CONTINENTAL RIFTS: SCIENTIFIC ADVANCES AND PROSPECTS FOR FUTURE ADVANCES

Understanding of continental rifting has advanced greatly in the past ~50 years: rifts are no longer characterized as simple symmetrical graben, bounded by steep normal faults, accommodating at most several kilometers or a few tens of percent extension. Major advances are observationally based rather than theoretically predicted. For example: reflection seismic imaging of normal faults bounding asymmetric half graben, geologic recognition of low-angle normal faults (LANFs) that individually accommodate tens of km of extension, and geodetic-seismic recognition of magmatic rifting events. New techniques and data sets (e.g., satellite geodesy and imaging, improved precision and intercalibration of isotopic dates, advances in exposure dating and model-age methods, seismic tomography and high-resolution DEMs) now drive observational advances, allow time to be sliced in ways previously unimaginable, and let us conceptualize mantle-to-surface links and feedbacks that inform rift tectonic studies, but also influence geodynamic thought more broadly. For a past example, high elevation of metamorphic core complex domes requires flow in a weak "crustal asthenosphere", a concept that can explain decoupling of crustal levels in other tectonic settings. Numerical models can test the physical/chemical feasibility of conceptual models.

Future rift studies should emphasize existing enigmas and new dilemmas raised by unexplained observations, and apply new methods and new combinations of methods. Long-standing problems include: (1) Formation of, and creep vs. seismogenic slip on, LANFs (explanations will improve understanding of fault-mechanics, crustal strength and seismic risk). (2) Many rifts expose midcrustal rocks with minimal overprinting (should inform us about crustal architecture and space-time evolution of crustal strength). (3) Many relationships between magmatism and rifting are documented but many explanations are conjectural. Neotectonic and geodetic studies of rifts seem to lag behind those of contractional orogens, where relief is typically larger and tectonic and erosion rates are faster, but new satellite, geophysical and dating methods open new doors of opportunity. "Nested" numerical models should help bridge and link the time and space scales of observations and processes.