Steven C. Cande

Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic

Recently reported radioisotopic dates and magnetic anomaly spacings have made it evident that modification is required for the age calibrations for the geomagnetic polarity timescale of Cande and Kent (1992) at the Cretaceous/Paleogene boundary and in the Pliocene. An adjusted geomagnetic reversal chronology for the Late Cretaceous and Cenozoic is presented that is consistent with astrochronology in the Pleistocene and Pliocene and with a new timescale for the Mesozoic. The age of 66 Ma for the Cretaceous/Paleogene (K/P) boundary used for calibration in the geomagnetic polarity timescale of Cande and Kent (1992) (hereinafter referred to as CK92) was supported by high precision laser fusion Ar/Ar sanidine single crystal dates from nonmarine strata in Montana. However, these age determinations are now

A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the worlds ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine-scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Ariantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points

Revised magnetic polarity time scale for Late Cretaceous and Cenozoic time

A revision of the Heirtzler and others magnetic reversal time scale is presented. In addition to incorporating published studies which have increased the resolution and accuracy of their time scale, we have revised the relative lengths of anomalies 4A to 5 and 29 to 34 and have eliminated anomaly 14. We have calibrated the time scale by choosing an age of 3.32 m.y. B.P. for the older reversal boundary of anomaly 2A and 64.9 m.y. B.P. for the older reversal boundary of anomaly 29. The resulting magnetic reversal time scale is in reasonable agreement with the biostratigraphic ages from Deep Sea Drilling Project (DSDP) drill holes.

Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models

The oceanic Cocos Plate subducting beneath Costa Rica has a complex plate tectonic history resulting in segmentation. New lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the East Pacific Rise from lithosphere formed at the Cocos-Nazca spreading center. They also define two early phase Cocos-Nazca spreading regimes and a major propagator. In addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of Cocos Plate over the Galapagos hot spot. The subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab. Newly identified seafloor-spreading anomalies show oceanic lithosphere formed during initial breakup of the Farallon Plate at 22.7 Ma and opening of the Cocos-Nazca spreading center. A revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the Cocos-Nazca spreading center. At 19.5 Ma a major ridge jump reshaped its geometry, and after ∼14.5 Ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere. A suspected cause of ridge jumps is an interaction of the Cocos-Nazca spreading center with the Galapagos hot spot.

A plate-kinematic framework for models of Caribbean evolution

Abstract We define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the Caribbean region. Newly determined poles of rotation defining the approximate spreading histories of the central North and the South Atlantic oceans are given. For the late Jurassic-Early Cretaceous anomaly sequence of the central North Atlantic, we have used previously published ∗ definitions of fracture-zone traces and magnetic anomaly picks, redetermining the pole positions and angular rotations for various isochrons on an Evans and Sutherland interactive graphics system. For magnetic anomalies younger than the Cretaceous Quiet Period in both oceans, we (1) used Seasat altimeter data to help define fracture-zone traces, and (2) identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones. By the finite difference method, the relative paleopositions and the relative motion history between North and South America were computed. This analysis defines the size and shape (and the rate at which the size and shape changed) of the interplate region between North and South America since the Middle Jurassic. Thus, a plate-kinematic framework is provided for the larger plates pertaining to the Caribbean region, in which can be derived more detailed scenarios for Gulf of Mexico and Caribbean evolution. North and South America diverged to approximately their present relative positions from Late Triassic? to Early Campanian (about 84 m.y. ago) time. This is the period during which the Gulf of Mexico and a Proto-Caribbean seaway were formed. Since the Campanian, only minor relative motion has occurred; from Early Campanian through to Middle Eocene times. South America diverged only another 200 km, and since the Middle Eocene, minor N-S convergence has occurred. These very minor post-Early Campanian motions have probably been accommodated by imperfect shear and compression along the Atlantic fracture zones to the east of the Lesser Antilles, and along the northern and southern borders of the Caribbean Plate. Accordingly, it is suggested that from Campanian time to the present, the relative motions between the North and South American plates have had only minor effects on the structural development of the Caribbean region. Primarily using the data of Engebretson et al. ∗∗ , the convergence history of Pacific plates with North America was calculated for two points near the western Caribbean. By completing finite difference solutions, the convergence history of the Pacific plates with the Caribbean and South American plates can be approximated. The direction and rate of convergence of the Pacific plates with the Americas may have controlled the style of subduction and possible microplate migration along the North American, South American and western Caribbean boundaries that define the eastern Pacific plate margin.

Cenozoic motion between East and West Antarctica

The West Antarctic rift system is the result of late Mesozoic and Cenozoic extension between East and West Antarctica, and represents one of the largest active continental rift systems on Earth. But the timing and magnitude of the plate motions leading to the development of this rift system remain poorly known, because of a lack of magnetic anomaly and fracture zone constraints on seafloor spreading. Here we report on magnetic data, gravity data and swath bathymetry collected in several areas of the south Tasman Sea and northern Ross Sea. These results enable us to calculate mid-Cenozoic rotation parameters for East and West Antarctica. These rotations show that there was roughly 180 km of separation in the western Ross Sea embayment in Eocene and Oligocene time. This episode of extension provides a tectonic setting for several significant Cenozoic tectonic events in the Ross Sea embayment including the uplift of the Transantarctic Mountains and the deposition of large thicknesses of Oligocene sediments. Inclusion of this East–West Antarctic motion in the plate circuit linking the Australia, Antarctic and Pacific plates removes a puzzling gap between the Lord Howe rise and Campbell plateau found in previous early Tertiary reconstructions of the New Zealand region. Determination of this East–West Antarctic motion also resolves a long standing controversy regarding the contribution of deformation in this region to the global plate circuit linking the Pacific to the rest of the world.

