A. G. Crosby

Accurate measurements of residual topography from the oceanic realm

In the oceans, our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped. These anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle. Here we analyze a global database of seismic reflection and wide-angle profiles from heavily sedimented oceanic crust, which abuts continental lithosphere. At 449 locations, we calculated water-loaded subsidence, compared it with a reference age-depth relationship, and determined residual depth. We then combined these spot measurements of residual depth with observations from mid-oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth. Our results suggest that the amplitude of residual depth varies by up to ±1 km with wavelengths of order 103 km. We compare our residual depths with free-air gravity and seismic tomographic anomalies. Our results show that residual depths correlate with long-wavelength gravity anomalies. In contrast, correlations between residual depths and vertically averaged shear velocity anomalies within the upper and/or the lower mantle are weaker. The largest discrepancies occur at short (∼1000 km) wavelengths. These combined observations suggest that residual depth anomalies could be generate by density variations within a thin (∼102 km) low-viscosity layer beneath the lithosphere. Our global compilation should play a significant role in helping to refine predictive geodynamical models.

Self-consistent strain rate and heat flow modelling of lithospheric extension: application to Newfoundland-Iberia conjugate margins

ABSTRACT The formation of highly extended continental margins is much debated and it remains unclear whether these margins form by uniform or depth-dependent stretching of lithosphere. The existence and form of depth dependency at deep-water margins is of considerable importance to hydrocarbon exploration. Recent discussion has centred on the amagmatic Newfoundland-Iberia conjugate margins where dense seismic wide-angle and deep reflection surveying has yielded well-resolved images of the crust and lithospheric mantle. We have tackled the problem of depth-dependency in three steps. First, we have reconstructed water-loaded subsidence histories by making simple assumptions about changes in water depth through time. Secondly, we have used these reconstructed subsidence histories to determine the spatial and temporal variation of lithospheric strain rate. Crucially, we make no prior assumptions about the existence and form of depth dependency. Inverse modelling has yielded excellent fits to both reconstructed subsidence and crustal thickness observations. Strain rate distributions are depth-dependent, suggesting that lithospheric mantle thins over a wider region than the crust. In order to avoid generating substantial volumes of syn-rift melt, crustal strain rates greatly exceed lithospheric mantle strain rates beneath highly-extended parts of the crust. These strain rate distributions control the growth and heat flow history of the conjugate margins through space and time.