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A forward model for the architecture of SDR complexes and its implications for their mode of formation

Author(s): Frank Peel Marine Geosciences Group, National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton, S014 3ZH, United Kingdom
Bramley Murton Marine Geosciences Group, National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton, S014 3ZH, United Kingdom

SDR complexes are common in magma-rich continental rift margins, developing in the period between continental breakup and oceanic spreading. Their genesis is poorly understood, because we do not see them forming today, although ancient systems are well-imaged on seismic data.

A simple forward model successfully simulates observed SDR architecture. We assume that: (i) the system can be modelled in 20 (out of plane effects are ignored); (ii) the rate of plate separation is constant: (iii) there is a consistent relationship between the supplied magma volume and the gross cross-sectional shape of the lava flows (larger volume flows travel further and have a lower taper angle).

An empirically derived shape function, matched to seismic observations, defines the geometries of SDR packages; each package represents a composite of many flows produced during a time increment. This shape changes according to the volume of the flow. Packages are stacked vertically to create the overall SDR geometry. Our first-pass model assumes 10 Airy isostasy, with no flexural rigidity. By varying the magma supply per unit time, we optimize the match between the predicted SOR architecture and the observed real-world geometry.

The model indicates that the total thickness of the SDR package should vary significantly in a dip transect. This cannot be tested directly against available seismic data because the predicted base of the SDR complex is too deep. However, there should be a predictable relationship between the geometry of the SDR complex and the bathymetry developed on the top of the complex. This can be tested against the observed bathymetry of the immediate post-SDR sediments; if the model is true, it should provide a method of predicting the paleobathymetry, with implications for hydrocarbon exploration.

An alternate genetic model explains SDR shape as a rollover geometry generated by movement on a curved fault surface. Where this has been tested against real world geometries it does not give as good afit as the extrusion/isostasy model.

All genetic SDR models indicate that, at time of formation, the oceanward-end of the SDR wedge is supported on a wide magma chamber. A simple explanation for why the wedge does not simply collapse down into the chamber may be that it is supported buoyantly on an magma chamber containing both liquid and crystal mush, with a vertical density gradient; the eruption of large volume (many km^3) flows indicates that the erupting fluid is less dense than the wedge; in major expulsive events the upper, lighter, layer is removed, and the overlying solid wedge subsides until it reaches density equilibrium.


Title:
A forward model for the architecture of SDR complexes and its implications for their mode of formation
Type:
other
Origin:
Academia
Day:
2
Session:
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Daily sequence no.:
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Lead author last name:
Peel
Lead author first name:
Frank
Affiliation(s):
Marine Geosciences Group, National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton, S014 3ZH
Country:
United Kingdom
Abstract status:
Author details missing
UID:
88