Dennis E. Hayes

Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous☆

Abstract A comprehensive study of the last 75 m.y. of plate tectonics history has been undertaken for the region south of 30°S in the South Pacific, Southeast Indian Ocean and the Tasman Sea. Some aspects of plate boundary evolution have been clarified by our compilation and examination of available marine geophysical data. Reidentification of magnetic lineations in the southern Tasman basin shows that the controversial interval of subduction of Tasman basin crust along the east Australian margin that was previously proposed is no longer necessary. A comparison of Cenozoic magnetic lineations from both sides of the easternmost spreading segment of southeast Indian ridge indicates that a portion of the Indian plate younger than anomaly 10 (32 m.y.B.P.) is missing. We suggest that the missing crust was either subducted beneath or captured by the Pacific plate. Older lineations on the Indian plate out to about anomaly 21 have greater along-strike lengths than their counterparts on the Antarctic plate. The difference is due to an interval of crustal accretion at the Indian—Pacific plate boundary in the Early to Middle Tertiary. In the South Pacific, the Antarctic plate may not have extended northeast of the Eltanin fracture zone system prior to anomaly 29 (69 m.y.B.P.). Subduction of oceanic lithosphere was probably occurring beneath the Antarctic peninsula and eastern Ellsworth Land parts of the Antarctic plate at that time. Between anomaly-29 time and a major reorganization of plate boundaries in the Late Oligocene, plate interactions occurred in the central South Pacific between the Pacific, Farallon, Antarctic, Aluk and possibly a fifth plate. Spreading rate calculations for the Early Oligocene indicate that a simple three-plate system involving the Pacific, Farallon and Antarctic plates is difficult to maintain unless highly asymmetric spreading occurred at the Farallon—Antarctic boundary in the Early Oligocene. Further to the southeast in the Bellingshausen basin, spreading occurred along segments of the Antarctic—Aluk plate boundary beginning at about anomaly-29 time. Collisions of segments of this boundary with the trench along the Antarctic peninsula occurred in the Early and Middle Tertiary and resulted in the total disappearance of portions of Aluk plate. The collisions brought Antarctic—Aluk ridge segments into contact with Antarctic—Aluk trench segments with resulting stabilization of the Antarctic continental margin. From the magnetic lineations mapped in the southern oceans, we calculated new finite rotations which determine the relative positions of the Australian, New Zealand, and Antarctic continental fragments at 5–10 m.y. intervals during the last 75 m.y. For the New Zealand region, finite rotations calculated from magnetic lineations in the Tasman Sea and southwestern Pacific indicate that three plates were active during the Late Cretaceous to Paleocene. Previous workers have proposed that one of the three plate boundaries occurred southward between East and West Antarctica approximately along the trend of the Transantarctic mountains. As no direct geologic or geophysical evidence in Antarctica supports this proposal and since sea-floor spreading magnetic evidence shows that an active plate boundary passed through the New Zealand region after the Paleocene, we prefer to carry the required plate boundary northward. The Indian—Pacific poles of relative motion were always close to this boundary during the Cenozoic and thus corresponding evidence for plate interactions in New Zealand continental geology is generally variable and sometimes subtle.

Seafloor Spreading in the Tasman Sea

Magnetic and seismic studies in the Central Tasman Sea show conclusively that it evolved by a process of seafloor spreading between 60 and 80 m.y. BP, suggesting that other marginal basins may have a similar history. A reconstruction of Australia, the Lord Howe Rise, New Zealand and Antarctica based on these data is presented and discussed.

Evolution of the Tasman Sea reappraised

Abstract We reexamined available marine magnetics data from the Tasman Sea and reidentified sea-floor spreading anomalies in the southern portion of the basin. From the revised magnetic lineations and fracture zones we calculated new finite rotations which descrobe the evolution of the basin in terms of a simple two-plate spreading system active between about 82 and 60 m.y. ago. Allowing for the probable continental origin of the Dampier ridge, the predicted displacement of the western margin of the Lord Howe rise relative to the eastern Australian margin in the northern Tasman basin is consistent with their observed separation. Thus, the controversial episode of subduction of Tasman basin crust at the east Australian margin proposed in earlier studies is no longer necessary.

Deep penetration seismic soundings across the northern margin of the South China Sea

Twenty reversed, two-ship expanding spread profiles (ESPs) with maximum source-receiver offsets of ∼100 km were collected in three transects across the rifted northern margin of the South China Sea. Source-receiver offset versus two-way travel time (X-T) data were mapped into the intercept time versus ray parameter (τ-p) domain, and velocity-depth solutions were obtained by a combination of τ-sum inversion in the τ-p domain and ray tracing in both the τ-p and X-T domains. Arrivals from the Moho were detected on 17 of the ESPs. The depths to Moho determined for individual ESP interpretations have reproducibilities of ±0.1 km to ±3 km; in most cases the Moho depth has been determined to within ±1.5 km. Moho depths determined in this investigation represent a significant improvement over previous estimates of Moho along the margin from gravity data. Variations in present-day crustal thickness (measured from top of prerift basement to Moho) are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent South China Sea Basin. The ESP interpretations reveal that across the eastern portion of the south China margin, the crust appears to thin more or less continuously toward the continent-ocean boundary. In the west, ESP interpretations also show a general trend of seaward crustal thinning but, in addition, indicate at least two instances of focused, localized crustal thinning. Crustal velocities and the relative proportion of upper crust (VP 6.4 km/s) are used to identify areas of the south China margin with similar and contrasting crustal structures. Variations in these properties are believed to result primarily from contrasting, prerift crustal structure across the margin. However, magmatic underplating during rifting, depth dependent extension, and Pleistocene igneous intrusions may also have contributed to the variations in present crustal structure. Reliable information about variations in crustal thickness and velocity structure across and along the south China margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin.

