R. D. Müller

Kinematics of the South Atlantic rift

Abstract. The South Atlantic rift basin evolved as a branch of a large Jurassic–Cretaceous intraplate rift zone between the African and South American plates during the final break-up of western Gondwana. While the relative motions between South America and Africa for post-break-up times are well resolved, many issues pertaining to the fit reconstruction and particularly the relation between kinematics and lithosphere dynamics during pre-break-up remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-break-up evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight-fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African Rift Zones, we have been able to indirectly construct the kinematic history of the pre-break-up evolution of the conjugate west African–Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic pre-salt sag basin and the Sao Paulo High. We model an initial E–W-directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities from 140 Ma until late Hauterivian times (≈126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ≈14 Myr-long stretching episode the pre-salt basin width on the conjugate Brazilian and west African margins is generated. An intermediate stage between ≈126 Ma and base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE–SW. From base Aptian onwards diachronous lithospheric break-up occurred along the central South Atlantic rift, first in the Sergipe–Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final break-up between South America and Africa occurred in the conjugate Santos–Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piaui-Ceara margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of these conjugate passive-margin systems and can explain the first-order tectonic structures along the South Atlantic and possibly other passive margins.

An open-source software environment for visualizing and refining plate tectonic reconstructions using high-resolution geological and geophysical data sets

We describe a powerful method to explore spatio-temporal relationships within geological and geophysical data sets by analyzing the data within the context of tectonic reconstructions. GPlates is part of a new generation of plate reconstruction software that incorporates functionality familiar from GIS software with the added dimension of geological time. Here we use GPlates to reconstruct geological terranes, geophysical grids, and paleomagnetic data within alternative tectonic models of the assembly of Western Australia and the configuration of Rodinia. With the ability to rapidly visualize a diverse range of geological and geophysical constraints within different reconstructions, users can easily investigate the implications of different tectonic models for reconciling a variety of observations and make more informed choices between different models and data.

Mid-Cretaceous seafloor spreading pulse: Fact or fiction?

Two main hypotheses compete to explain the mid-Cretaceous global sea-level highstand: a massive pulse of oceanic crustal production that occurred during the Cretaceous Normal Superchron (CNS) and the “supercontinent breakup effect,” which resulted in the creation of the mid-Atlantic and Indian ocean ridges at the expense of subducting old ocean floor in the Tethys and the Pacific. We have used global oceanic paleo-age grids, including now subducted ocean floor and two alternative time scales, to test these hypotheses. Our models show that a high average seafloor spreading rate of 92 mm/a in the Early Cretaceous that decreased to 60 mm/a during the Tertiary, with peaks of 86 mm/a and 70 mm/a at 105 Ma and 75 Ma ago, respectively, correspond to the two observed sea-level highstands in the Cretaceous. Calculations using GTS2004 produce lower seafloor spreading rates during the same period and diminish the mid-Cretaceous spreading pulse. Global ridge lengths increased in the earliest Cretaceous but stayed relatively constant through time. However, we find that the average age of the ocean basins through time is only weakly dependent on the choice of time scale. The expansive mid- and Late Cretaceous epicontinental seas, coupled with warm climates and oxygen-poor water masses, were ultimately driven by the younger average age of the Cretaceous seafloor and faster seafloor spreading rather than a vast increase in mid-ocean ridge length due to the breakup of Pangea or solely by higher seafloor spreading rates, as suggested previously.

Late Jurassic rifting along the Australian North West Shelf: margin geometry and spreading ridge configuration

