Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Rapid Subsidence at Outer Continental Margins and SDR Packages During the Rift-Drift Transition: The Roles of Landward-Dipping Faults and “Magmatic Detachment”
(1) Tectonic Analysis Ltd., Duncton, United Kingdom.
(2) BP America Inc, Houston, TX.
One aspect of especially magmatic continental rifting that is poorly acknowledged/ understood is very rapid subsidence of outer continental margins/ seaward dipping reflector (SDR) packages to 2 km depth subsea as seafloor spreading begins. Such subsidence is shown by 1) commonly angular contacts between SDRs and overlying marine strata and 2) deep water (~2km) formation of ocean crust adjacent to continent ocean transitions (COTs) as suggested by backstripping. Thermal subsidence cannot account for rotational subsidence of subaerial SDRs to the emplacement depth of ocean crust, because 36Ma are needed. Further, the often-angular contact between SDRs and onlapping marine strata suggests rapid (<3 Ma?) rotational subsidence at onset of spreading, requiring tectonic control. We also observe that COTs often contain landward dipping faults (LDFs) within the continental crust, and LDFs and/or high seismic velocity zones (denoting former magma chambers of mantle partial melt) beneath SDR packages at magmatic margins. We acknowledge plagioclase-spinel-garnet phase changes as accelerators of post-rift thermal subsidence, but suggest the roles of LDFs and sub-SDR magma chambers are greater and faster. We envision rifting involves detachment of continental crust off rising, ductile, often partially molten mantle welts, facilitated by LDFs and “magmatic detachment zones” of low shear strength. This allows fast, fault related subsidence at COTs, in mega-half grabens, the outer of which commonly contain SDRs as largely igneous half graben “fill”. Thinning sub-continental mantle extrudes and rises from beneath the diverging continents to 2-2.5 km subsea (by isostasy), but rarely higher, while continental crust and SDR packages are sloughed off the welts at listric LDFs and sub-SDR magma chambers, leading to SDR package rotation. Detachment may combine with seaward movement of original sub-SDR magma, akin to caldera collapse, to allow rapid subsidence/ drowning of SDR packages. Migrating magma may feed initial ocean crust formation at 2-9 km subsea, without structural discordance in the SDR to ocean crust transition surface. Where SDRs are absent (amagmatic margins), we see exhumed sub-continental mantle in the position that would normally underlie SDRs. Global examples are shown to support these concepts.