A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica

We propose that magnetic anomalies south of Australia and along the conjugate margin of Antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34. The original anomaly identification has two troublesome aspects: (1) it does not account for an “extra” anomaly between anomalies 20 and 21, and (2) it provides no explanation for the rough topography comprising the Diamantina Zone. With our revised identification there is no “extra” anomaly and the Diamantina Zone is attributed to a period of very slow spreading (∼4.5mm/yr half rate) between 90 and 43 m.y. The ages bounding the interval of slow spreading (90 and 43 m.y.) correspond to times of global plate reorganizations. Our revised identification opens up the possibility that part of the magnetic quiet zone south of Australia formed during the Cretaceous long normal polarity interval. Breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P. The “breakup unconformity” identified by Falvey in the Otway Basin may correspond to a eustastic sea level change.

Geophysics of the Pitman Fracture Zone and Pacific-Antarctic Plate Motions During the Cenozoic

Multibeam bathymetry and magnetometer data from the Pitman fracture zone (FZ) permit construction of a plate motion history for the South Pacific over the past 65 million years. Reconstructions show that motion between the Antarctic and Bellingshausen plates was smaller than previously hypothesized and ended earlier, at chron C27 (61 million years ago). The fixed hot-spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early Tertiary, either between East Antarctica and West Antarctica or between the North and South Pacific. A plate reorganization at chron C27 initiated the Pitman FZ and may have been responsible for the other right-stepping fracture zones along the ridge. An abrupt (8°) clockwise rotation in the abyssal hill fabric along the Pitman flowline near the young end of chron C3a (5.9 million years ago) dates the major change in Pacific-Antarctic relative motion in the late Neogene.

Magnetic evidence for slow seafloor spreading during the formation of the Newfoundland and Iberian margins

There is considerable debate concerning the nature and origin of the thin crust within the ocean^continent transition (OCT) zones of many passive non-volcanic continental margins, located between thinned continental and true oceanic crust. This crust is usually found to be underlain by upper mantle material of 7.2^7.4 km/s velocity at shallow depths (1^2 km). It has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading. One of the examples of occurrence of such a crust are the conjugate margins of Newfoundland and Iberia. Here we present an interpretation of magnetic data from these regions to show that their OCT zones are underlain by crustal material formed by slow seafloor spreading (6.7 mm/yr) soon after Iberia separated from the Grand Banks of Newfoundland in the late Jurassic. Similarities in the magnetic anomalies and velocity distributions from these regions with those from the Sohm Abyssal Plain, a region lying immediately south of the Newfoundland Basin and formed by seafloor spreading at a similar rate of spreading, give further support to such an interpretation. The idea that these regions were formed by unroofing of upper mantle during rifting of Iberia from Newfoundland may be likely but the presence of weak magnetic anomalies in these regions, which bear all the characteristics of seafloor spreading anomalies, makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading. The similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the North Atlantic and that of the Labrador Sea suggest that this OCT velocity structure may be the norm rather than the exception across those passive non-volcanic margins where the initial seafloor spreading was slow. Furthermore, the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges. fl 2000 Elsevier Science B.V. All rights reserved.

Southeast Pacific tectonic evolution from Early Oligocene to Present

Plate tectonic reconstructions of the Nazca, Antarctic, and Pacific plates are presented from late Oligocene to Present. These reconstructions document major plate boundary reorganizations in the southeast Pacific at dirons 6C (24 Ma), 6(o) (20 Ma), and 5A (12 Ma) and a smaller reorganization at chron 3(o) (5 Ma). During the chron 6(o) reorganization it appears that a ridge propagated into crust north of the northernmost Pacific-Antarctic Ridge, between the Chiloe fracture zone (FZ) of the Chile ridge and Agassiz FZ of the Pacific-Nazca ridge, which resulted in a northward jump of the Pacific-Antarctic-Nazca (PAC-ANT-NAZ) mid-ocean triple junction. During the chron 5A reorganization the Chile ridge propagated northward from the Valdivia FZ system to the Challenger FZ, through lithosphere formed roughly 5 Myr earlier at the Pacific-Nazca ridge. During this reorganization a short-lived microplate (the Friday microplate) existed at the PAC-ANT-NAZ triple junction. The PAC-ANT-NAZ triple junction jumped northward 500 km as a result of this reorganization, from a location along the Valdivia FZ to a location along the Challenger FZ. The chron 5A reorganization also included a change in spreading direction of the Chile and Pacific-Antarctic ridges. The reorganization at chron 3(o) initiated the formation of the Juan Fernandez and Easter microplates along the East Pacific rise. The manner of plate boundary reorganization at chron 6(o) and chron 5A (and possibly today at the Juan Fernandez microplate) included a sequence of rift propagation, transfer of lithosphere from one plate to another, microplate formation, and microplate abandonment and resulted in northward migration of the PAC-ANT-NAZ triple junction. The associated microplate differs from previously studied microplates in that there is no failed ridge.