Basement-cover relationships and tectonic significance of Mt Miroroa, western Hawke's Bay

Abstract

At Mt Miroroa , indurated Mesozoic basement rocks have been faulted over Late Tertiary alternating marine sandstone, siltstone and pebbly limestone. The fault plane strikes NW-SE and dips at 45° SW. The younger sediments are flat lying to the north, but towards the Miroroa Fault they are steepened on the southern limb of an ESE trending syncline, and in places arc overturned. The 3 km long Miroroa Fault is truncated by the northeast-trending Ruahine and Comet Faults. Considerable deformation is evident above and below the Miroroa Fault, although the shear plane is only 5 cm thick. Topographically bclow but subparallel to the Miroroa Fault, several normal faults have raised horst wedges of basement rocks into the Late Tertiary strata. Conjugate minor fault couples immediately below the decollement indicate maximum principal stress to be low-angle and oriented NE-SW. Strain ellipsoids constructed from deformed fossils beneath the Miroroa Fault confirm this orientation. Conjugate faults within a 10 km radius north of the fault indicate NE-SW shortening directions with a subordinate easterly component In terms of plate tectonic nomenclature, Mt Miroroa lies within the frontal ridge. Sites elsewhere in the frontal ridge of the North Island also indicate NE shortening. It is suggested that the Miroroa Fault either (1) began as a northwest trending normal fault, that was subsequently overturned to its present-day reverse geometry by continued NE-SW directed compression, or (2) developed as an en echelon or side stepping fault between the adjacent Cornet and Ruahine Faults.