The Argo and Gascoyne Abyssal plains in the easternmost Indian Ocean document the last stages of eastern Tethys evolution before the breakup of eastern Gondwana. Thus they provide crucial information not only for modelling the evolution of the eastern Tethys and Proto-Indian Ocean, but also to understand the complex geodynamic history of the North West Shelf. We have revisited the marine magnetic anomaly record of the Argo and Gascoyne Abyssal Plains in combination with other geological and geophysical data from the North West Shelf and southeast Asia. Based on the combined data, we have created a revised plate-tectonic model and a set of palaeogeographic reconstructions for the evolution of the North West Shelf for the early stages after the breakup. The main difference between this model and previously published models is that we have interpreted a complete section of anomalies, M25A – M22A, in the Gascoyne Abyssal Plain, northwest of the Exmouth Plateau. The magnetic anomalies have the same trend as in the Argo Abyssal Plain. Our new plate-tectonic reconstructions show that continental breakup in the Argo and northern Gascoyne Abyssal Plains, east and northwest of the Exmouth Plateau, respectively, started simultaneously in the Oxfordian with M25A identified as the oldest anomaly. In the Gascoyne Abyssal Plain, the oldest anomaly sequence, M25A – M22A (154.5 – 150.4 Ma) indicates that the ‘Argo’ spreading ridge continued around the northern margin of Greater India, and was probably linked with the Somali Basin. Sea-floor spreading continued until M14, separating the West Burma Block and possibly other smaller continental fragments like the Sikuleh Terrane of Western Sumatra from the northern Australian margin. A southward-directed ridge jump at M13 (134 Ma) transferred segments of Australian Plate oceanic crust to the West Burma Plate. Contemporaneously, an anticlockwise change in spreading direction fixed the West Burma Block relative to Greater India until its collision with the southern Eurasian margin.

Cause and evolution of intraplate orogeny in Australia

ABSTRACTIntraplate orogeny remains an enigmatic process in terms of plate tectonics. Regional continental stress fi elds agree well with stress orientations predicted by the main forces driv-ing and resisting plate motion. However, regional continental stress fi elds fail to explain major localized compressive plate deformation in continental interiors far away from plate bound-aries. Global or plate-wide intraplate stress models typically assume a laterally homogeneous plate rheology and are confi ned to modeling the present, without addressing the antiquity of the current stress fi eld. An understanding of intraplate stress and deformation requires the linking of spatial variations in continental rheology with time-dependent plate geometry and driving forces. Here we demonstrate how the complex interplay between juxtaposed weak and strong geological provinces and changes in far-fi eld plate boundary forces has caused intra-plate orogenesis and tectonic reactivation in southeastern Australia during the Tertiary. Our fi ndings are contrary to the opinion that continental interiors are insensitive to compression at plate margins, and help to explain the mechanisms causing intraplate orogenesis.Keywords:

Middle Miocene tectonic boundary conditions for use in climate models

Utilizing general circulation models (GCMs) for paleoclimate study requires the construction of appropriate model boundary conditions. We present a middle Miocene paleotopographic and paleobathymetric reconstruction geographically constrained at 15 Ma for use in GCMs. Paleotopography and paleogeography are reconstructed using a published global plate rotation model and published geological data. Paleobathymetry is reconstructed through application of an age-depth relationship to a middle Miocene global digital isochron map, followed by the overlay of reconstructed sediment thickness and large igneous provinces. Adjustments are subsequently made to ensure our reconstruction may be utilized in GCMs.

Australian paleo-stress fields and tectonic reactivation over the past 100 Ma

Even though a multitude of observations suggest time-dependent regional tectonic reactivation of the Australian Plate, its large-scale intraplate stress field evolution remains largely unexplored. This arises because intraplate paleo-stress models are difficult to construct, and that observations of tectonic reactivation are often hard to date. However, because the Australian plate has undergone significant changes in plate boundary types and geometries since the Cretaceous, we argue that even simple models can provide some insights into the nature and timing of crustal reactivation through time. We present Australian intraplate stress models for key times from the Early Cretaceous to the present, and link them to geological observations for evaluating time-dependent fault reactivation. We focus on the effect time-dependent geometries of mid-ocean ridges, subduction zones and collisional plate boundaries around Australia have on basin evolution and fault reactivation through time by reconstructing tectonic plates, restoring plate boundary configurations, and modelling the effect of selected time-dependent plate driving forces on the intraplate stress field of a rheologically heterogeneous plate. We compare mapped fault reactivation histories with paleo-stress models via time-dependent fault slip tendency analysis employing Coulomb-Navier criteria to determine the likelihood of strain in a body of rock being accommodated by sliding along pre-existing planes of weakness. This allows us to reconstruct the dominant regional deformation regime (reverse, normal or strike-slip) through time. Our models illustrate how the complex interplay between juxtaposed weak and strong geological plate elements and changes in far-field plate boundary forces have caused intraplate orogenesis and/or tectonic reactivation in basins and fold belts throughout Australia.

