TIMESCALES OF CONTINENTAL RIFTING

We quantify the time and length scales of faulting and magmatism that produce the time-averaged rift structures imaged in rifts and passive margins worldwide. Seismic and geodetic strain patterns during discrete, intense rifting episodes in magmatic and amagmatic sectors of the East African rift zone that span middle to late-stage rifting are compared and contrasted. We also examine the longer term rifting cycle and its relation to changing far-field extension directions with examples from the Rio Grande rift zone and Baikal rift. Over time periods of millions of years, periods of rotating regional stress fields are marked by a lull in magmatic activity and a temporary halt to tectonic rift opening. During periods of rift opening, the interseismic opening rate is low (~1-4 mm/yr) and rift opening mainly occurs by slip on border faults. In contrast, within rift sectors with upper crustal magma chambers beneath the central rift valley seismic energy release accounts for a small fraction of the deformation; most of the strain is accommodated by magma intrusion and slow-slip. Magma intrusion processes appear to decrease the time period between rifting episodes, effectively accelerating the rift to rupture process.

This comparison demonstrates that intense rifting events, both seismic and magmatic, produce the long-term fault displacements and maintain the along-axis rift architecture through repeated rifting episodes.