Tectonic history of the Cretaceous Mokoiwi Formation and adjacent units, northeastern North Island, New Zealand

Abstract

The Mokoiwi Formation forms a tectonic inlier of variably disrupted, complexly faulted and folded strata. It consists of three members: 1) an Albian (Motuan) thin-bedded flysch sequence, 2) the Taitai Sandstone, a deformational breccia, and 3) the Albian (Motuan) Rip Volcanics, a spilite-keratophyre-carbonate-chert assemblage. The inlier covers approximately 100 square kilometers and is separated by faults from surrounding Upper Cretaceous and Lower Tertiary strata. Boundary faults to the southwest and northeast have parallel NW trends but opposing dip directions. The Northern boundary fault dips steeply to the SW while the southwestern Hukanui boundary fault dips shallowly to the NE. The inlier is, therefore, wedge-shaped in NE-SW cross-section with its thickest portion lying to the northeast. These two boundary faults are interpreted as a continuous fault surface that has been folded during a late-phase, NW trending fold event. The eastern boundary of the inlier is apparently folded and faulted and dips generally westward. To the NW, the inlier may onlap onto Jurassic-Cretaceous strata as depicted by Kingma and Speden (1978). However, the contact is obscured by younger deformation. Three features dominate the internal structure of the Mokoiwi Formation: 1) the large percentage of relatively undeformed, overturned bedding (67% of all bedding measurements with known younging directions), 2) the occurrence of broken formation throughout the sequence, accounting for 25% of all planar fabric measurements, and 3) subhorizontal folds in bedding and broken formation, locally with NW-SE trends. The widespread occurrence of little deformed, overturned bedding within the Mokoiwi Formation is difficult to explain. It cannot simply represent an overturned limb of a large recumbent fold because such overturned limbs are usually highly attenuated. Mesoscopic structures suggest that two or more separate episodes of folding result in the abundant overturned bedding. Broken formation results mainly from stratal disruption with strong layer-parallel extension and subordinate layer-parallel shortening. The planar fabric within the broken formation is usually subparallel to the adjacent bedding and is defined by two distinct elements: 1) fragmentation and separation of sandstone units, resulting in parallel alignment of elongate and irregular lozenges or phacoids, and 2) an anastomosing foliation formed by the disrupted mudstone matrix. Folds within the Mokoiwi Formation are complex and discontinuous. Coherent bedding and broken formation are commonly folded about the same fold axis, suggesting broken formation predates folding. In a few cases, broken formation results from hinge collapse of tight to isoclinal folds. Intervals of disrupted mudstone, with or without sandstone lozenges, locally behave as decollement zones during disharmonic folding. The majority of folds have open monoclinal profiles. A large domain with a NW structural grain, which includes all of the Rip Volcanics, lies in the northeastern portion of the inlier. The NW trend is highly oblique to the N to NNE trending structural fabric of the Jurassic-Cretaceous units to the southwest. Similar NW trends have been found to the south by Stoneley (1968) and Black (1980) in areas of mid-Tertiary and younger decollement. The major Taitai Sandstone bodies of Mt. Aorangi, Mt. Wharekia, and portions of Mt. Taitai have dominant NE trending fold axes. Alternating sandstone/mudstone sequences adjacent to these mountains commonly have complex interference fold patterns with non-cylindrical folding. Intervals of NE trending folds are rare within the Mokoiwi alternating sequence. Three tectonostratigraphic basement terranes are defined within northeastern North Island, two greywacke terranes and a seamount terrane that includes the Matakaoa Volcanics. The greywacke terranes are differentiated by their age of emplacement. To the SW, emplacement of the Koranga subduction-accretion complex terrane was apparently restricted to the latest Jurassic and possibly the Early Cretaceous. To the NE, the Oponae-Waitahaia subduction-accretion complex terrane formed during the Albian and perhaps the Cenomanian. NE of these two greywacke terranes is the Cretaceous-Tertiary Matakaoa seamount terrane, much of which lies offshore to the north of East Cape. The two greywacke terranes accreted to each other during the middle Cretaceous while the Matakaoa terrane accreted during the middle Tertiary. The region has been affected by a middle to late Tertiary fold-and-thrust belt, apparently- related to accretion of the Matakaoa Volcanics and to continued subduction along the Hikurangi Trough. These volcanics and related units of the Matakaoa terrane that lie beneath the adjacent continental shelf are exotic to New Zealand. Correlation of autochthonous strata overlying the greywacke terranes with allochthonous strata suggest that most of the displaced middle Cretaceous to Lower Tertiary sequences are para-autochthonous