Early Miocene Thrust Tectonics on Raukumara Peninsula, Northeastern New Zealand

Abstract

Raukumara Peninsula lies at the northeastern end of the East Coast Deformed Belt, a province of deformed Late Mesozoic-Late Cenozoic rocks on the eastern edges of the North Island and northern South Island of New Zealand. Late Cenozoic deformation in this province is associated with westward subduction of the Pacific Plate, which started at about the beginning of the Miocene. Early Miocene tectonism on Raukumara Peninsula took place in a hitherto little-known thrust belt, the East Coast Allochthon. The configuration, evolution and origin of this thrust belt are the subjects of this thesis. The thrust belt extends 110 km from the thrust front in the southwest to the northeastern tip of Raukumara Peninsula. Internal structures strike northwest, perpendicular to the present trend of the continental margin but parallel to the Early Miocene trend suggested by plate reconstructions and paleomagnetic studies. The structure and kinematic evolution of the thrust belt were investigated by detailed mapping of three key areas in its central part and by analysis of previous work throughout the region. Gross differences in structure lead to the division of the belt into three zones: southern, central and northern. Deformation in the southern and central zones (the southwestern two-thirds of the system) was thin-skinned, involving southwestward transport of thrust sheets above a decollément horizon at the top of the Maastrichtian-Paleocene Whangai Formation. The decollément is exposed in the northwest due to southeastward tilting accompanying post-Miocene uplift of the Raukumara Range. Deformation in the northern zone involved reactivations of northeast-directed Cretaceous thrusts as well as southwestward emplacement of allochthonous sheets. Stratigraphic relationships show that thrusting took place during = 6 m.y. in the earliest Miocene. The 18 km wide southern zone is an emergent imbricate fan of rocks detached from above the Whangai Formation in a piggy-back sequence and transported less than about 18 km at rates of 2.6-3.6 mm/yr (± 20%-100%). The central and northern zones include rocks older than Whangai Formation. The sheets of the central zone and the southwest-directed sheets of the northern zone make up three major allochthonous units: the Waitahaia allochthon, consisting predominantly of mid-Cretaceous flysch above the Waitahaia Fault and equivalent structures, at the bottom of the thrust pile; the Te Rata allochthon, of Late Cretaceous-Early Tertiary continental margin sediments above the Te Rata Thrust, in the middle; and the Matakaoa sheet, an ophiolite body of mid-Cretaceous-Eocene basaltic and pelagic sedimentary rocks, at the top and back of the thrust belt. The Waitahaia allochthon was emplaced first and was subsequently breached by the Te Rata Thrust. The mid-Cretaceous rocks of the Waitahaia allochthon are mostly overturned, a result of the southwest-directed Early Miocene thrusting overprinting a Cretaceous structure of predominantly southwestward dips. The Te Rata allochthon comprises a complex pile of thrust sheets and slices with a general older-on-younger stacking order but with common reversals. Synorogenic sedimentary rocks occur within it. The complexity of internal structure of these two allochthons suggests they have undergone more than the 50% shortening estimated for the southern zone. The minimum southwestward displacement of the Te Rata allochthon is 60 km. The minimum displacements of the Waitahaia and Matakaoa allochthons are 55-195 km and 115-530 km respectively, depending on whether the Te Rata allochthon originally lay in front of the original position of the Waitahaia allochthon or was originally the upper part of the Waitahaia allochthon, and on the amounts of internal shortening of the allochthons. Over the = 6 m.y. period of thrusting, these estimates imply displacement rates for the Matakaoa sheet of 19-88 mm/yr. The average plate convergence rate at East Cape for the period 36-20 Ma is estimated at 25-30 mm/yr; the rate for the Early Miocene—when subduction was active—may have been faster. Reasonable displacement rates for the Matakaoa sheet would result if the Te Rata allochthon was originally the upper part of the Waitahaia allochthon and if both allochthons have been shortened somewhat less than 50%. The emplacement mechanism of the Matakaoa ophiolite is elucidated by comparison with Northland, northwest along strike from Raukumara Peninsula, onto which correlative rocks were emplaced at the same time. The thinness of the Northland ophiolite bodies, their composition of rocks typical of the uppermost levels of oceanic crust, and the start of andesitic volcanism accompanying their obduction show that they were emplaced as a thin flake of oceanic crust which peeled off the downgoing slab during the inception of southwestward subduction. The reason the ophiolites were initially peeled from the slab is probably that their upper levels prograded southwestward over sediments of the Northland-Raukumara continental margin. In such a situation, initial compression would have led to formation of a northeast-dipping thrust at the volcanic/sediment interface; this thrust would then have propagated back into the downgoing plate with continued convergence, allowing the ophiolites to climb up the continental slope pushing the allochthonous sedimentary sheets ahead of them.