Anatomy of a continent-backarc transform — The Vening Meinesz Fracture Zone northwest of New Zealand

The northwestern continental margin of New Zealand offers one of the finest examples of a continent-backarc transform. This transform, part of the Vening Meinesz Fracture Zone (VMFZ), accommodated about 170 km of sea-floor spreading in the Norfolk backare basin together with eastward migration of a volcanic arc, the Three Kings Ridge, in the Mid- to Late Miocene. Before the onset of spreading, strain along the VMFZ may have been linked to a major Early Miocene obduction event — the emplacement of the Northland Allochthon. The transform is manifested by a belt up to 50 km wide of left-stepping, linear fault scarps up to 2000 m high within an approximately 100 km-wide deformed zone. A marginal ridge, the Reinga Ridge, which includes a faulted, folded and uplifted Miocene sedimentary basin, occurs within the high-standing continental side of the deformed zone, whereas a narrow strip of linear detached blocks occupies the deep backarc oceanic side. Prespreading uplift and erosion of crust in the proto-backarc region, are volcanism, and obduction of the allochthon, supplied clastic sediments to the basin on the continental side. This basin was complexly deformed as the transform evolved. The transform was initiated as a dextral strike-slip fault zone, which developed right-branching splays and left-steps along its length, uplifting and cutting the continental margin into left-hand, en echelon blocks and relays. Folds formed locally within relay blocks and at the distal ends of the splays. Only the high continental side of this zone (the Reinga Ridge) remains, the formerly adjacent crust (the Three Kings Ridge) having been displaced towards the southeast. As the Three Kings block moved and the Norfolk Basin opened, opposing rift margins of the backarc basin foundered to form terraces. The oceanic side of the transform also subsided to produce the belt of detached blocks (some laterally displaced by strike slip) and linear troughs along the main escarpment system.     

Seismic stratigraphy and structure of the Northland Plateau and the development of the Vening Meinesz transform margin,SW Pacific Ocean

The Northland Plateau and the Vening Meinesz “Fracture” Zone (VMFZ), separating southwest Pacific backarc basins from New Zealand Mesozoic crust, are investigated with new data. The 12–16 km thick Plateau comprises a volcanic outer plateau and an inner plateau sedimentary basin. The outer plateau has a positive magnetic anomaly like that of the Three Kings Ridge. A rift margin was found between the Three Kings Ridge and the South Fiji Basin. Beneath the inner plateau basin, is a thin body interpreted as allochthon and parautochthon, which probably includes basalt. The basin appears to have been created by Early Miocene mainly transtensive faulting, which closely followed obduction of the allochthon and was coeval with arc volcanism. VMFZ faulting was eventually concentrated along the edge of the continental shelf and upper slope. Consequently arc volcanoes in a chain dividing the inner and outer plateau are undeformed whereas volcanoes, in various stages of burial, within the basin and along the base of the upper slope are generally faulted. Deformed and flat-lying Lower Miocene volcanogenic sedimentary rocks are intimately associated with the volcanoes and the top of the allochthon; Middle Miocene to Recent units are, respectively, mildly deformed to flat-lying, calcareous and turbiditic. Many parts of the inner plateau basin were at or above sea level in the Early Miocene, apparently as isolated highs that later subsided differentially to 500–2,000 m below sea level. A mild, Middle Miocene compressive phase might correlate with events of the Reinga and Wanganella ridges to the west. Our results agree with both arc collision and arc unzipping regional kinematic models. We present a continental margin model that begins at the end of the obduction phase. Eastward rifting of the Norfolk Basin, orthogonal to the strike of the Norfolk and Three Kings ridges, caused the Northland Plateau to tear obliquely from the Reinga Ridge portion of the margin, initiating the inner plateau basin and the Cavalli core complex. Subsequent N115° extension and spreading parallel with the Cook Fracture Zone completed the southeastward translation of the Three Kings Ridge and Northland Plateau and further opened the inner plateau basin, leaving a complex dextral transform volcanic margin.     

Inverse Vening Meinesz formula and deflection-geoid formula: applications to the predictions of gravity and geoid over the South China Sea

Using the spherical harmonic representations of the earth's disturbing potential and its functionals, we derive the inverse Vening Meinesz formula, which converts deflection of the vertical to gravity anomaly using the gradient of the H function. The deflection-geoid formula is also derived that converts deflection to geoidal undulation using the gradient of the C function. The two formulae are implemented by the 1D FFT and the 2D FFT methods. The innermost zone effect is derived. The inverse Vening Meinesz formula is employed to compute gravity anomalies and geoidal undulations over the South China Sea using deflections from Seasat, Geosat, ERS-1 and TOPEX//POSEIDON satellite altimetry. The 1D FFT yields the best result of 9.9-mgal rms difference with the shipborne gravity anomalies. Using the simulated deflections from EGM96, the deflection-geoid formula yields a 4-cm rms difference with the EGM96-generated geoid. The predicted gravity anomalies and geoidal undulations can be used to study the tectonic structure and the ocean circulations of the South China Sea. Received: 7 April 1997 / Accepted: 7 January 1998