Rifted Margin Crustal Architecture and Magmatic Type from Time-Domain Seismic Reflection Data Using the Warner 10 second Moho TWTT Rule: A New First-Approach Methodology

We present a new first-approach methodology, applicable to thermally re-equilibrated rifted margins, to determine margin crustal architecture and magmatic type from the TWTT of top basement of time-domain seismic reflection data. The method invokes Warner’s 10s Moho rule (Warner, 1987) to give the TWTT crustal basement thickness from top basement TWTT from which we determine crustal basement thickness. It does not require the Moho to be seismically imaged or sediment thickness to be known.

Determining rifted margin crustal thickness and assessing whether a margin is magma-normal, magma-starved or magma-rich is fundamental to understanding margin structure and formation processes. This is often a difficult task compounded by the absence of clear or unambiguous seismic Moho.

Warner observed that, for a thermally re-equilibrated margin, the Moho seismic reflection is approximately flat at ~10s TWTT and is constant irrespective of the complexity of geology above. Moho TWTT is at 10s for unthinned continental crust, oceanic crust, and for crust in between, and applies equally to magma-rich, magma-starved and magma-normal rifted margins.

We apply the new methodology using Warner’s 10s Moho rule to map crustal basement thickness for the Campos and Santos rifted margins offshore Brazil from TWTT of top basement observed on seismic reflection data. We show that the resulting map of crustal thickness determined from top basement TWTT shows a good correlation with that determined using gravity inversion.

Modelling shows that different magmatic-margin types have distinct shapes of top basement TWTT that is independent of sediment thickness. The lateral transition from downward-sloping to flat top basement TWTT corresponds to the oceanward taper of thinned continental crust to boxed-shaped oceanic crust, providing an estimate of the landward-limit of oceanic crust (LaLOC). Magma-starved margins show a step-up of top basement TWTT onto oceanic crust. For margins with magma, lateral inflections in the TWTT of base sediment provide information of the onset of magmatic-volcanic addition and the formation of hybrid crust consisting of thinned continental crust plus new magmatic crust. For magma-normal margins this lateral inflections of TWTT corresponds to the start of deep-water volcanics (SDRs) at 6-7s TWTT. For magma-rich margins (with sub-aerially erupted volcanic SDRs) this TWTT inflection occurs at 2-3s.

We interpret the top basement TWTT profiles on the Southern Campos Margin to indicate a slightly magma-poor margin. The thinnest crust occurs between thinned continental crust and normal-thickness oceanic crust, consistent with a simple isostatic model where maximum decompression melting to form oceanic crust does not occur until after continental crust separation.

On the SW Santos Margin, we interpret the top basement TWTT profiles to indicate a slightly magma-rich margin. A broad region separates the end of the crustal thinning taper and the LaLOC. A simple isostatic model can generate this top basement TWTT shape as a broad region of hybrid crust or thicker-than-normal early oceanic crust.

Top basement TWTT cannot reliably identify the margin domain transition between the necking zone and hyperextended crust. This transition coincides with the onset of normal decompression melting and the start of hybrid crust.