accretionary wedge, particularly the part onshore, is currently undergoing gravitationally-driven collapse expressed as deep-seated landslides and normal faulting. Many folds in the basin are fault-cored, several of which have been targeted in recent years by petroleum exploration companies (e.g. Hukarere-, Whakatu-and Kereru-). Most deformation of the forearc basin fill in central Hawke’s Bay is post early Nukumaruan (2.4 Ma) and much of this has occurred since the early Pleistocene (.8 Ma). Dextral-slip on Mohaka and Ruahine Faults since the Early Pliocene is likely to be less than 0 km. Significant unconformities in the basin fill reflect early phases of development of oblique-slip faults in the axial ranges. New dextral oblique-slip faults are developing in the basin fill to the east of the main oblique-slip faults bordering the ranges.