Mesozoic magnetic lineations and the magnetic quiet zone off northwest Africa

An extensive compilation of recently acquired geophysical reconnaissance data has allowed the Mesozoic magnetic lineations (The Eastern Keathley sequence) to be identified and mapped in detail for the area off northwest Africa lying between Madeira and the Cape Verde Islands. These anomalies were generated as one limb of a symmetric spreading center (Paleo Mid-Atlantic Ridge) from about 107 to 153 m.y.B.P. Offsets in the lineation pattern serve to identify fracture zone traces whose trends are approximately east-west. The seaward boundary of the marginal quiet zone does not precisely define an isochron due to the presence of a variable width transition zone of intermediate amplitude magnetic anomalies. Crust underlying the marginal quiet zone was generated, at least in part, during the Jurassic, Graham normal polarity epoch. The quiet zone boundary is not offset significantly on opposite sides of the Canaries lineament as previously suggested. A possible counterpart of the U.S. east coast magnetic anomaly is observed in some areas near the shelf/slope break of Spanish Sahara and Mauritania. The presence of relatively high-amplitude (but not-correlatable) magnetic anomalies seaward of the Mesozoic sequence and presumably generated during the Cretaceous, Mercanton normal polarity epoch remains a paradox.

Gravity, heat flow, and seismic constraints on the processes of crustal extension : Northern margin of the South China Sea

Multichannel seismic data and gravity data have been used to construct crustal thickness profiles for three transects (eastern, central, western) across the rifted northern margin of the South China Sea. The present-day crustal configuration of the margin is then interpreted by modeling the effects of two end-member classes of extension processes, pure shear and simple shear. The applicability of each of these processes to the extension of the south China margin has been evaluated by comparing model predictions of subsidence and heat flow with observations across the margin. Neither of these end-member models satisfactorily fits the observed data on the eastern and central transects across the south China margin when typical values for standard input parameters are used; the resulting heat flow is significantly underestimated by both models. In the case of a pure shear model, heat flow observations may be matched either by assuming an uncommonly thin initial steady state lithospheric thickness (∼60 km) or by assuming an unusually large crustal radiogenic heat production within the original, unextended continental crust. A perhaps more reasonable alternative scenario presumes the existence of an initially slightly thinner than “normal” steady state lithosphere (thicknesses of ∼90–100 km) in conjunction with a significant amount of upper crustal radiogenic heat production. Such heat production could be accommodated by the presence of Cretaceous granitic bodies (hypothesized) within the basement beneath the south China margin. In the case of a simple shear model, however, the observed high heat flow on the rifted south China margin may only be matched if the steady state lithospheric thickness is assumed to be uncommonly thin (∼60 km). Because the observed geophysical data characterizing lithospheric extension may be matched using more realistic input parameters in the pure shear case, pure shear extension is preferred over simple shear extension as the dominant mechanism for explaining the large-scale rifting of the south China margin. For extension within the crust, however, combinations of both processes are not only possible, but probable, given published seismic evidence for through-going crustal faults on the south China margin.

Echo character of the East Brazilian continental margin and its relationship to sedimentary processes

Abstract The nature and regional distributions of various types of bottom echoes recorded on 3.5-kHz echograms from the East Brazilian continental margin (8–30°S) provide valuable information about sedimentary processes which have been active on a regional scale. The ten types of echoes observed fall into two major classes: distinct and indistinct. Indistinct echoes have two sub-classes; prolonged and hyperbolic. A qualitative correlation is observed between three types of distinct and indistinct-prolonged echoes and the relative abundance of coarse, bedded sediment (silt, sand, gravel) in piston cores. Regions returning distinct echoes with continuous parallel sub-bottoms contain little or no coarse sediment; regions returning indistinct very prolonged echoes with no sub-bottoms contain very high concentrations of coarse sediment; and regions returning indistinct semiprolonged echoes with intermittent sub-bottoms contain moderate or intermediate amounts of coarse sediment. Thus the regional distributions of these three echo types reflect the dispersal of coarse terrigenous sediment throughout the region. High concentrations of coarse sediment are restricted to relatively small areas which are generally proximal to large deep-sea channels, whereas very low concentrations occur in distal regions such as the lowermost continental rise and adjacent abyssal plain. Moderate concentrations of coarse sediment occur throughout most of the continental rise. Five of the six types of hyperbolic echoes observed are reflected from erosional/depositional bed forms. Although some of these bed forms (especially on the upper continental rise) have probably been produced by gravity-controlled mass flows (turbidity currents, slumps, etc.) the fact that the most extensive and widespread regions of hyperbolic echoes occur in distal regions beneath the present axis of flow of the Antarctic Bottom Water suggests that most of these bed forms are the result of sediment reworking by the contour-following bottom currents of this water mass.

