The African continental divide: Indian versus Atlantic Ocean spreading during Gondwana dispersal

ABSTRACT It is well established that plate-tectonic processes operate on a global scale and that spatially separate but temporally coincident events may be linked. However, identifying such links in the geological record and understanding the mechanisms involved remain speculative. This is particularly acute during major geodynamic events, such as the dispersal of supercontinents, where multiple axes of breakup may be present as well as coincidental collisional events. To explore this aspect of plate tectonics, we present a detailed analysis of the temporal variation in the mean half rate of seafloor spreading in the Indian and Atlantic Oceans, as well as plate-kinematic attributes extracted from global plate-tectonic models during the dispersal of Gondwana since ca. 200 Ma. Our analysis shows that during the ~20 m.y. prior to collision between India and Asia at ca. 55 Ma, there was an increase in the mean rate of seafloor spreading in the Indian Ocean. This manifests as India rapidly accelerating toward Asia. This event was then followed by a prompt deceleration in the mean rate of Indian Ocean seafloor spreading after India collided with Asia at ca. 55 Ma. Since inception, the mean rate of seafloor spreading in the Indian Ocean has been generally greater than that in the Atlantic Ocean, and the period of fastest mean half spreading rate in the Indian Ocean was coincident with a slowdown in mean half seafloor spreading rate in the competing Atlantic Ocean. We hypothesize that faster and hotter seafloor spreading in the Indian Ocean resulted in larger ridge-push forces, which were transmitted through the African plate, leading to a slowdown in Atlantic Ocean spreading. Following collision between India and Asia, and a slowdown of Indian Ocean spreading, Atlantic spreading rates consequently increased again. We conclude that the processes in the Indian and Atlantic Oceans have likely remained coupled throughout their existence, that their individual evolution has influenced each other, and that, more generally, spreading in one basin inevitably influences proximal regions. While we do not believe that ridge push is the main cause of plate motions, we consider it to have played a role in the coupling of the kinematic evolution of these oceans. The implication of this observation is that interaction and competition between nascent ocean basins and ridges during supercontinent dispersal exert a significant control on resultant continental configuration.

From crisis to normal science, and back again: Coming full “Kuhn cycle” in the career of Warren B. Hamilton

ABSTRACT In this paper, I use Thomas S. Kuhn’s model of scientific change to frame a brief, broad-brushed biographical sketch of the career of Warren B. Hamilton. I argue that Hamilton’s career can usefully be interpreted as encompassing a full “Kuhn cycle,” from a period of crisis in his early work, to one of normal science in midcareer, and back to something resembling crisis in his later research. Hamilton entered the field around mid-twentieth century when earth science can plausibly be described as being in a period of crisis. The then dominant fixist paradigm was facing an increasing number of difficulties, an alternative mobilist paradigm was being developed, and Hamilton played an important role in its development. The formulation of plate tectonics in the 1960s saw the overthrow of the fixist paradigm. This inaugurated a new phase of normal science as scientists worked within the new paradigm, refining it and applying it to different regions and various geological phenomena. Hamilton’s midcareer work fits largely into this category. Later, as the details of the plate-tectonic model became articulated more fully, and several of what Hamilton perceived as weakly supported conjectures became incorporated into the paradigm, problems began again to accumulate, and earth science, in Hamilton’s estimation, entered a new period of crisis. Radically new frameworks were now required, and Hamilton’s later work was dedicated principally to developing and articulating these frameworks and to criticizing mainstream views.

