Wavelet and multitaper coherence methods for assessing the elastic thickness of the Irish Atlantic margin

Abstract

S U M M A R YThere have been some inconsistencies in estimates of the effective elastic thickness of the continental lithosphere Te based upon admittance or coherence relationships between gravity and topography. This paper compares multitaper and wavelet methods to analyse the coherence between Bouguer gravity and bathymetric data over the Irish Atlantic margin. The analyses show that similar lateral Te variations can be recovered from the data, but demonstrate that the size of the data window can give rise to a significant downward bias in Te estimates. A seismically constrained 3-D gravity inversion over the Rockall basin shows the presence of surface and subsurface loads whose ratio is loosely correlated with load ratio variations generated from the wavelet coherence method. The Te and load ratio, f variations can be plausibly related to major geological structures on the margin. If the load ratio variations can be interpreted geologically, it implies that spectral based methods to estimate effective elastic thickness must incorporate subsurface loads within the underlying theoretical model.On the Irish Atlantic margin, Te is generally low (6-18 km) and is associated with a NE-SW Caledonian trend. The weakest lithosphere is in the southern Rockall basin, Porcupine bank and Porcupine basin and the strongest lithosphere is along the Rockall-Hatton region. The low Te values are consistent with results from other passive margins. The reasons for such low Te values on the Irish Atlantic margin remain unclear, but may be the consequence of Te being frozen into the lithosphere when loads were emplaced during continental breakup and temperature gradients were high. The process of sedimentation and the presence of fluids may be contributory factors. There is an indication of a geological and rheological divide between the Rockall-Hatton region and the Rockall basin, possibly associated with the Caledonian orogenic front.

Assessment of flexural analysis applied to the Sumatra-Java subduction zone

“…The equivalent topography is the height or depth the crust will assume in the absence of ice or water present and under isostatic conditions (Daly et al 2004) which is given by:…”

India–Madagascar paleo-fit based on flexural isostasy of their rifted margins

“…In order to avoid separate analyses and inversions on land and ocean areas, we scaled the sediment-corrected ocean bathymetry (h) to equivalent topography (h′) using the equation, h′ = (ρ c − ρ w )h / ρ c , where ρ c and ρ w are the densities of crust and water, respectively. The equivalent topography represents the bathymetry that one would assume if there were no water present under isostatic conditions (Daly et al, 2004;Kirby and Swain, 2008). Accordingly the derived equivalent topography for the WCMI (Fig.…”

“…Stern and Brink (1989) estimated a Te of~19 km in the Ross Sea where rifting occurred at about 60 Ma, whereas in the Valencia trough where there is a comparatively young rift age of 20 Ma, elastic thickness estimates are~5 km (Watts and Torné, 1992). Daly et al (2004) computed the elastic thickness of the Irish Atlantic margin using a multitaper coherence method between scaled bathymetry and Bouguer gravity and obtained Te values of~6-18 km. Wyer and Watts (2006) applied flexural back-stripping and gravity modeling techniques to calculate the gravity anomaly associated with rifting and sedimentation along the eastern continental margin of the USA.…”

Spatial variations of effective elastic thickness of the lithosphere in Central America and surrounding regions

“…Since this study focuses on spatial variations of re and its geody namic implications for Central America and surrounding regions, we do not discuss here the differences in absolute values between both maps produced by the methods, and the interested reader can find a more thorough comparison between the wavelet and multitaper methods in, for example, Daly et al (2004), Audet et al (2007), Perez-Gussinye et al (2007, 2009a, and Kirby and Swain (2011).…”