A geophysical investigation of the Peru-Chile Trench☆

Abstract Twenty-six profiles of topography, free-air gravity anomaly, and total intensity magnetic anomaly across the Peru-Chile Trench are presented and discussed. The trench is obliterated as a topographic feature south of about 40°S, but a well-defined negative gravity belt and seismic reflection profiles indicate that the trench continues as a structural feature at least into the Drake Passage. The trench can be broadly divided into two main provinces, the main sediment-free province which extends from 8°S to 32°S and the sedimentary province which extends from 33°S to 57°S. Three other minor provinces are also defined. A free-air gravity anomaly map of the offshore area of the entire west coast of South America is presented. A secondary negative gravity belt is found that roughly parallels the prominent negative gravity belt associated with the trench. This secondary belt is thought to be a reflection of the Bolivar Geosyncline which lies very near the present coast of Colombia, Ecuador, and Peru. Studies incorporating seismic refraction and reflection measurements and gravity measurements indicate a pronounced crustal thinning beneath the offshore flank of the trench. Crustal thicknesses beneath the axis of the trench appear to be normal for the transition region of an ocean-continent margin. Seismic refraction measurements and continuous gravity observations are in close mutual agreement. It is suggested that the trench may originate by high angle normal faulting near the base of the continental slope accompanied by a downward flexure of the crust further offshore. The gravity data indicate that there are no radical variations in the crustal structure along the entire length of the western continental margin of South America. The magnetic anomalies cannot be correlated over large distances. The magnetic signature is considerably smoother over the shoreward flank of the trench than over the offshore flank and the trench axis roughly defines the boundary. This points to a sharp contrast in the magnetic state of the material underlying the two flanks of the trench. The relatively large amplitudes of some observed magnetic anomalies strongly suggest that they are not produced primarily by induced magnetization of bodies in the crust. In certain areas the observed anomalies correlate well with the basement topography. There is good evidence that some of these anomalies are caused by remanent magnetization of magnitude many times larger than the predicted induced magnetization and in a direction opposite to the earths present field.

The evolution of the Parece Vela Basin, eastern Philippine Sea

Abstract The Parece Vela Basin is a back-arc basin. It is approximately 5000 m deep and is divided into two topographic provinces by the north-trending Parece Vela Rift. The western province is thinly sedimented and topographically rough. The eastern province is blanketed by a thick apron of volcaniclastic sediments which were derived from the West Mariana Ridge. The Parece Vela Rift is composed of a series of discrete deeps and troughs with depths commonly of 6 km and locally exceeding 7 km. Petrologic and seismic refraction data indicate that the Parece Vela Basin is of oceanic character. Low-amplitude, nort-trending, lineated magnetic anomalies are present in the basin and appear symmetric about a line near the Parece Vela Rift. In the central latitudes of the basin seafloor spreading anomalies 10 (30 m.y. B.P.) to 5E or 5D (18 or 17 m.y. B.P.) can be identified. The uncertainty in identifying the youngest anomaly may be due to ridge jumps near the end of spreading. Spreading may have started slightly later in the northern end of the basin. Anomalies in the eastern province are disrupted and are difficult to correlate. DSDP results indicate post-spreading volcanism on the eastern side of the basin and this may have degraded the anomalies. The age obtained in the western province of the basin at DSDP Site 449 (∼25m.y. B.P.) is in close agreement with that obtained from the magnetic data (∼26m.y. B.P.). It is hypothesized that subduction was occurring at a west-dipping subduction zone east of the Palau-Kyushu Ridge in the Early Oligocene. This volcanic arc split about 31 or 32 m.y. ago and interarc spreading was initiated between the Palau-Kyushu Ridge (which then became a remnant arc) and the West Mariana Ridge. The Parece Vela Basin formed between the ridges by two-limb seafloor spreading. Spreading stopped about 17 or 18 m.y. ago. Like certain other marginal basins, the Parece Vela Basin is deeper than predicted from depth vs. age curves. The average heat flow for the Parece Vela Basin is in agreement with that predicted from heat flow vs. age curves. The origin of the Parece Vela Rift is unclear. It may represent the extinct spreading center or may be a postspreading feature.

Magnetic boundaries in the north atlantic ocean.

Magnetic boundaries parallel the continental slope and separate undisturbed from disturbed magnetic regions on both sides of the North Atlantic. The boundaries lie 2000 to 2500 kilometers from the axis of the mid-Atlantic ridge and roughly equidistant from it. The undisturbed zone, lying on the continental side of the boundaries, may reflect the long period of no reversals in magnetic polarity that occurred during the late Paleozoic.