The role of the pre-Alpine polycrystalline basement in the paleogeographic configuration of multiple Neotethyan oceanic basins

The study provides a deeper understanding of the early Mesozoic paleogeogeographic spatial-temporal relationship by studying the two Adria-Europe intervening basement blocks. The Drina-Ivanjica and Pelagonian crustal fragments play important role in the internal early Alpine oceanic constitution further controlling the late Jurassic emplacement of Tethyan Dinaric-Hellenic ophiolites. The proposed paleogeographic reassessment is driven by the new paleocontinental inheritance data associated with the Variscan – pre-Variscan basement terranes. The recently published data suggest an Avalonian-type inheritance of the Pelagonian basement block which indicates a different pre-Variscan plate-tectonic journey, including separate spatial arrangement during Variscan amalgamation. In turn, Cadomian-type basement inheritance has been documented within the sliced Adria microplate. Thus, the Avalonian inheritance place the Pelagonian block away from the Apulia/Adria (Dinarides). In the investigated context of Paleozoic-Mesozoic paleogeographic transition, the Pelagonian block may represent a segment of the Cimmerian ribbon continent or southernmost segment of the Variscan Europe. With regards the nearby Adria microplate, a Triassic-Jurassic oceanic opening led to the decoupling (spreading away from the main Adria microplate) of the Drina-Ivanjica block. The rifting is in line with the simultaneous yet opposite or westward-directed drift of the Pelagonides. The breakup of south European Variscan configuration eventually result in the spatial alignment of the two basement fragments referred to as the “Drina–Pelagonide continental splinter”. By linking the paleogeographic pre-Jurassic–Jurassic relationship between these continental units, the two landlocked Neotethyan Vardar s.l. basins are extrapolated, “Dinaric Tethys” / Inner Dinaric-(Mirdita-Pindos) and the main Vardar Ocean (Western Vardar Zone).

Seismic reflections from a lithospheric suture zone below the Archaean Yilgarn Craton

Seismic reflectors in the uppermost mantle, which can indicate past plate tectonic subduction, are exceedingly rare below Archaean cratons, and restricted to the Neoarchaean. Here we present reprocessed seismic reflection profiles from the northwest Archaean Yilgarn Craton and the Palaeoproterozoic Capricorn Orogen of western Australia that reveal the existence of a ~4 km thick south-dipping band of seismic reflectors that extends from the base of the Archaean crust to at least 60 km depth. We interpret these reflectors, which lie south of a ~50 km deep crustal root, as a relict suture zone within the lithosphere. We suggest that the mantle reflectors were created either by subduction of an oceanic plate along the northern edge of the Yilgarn Craton, which started in the Mesoarchaean and produced the rocks in northern Yilgarn greenstone belts that formed in a supra-subduction zone setting, or, alternatively, by underthrusting of continental crust deep into the lithosphere during the Palaeoproterozoic.

Thermochronologic constraints on the origin of the Great Unconformity

The origin of the phenomenon known as the Great Unconformity has been a fundamental yet unresolved problem in the geosciences for over a century. Recent hypotheses advocate either global continental exhumation of more than 3–4 km during Cryogenian (717–635 Ma) snowball Earth glaciations, or alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1000–540 Ma) due to plate tectonic reorganization from supercontinent Rodinia assembly and breakup. To test these hypotheses, the temporal pattern of Neoproterozoic thermal histories were evaluated for four North American locations using previously published medium-to-low temperature thermochronology and geologic information. We present inverse time-temperature simulations within a Bayesian modelling framework that record a consistent signal of relatively rapid, high magnitude cooling of ~120–200°C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ~3–5 km of unroofing during snowball Earth glaciations, but also demonstrate that plate tectonic drivers, with the potential to cause both exhumation and burial, may have significantly influenced the thermal history in regions that were undergoing deformation concomitant with glaciation. In the cratonic interior, however, glaciation remains the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion revealed by our thermochronological inversions. To obtain a full picture of the extent and synchroneity of such erosional exhumation, studies on stable cratonic crust below the Great Unconformity must be repeated on all continents.

Supplemental Material: Late Cenozoic tephrochronology of the Mount Diablo area within the evolving plate-tectonic boundary zone of northern California

Terminology relating to tephra and tephra layer nomenclature, methods of sampling tephra in the field, laboratory treatment of tephra samples for analysis, methods of chemical analysis of tephra and radiometric dating (40Ar/39Ar), and methods of data evaluation<br>