Early to Middle Miocene monsoon climate in Australia

The present-day Australian monsoon delivers substantial moisture to the northern regions of a predominantly arid continent. However, the pre-Quaternary history of the Australian monsoon is poorly constrained due to sparse and often poorly dated paleoclimate proxy evidence. Sedimentological and paleontological data suggest that warm, humid, and seasonal environments prevailed in central and north Australia during the Miocene, though it is unclear whether these were products of the Australian monsoon. We perform a series of sensitivity experiments using an atmospheric general circulation model, combined with an offl ine equilibrium vegetation model, to quantitatively constrain the areal extent of the Miocene monsoon. Our results suggest a weaker than modern monsoon climate during the Miocene. This result is insensitive to atmospheric CO 2 , although somewhat sensitive to vegetation interactions and the presumed distribution of inland water bodies. None of our Miocene experiments exhibit precipitation rates greater than modern over north Australia, in disagreement with paleoclimate record interpretations. Vegetation modeling indicates that inferred precipitation values from fossil fl ora and fauna could only support Miocene vegetation patterns if atmospheric CO 2 was twice the modern concentration. This suggests that elevated CO 2 was critical for sustaining Miocene vegetation.

Circum‐Arctic mantle structure and long‐wavelength topography since the Jurassic

The circum-Arctic is one of the most tectonically complex regions of the world, shaped by a history of ocean basin opening and closure since the Early Jurassic. The region is characterized by contemporaneous large-scale Cenozoic exhumation extending from Alaska to the Atlantic, but its driving force is unknown. We show that the mantle flow associated with subducted slabs of the South Anuyi, Mongol-Okhotsk, and Panthalassa oceans have imparted long-wavelength deflection on overriding plates. We identify the Jurassic-Cretaceous South Anuyi slab under present-day Greenland in seismic tomography and numerical mantle flow models. Under North America, we propose the “Farallon” slab results from Andean-style ocean-continent convergence around ~30°N and from a combination of ocean-continent and intraoceanic subduction north of 50°N. We compute circum-Arctic dynamic topography through time from subduction-driven convection models and find that slabs have imparted on average <1–16 m/Myr of dynamic subsidence across the region from at least 170 Ma to ~50 Ma. With the exception of Siberia, the main phase of circum-Arctic dynamic subsidence has been followed either by slowed subsidence or by uplift of <1–6 m/Myr on average to present day. Comparing these results to geological inferences suggest that subduction-driven dynamic topography can account for rapid Middle to Late Jurassic subsidence in the Slave Craton and North Slope (respectively, <15 and 21 m/Myr, between 170 and 130 Ma) and for dynamic subsidence (<7 m/Myr, ~170–50 Ma) followed by dynamic uplift (<6 m/Myr since 50 Ma) of the Barents Sea region. Combining detailed kinematic reconstructions with geodynamic modeling and key geological observations constitutes a powerful tool to investigate the origin of vertical motion in remote regions.

Seawater chemistry driven by supercontinent assembly, breakup, and dispersal

Global oceans are known to have alternated between aragonite and calcite seas. These oscillations refl ect changes in the Mg/Ca ratios of seawater that control biomineralization and the composition of marine carbonates, and are thought to be caused mainly by the time dependence of crustal accretion at mid-ocean ridge crests and the associated high-temperature mid-ocean ridge fl uid fl ux. Here we use global ocean basin reconstructions to demonstrate that the fl uctuations in hydrothermal ocean inputs are instead caused by the gradual growth and destruction of mid-ocean ridges and their relatively cool fl anks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge fl ank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fl uid fl ux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcifi cation from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their fl anks along the Pacifi c rim.