During the development of continental rifts, strain accommodation shifts from border faults to intra-rift faults. This transition represents a critical process in the evolution of rift basins in the East African Rift, resulting in the focusing of strain and, ultimately, continental breakup. An analysis of fault and fluid systems in the younger than 7 Ma Natron and Magadi basins (Kenya-Tanzania border) reveals the transition as a complex interaction between plate flexure, magma emplacement, and magmatic volatile release. Rift basin development was investigated by analyzing fault systems, lava chronology, and geochemistry of spring systems. Results show that extensional strain in the 3 Ma Natron basin is primarily accommodated along the border fault, whereas results from the 7 Ma Magadi basin reveal a transition to intra-rift fault-dominated strain accommodation. The focusing of strain into a system of intra-rift faults in Magadi also occurred without oblique-style rifting, as is observed in Ethiopia, and border fault hanging-wall flexure can account for only a minor portion of faulting along the central rift axis (~12% or less). Instead, areas of high upper crustal strain coincide with the presence of hydrothermal springs that exhibit carbon isotopes and N 2-HeAr abundances indicating mixing between mantle-derived (magmatic) fluids and air saturated water. By comparing the distribution of fault-related strain and zones of magmatic fluid release in the 3 Ma Natron and 7 Ma Magadi basins, we present a conceptual model for the evolution of early-stage rifting. In the first 3 m.y., border faults accommodate the majority of regional extension (1.24-1.78 mm yr-1 in Natron at a slip rate ranging 1.93-3.56 mm yr-1), with a significant portion of intra-rift faulting (38%-96%) driven by flexure of the border fault hanging wall. Fluids released from magma bodies ascend along the border fault and then outward into nearby faults forming in the flexing hanging wall. By 7 m.y., there is a reduction in the amount of extension accommodated along the border fault (0.40-0.66 mm yr-1 in Magadi at a slip rate ranging from 0.62 to 1.32 mm yr-1), and regional extension is primarily accommodated in the intra-rift fault population (1.34-1.60 mm yr-1), with an accompanying transition of magmatic volatile release into the rift center. The focusing of magma toward the rift center and concomitant release of magmatic fluids into the flexing hanging wall provides a previously unrecognized mechanism that may help to weaken crust and assist the transition to intra-rift dominated strain accommodation. We conclude that the flow of magmatic fluids within fault systems plays an important role in weakening lithosphere and focusing upper crustal strain in early-stage continental rift basins prior to the establishment of magmatic segments.

[1] Geochronologic and geological data define a 600 ka age for the current volcanic front in Costa Rica. In Nicaragua, this age is less constrained but is likely within the range 600 ka to 330 ka. In Costa Rica, the new geochronologic data significantly improve estimates of the volumes of the volcanoes because they define the contact between the active volcanoes and the previous volcanic front, which is substantially older (2.2 to 1.0 Ma). In addition, the contrast in extrusive volcanic flux between western Nicaragua (1.3 Â 10 10 kg/m/Myr) and central Costa Rica (2.4 Â 10 10 kg/m/Myr) is greatly reduced from previous estimates and now within the range of error estimates. We estimate the subducted component of flux for Cs, Rb, Ba, Th, U, K, La, Pb, and Sr by subtracting estimated mantle-derived contributions from the total element flux. An incompatible element-rich OIB source for the Cordillera Central segment in Costa Rica makes the subducted element flux there highly sensitive to small changes in the modeled mantle-derived contribution. For the other three segments studied, the estimated errors in concentrations of highly enriched, subductionderived elements (Cs, Ba, K, and Pb) are less than 26%. Averaged over the time of the current episode of volcanism, the subduction-derived fluxes of Cs, Ba, K, Pb, and Sr are not significantly different among the four segments of the Central American volcanic front in Nicaragua and Costa Rica. The subductionderived fluxes of Th and La appear to increase to the SE across Nicaragua and Costa Rica, but the estimated errors in their subduction-derived concentrations are very high, making this variation questionable. The lack of change in the fluxes of Cs, Ba, K, Pb, and Sr argues that the well-defined regional variation in Ba/La is the result of changes in the mode or mechanics of fluid delivery into the mantle wedge, not the total amounts of fluids released from the slab. Concentrated or focused fluids in Nicaragua lead to high degrees of melting. Diffuse fluids in Costa Rica cause lower degrees of melting.

Landscape evolution in the eastern Mojave Desert is recorded by sys1ematic changes in Pliocene to latest Pleistocene volcanic: land• forms that show discrete periods of eolian deposition, surface stabilization, drainage-network expansion, and erosion on basaltic lava nows. These processes are documented by K•Ar dating in conjunction with morphometric, sedimentologic. pedologic, and geophysica1 stud

The Iberia Abyssal Plain segment of the West Iberia margin was drilled during Ocean Drilling Program Legs 149 and 173 and has been extensively studied geophysically. We present new microstructural investigations and new age data. These, together with observed distribution of upper-and lower-crustal and mantle rocks along the ocean-continent transition suggest the existence of three detachment faults, one of which was previously unrecognized. This information, together with a simple kinematic inversion of the reinterpreted seismic section Lusigal 12, allows discussion of the kinematic evolution of detachment faulting in terms of the temporal sequence of faulting, offset along individual faults, and thinning of the crust during faulting. Our study shows that the detachment structures recognized in the seismic profile became active only during a final stage of rifting when the crust was already considerably thinned to c. 12km. The total amount of extension accommodated by the detachment faults is of the order of 32.6km corresponding to a [3 factor of about two. During rifting, the mode of deformation changed oceanwards. Initial listric faulting led to asymmetric basins, accommodating low amounts of extension, and was followed by a situation in which the footwall was pulled out from underneath a relatively stable hanging wall accommodating high amounts of extension. Deformation along the latter faults resulted in a conveyor-belt type sediment accumulation in which the exhumed footwall rocks were exposed, eroded and redeposited along the same active fault system.

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