ALBERT

Description: The reconstructed positions of many of the Indian Ocean intraplate volcanic features are inconsistent with the present positions of the hotspots to which they have been ascribed. The discrepancy can be explained by the past motion of the hotspots themselves, and in the errors introduced in an absolute plate model based on fixed hotspots. Previous models for the motion of hotspots in the Indian Ocean (e.g. [Steinberger and O’Connell, in: The History and Dynamics of Global Plate Motions, 2000, pp. 377–398, and Geophys. J. Int. 132 (1998) 412–434; Antretter et al., Earth Planet. Sci. Lett. 203 (2002) 635–650]) have had success in matching paleolatitudes of the Kerguelen hotspot, but not the Marion or Reunion hotspots. We calculate the motion of the Indian Ocean hotspots using a method described in Steinberger and O’Connell (op. cit.), employing a plate motion model revised using identification of Mesozoic magnetic anomalies in the Enderby Basin [Gaina et al., in: The Evolution and Dynamics of the Australian Plate, in press]. We find a motion of not, vert, similar7° south for the Kerguelen plume, similar to previous models [Antretter et al., op. cit.]. This motion alleviates the discrepancies in the position of the Kerguelen plume and its purported involvement in the formation of the Ninetyeast Ridge, Rajmahal Traps, Bunbury basalts and assorted volcanics of the Western Australian margin. We find a motion of 5° north and 7° south for the Reunion and Marion plumes, respectively. The motion of the Reunion hotspot is consistent with paleolatitude estimates. The apparent fixity of the Marion hotspot [Torsvik et al., Earth Planet. Sci. Lett. 164 (1998) 221–232] is not well-constrained, but may be due to the combination of hotspot motion and true polar wander acting in opposing directions. We present revised absolute finite rotations for the Indian plate for the last 65 Ma based on the motion of these hotspots, which place India farther north in the past than for fixed hotspot models.

Description: It is widely accepted that substantial relative motion has occurred between the Indo-Atlantic and Pacific hot spots since the Late Cretaceous. At the same time, a fixed Indo-Atlantic hot spot reference frame has been argued for and used since the advent of plate tectonics, implying relatively little motion between the hot spots in this domain since about 130 Ma. Most plumes purported to have caused these hot spots, while being advected in the global-scale mantle flow field, are assumed to move an order of magnitude more slowly than plates. However, the lifetime of a plume may be over ∼100 Myr, and the integrated motion of a plume is expected to be significant over these times. The uncertainties inherent in hot spot reconstructions are of a magnitude similar to the expected plume motion, and so any differences between a fixed and moving frame of reference must be discernible beyond the level of these uncertainties. We present a method for constraining hot spot reconstruction uncertainties, similar to that in use for relative plate motion. We use a modified Hellinger criterion of fit for the hot spot problem, using track geometries and radiometric dating, and derive covariance matrices for our Indo-Atlantic rotations for the last 120 Myr. However, any given mantle convection model introduces additional uncertainties into such models, based on its model parameters and starting conditions (e.g., choice of global tomography model, viscosity profile, nature of mantle phase transitions). We use an interactive evolutionary approach, where we constrain the hot spot motion resulting from convection models to fit paleomagnetic constraints, and converge on an acceptable motion solution by varying unknowns over several generations of simulations. Our hot spot motion model shows large motion (5–10°) of the Indo-Atlantic hot spots for times 〉80 Ma, consistent with available paleomagnetic constraints. The differences between the fixed and moving hot spot reference frames are not discernible over the level of uncertainty in such rotations for times 〈80 Ma.

Description: Mantle induced dynamic topography may have a significant effect on the accomodation space in sedimentary basins, especially close to “slab burial grounds”, but its magnitude and relevance is controversial. We use five Cenozoic stratigraphic sections and 56 wells from the northern South China Sea margin to assess all mechanisms contributing to Late Tertiary tectonic subsidence in this area and to ground-truth the dynamic topography output of a mantle convection model. The computed dynamic topography decreases on average by about 300 m from 20 Ma to the present in the northern South China Sea area. Post-rift anomalous subsidence is computed as the discrepancy between observed and predicted post-rift subsidence based on modeling exponential thermal cooling following earlier lithospheric thinning events. In shallow water areas its magnitude is similar to that predicted by the mantle convection model, confirming that the change in dynamic topography through time since 20 Ma agrees well with subsidence unaccounted for by a conventional rift model. However, this mechanism does not explain the rapid increase of anomalous subsidence towards the deep water areas in the Qiongdongnan and Pearl River Mouth basins. In the deep water (〉500 m) part of the Qiongdongnan and Pearl River Mouth basins, post-rift anomalous subsidence ranges from 900 to 1200 m, whereas in the continental shelf area of the basins it ranges from 700 to 300 m and decreases landwards, to less than 250 m in the Beibuwan Basin. This rapid subsidence event, preceded by minor uplift, is interpreted as a thermal cooling episode induced by a late magmatic event. In the Yinggehai Basin anomalous subsidence during the post-rift stage is about 300–500 m in the northwestern part of the basin, substantially less than that in the middle and southeastern parts, where it ranges from 900 to 1200 m. The distinct subsidence anomaly in the Yinggehai Basin may originate from a combination of the dextral movement of marginal faults since the Late Miocene and the effect of dynamic topography.