Rifted margins and extension rates

Rifted margins are commonly classified as either magma-rich or magma-poor, although arguably a gradual transition may exist between the two end members. Magma-poor margins may form during ultraslow to slow extension, and conversely magma-rich margins during more rapid extension. Numerical modelling to some extent suppmis this general notion. Nevertheless, these relationships are based on limited data sets. Rates for magma-poor margins are mainly based on the Iberia margin where the syn-rift development has been addressed by use of a sequence of ODP boreholes that span the margin in the dip direction. However, the boreholes were targeted on the updip end of the half grabens and only pmiially sampled the synrift successions. Exhumation rates have been based on cooling ages oflower crust in the outer hyperextended margin, weak magnetic anomalies of debated nature within the exhumed mantle, and ages of sediments overlying the exhumed mantle.

We know of no well-constrained extension estimates for magma-rich margins. Probably the NE Atlantic margins represent the best documented case, but there the pre-break-up extension rate is poorly constrained. Subsequent seafloor spreading rates are well-constrained by tightly spaced magnetic isochrons, which reveal a rapid initial spreading (ca 25 mm/yr half rate) that soon diminished. Initial s.eafloor spreading in the southem South Atlantic was also rapid. These rapid initial seafloor spreading rates along magma-rich margins have possibly led to the general perception that such margins always form during rapid extension. However, the concept needs more rigorous testing with a better worldwide database. Other magma-rich margins, such as the eastern US seaboard (Central Atlantic) formed during the magnetically quiescent period in the Early-Middle Jurassic. Currently favored ages of break-up and the first isochron indicate ultraslow to slow spreading, but notably the age of break-up is poorly constrained.

The assumption that there is a direct proportionality between extension rate and magma production appears to be supported by the limited data set, but the reasons why this is the case is not yet clear. We present some ideas and critical data in an effort to illuminate the underlying processes with emphasis on the following questions:

- Do available data really support the notion that magma-poor and- rich margins form during slow and fast extension respectively? - How might geological elements missing in many models (small-scale convection, lithospheric composition, or structural inheritance) affect numerical models? - Is hot, rising asthenosphere required to generate the observed melt?