We investigate the spatiotemporal patterns of strain accommodation during multiphase rift evolution in the Shire Rift Zone (SRZ), East Africa. The NW-trending SRZ records a transition from magma-rich rifting phases (Permian-Early Jurassic: Rift-Phase 1 (RP1), and Late Jurassic-Cretaceous: Rift-Phase 2 (RP2)) to a magma-poor phase in the Cenozoic (ongoing: Rift-Phase 3 (RP3)). Our observations show that although the rift border faults largely mimic the pre-rift basement metamorphic fabrics, the rift termination zones occur near crustal-scale rift-orthogonal basement shear zones (Sanangoe (SSZ) and the Lurio shear zones) during RP1-RP2. In RP3, the RP1-RP2 sub-basins were largely abandoned, and the rift axes migrated northeastward (rift-orthogonally) into the RP1-RP2 basin margin, and northwestward (strike-parallel) ahead of the RP2 rift-tip. The northwestern RP3 rift-axis side-steps across the SSZ, with a rotation of border faults across the shear zone and terminates further northwest at another regional-scale shear zone. We suggest that over the multiple pulses of tectonic extension and strain migration in the SRZ, pre-rift basement fabrics acted as: 1) zones of mechanical strength contrast that localized the large rift faults, and 2) mechanical ‘barriers’ that refracted and possibly, temporarily halted the propagation of the rift zone. Further, the cooled RP1-RP2 mafic dikes facilitated later-phase deformation in the form of border fault hard-linking transverse faults that exploited mechanical anisotropies within the dike clusters and served as mechanically-strong zones that arrested some of the RP3 fault-tips. Overall, we argue that during pulsed rift propagation, inherited strength anisotropies can serve as both strain-localizing, refracting, and transient strain-inhibiting tectonic structures.

Stick-slips are spontaneous, unstable slip events during which a natural or man-made system transitions from a strong, sticking stage to a weaker, slipping stage. Stick-slips were proposed by Brace and Byerlee (1966) as the experimental analogue of natural earthquakes. We analyze here the mechanics of stick-slips along brittle faults by conducting laboratory experiments and by modeling the instability mechanics. We performed tens of shear tests along experimental faults made of granite and gabbro that were subjected to normal stresses up to 14.3 MPa and loading velocities of 0.26-617 micron/s. We observed hundreds of spontaneous stick-slips that displayed shear stress drops up to 0.66 MPa and slip-velocities up to 14.1 mm/s. The pre-shear and post shear fault surface topography were mapped with atomic force microscopy at pixel sizes as low as 0.003 micron^2. We attribute the sticking phase to the locking of touching asperities and the slipping phase to the brittle failure of these asperities, and found that the fault asperities are as strong as the inherent strength of the host rock. Based on the experimental observations and analysis, we derived a mechanical model that predicts the relationships between the measured stick-slip properties (stress-drop, duration, and slip-distance) and asperity strength.

Rotary shear (RS) experiments have been used to characterize the deformational behavior of materials and attempt to understand earthquakes. Typical RS experiments test materials under a prescribed slipping velocity and normal load. Yet, in natural earthquakes, fault nucleation, growth, termination, and slipping velocity are not predefined, but a result of the stored and released energy around the seismic fault. Here we present new measurements performed with a RS apparatus designed to be more representative of a natural system. The device uses a clock spring that when loaded by a motor imposes a linearly increasing torque to the sample. Thus, events occur spontaneously when the shear stress exceeds the static shear stress acting on the surfaces in contact. We report the results of experiments using solid poly(methyl methacrylate) (PMMA) and granular samples of polyvinyl chloride powder (figure), glass microbeads, silica powder, and crushed quartz. PMMA experiments were started at a spring loading rate (SLR) of ~2.5 RPM, where we observed low amplitude stick-slip events occurring at regular recurrence intervals. The SLR was then increased to ~12.5 RPM where after a transition period, temperatures during slip events exceeded the melting point of PMMA (~160ºC). This formed a melt layer that cooled and bonded the slipping surfaces. The friction coefficient just before rupture and the amount of weakening increased as a function of the amount of melt produced. Granular experiments were conducted at a SLR of ~2.5 RPM and variable normal stresses (0.1-0.5 MPa). The granular samples show strain hardening just before rupture, followed by strain softening and marked changes in behavior with varying water content. Since the behavior of PMMA is comparable to that of rocks at depth (McLaskey and Glaser, 2011), results of PMMA tests yield insight into precursory and coseismic events, fault strengthening/weakening mechanisms, and perhaps, the formation of pseudotachylite glass. Experiments with granular samples allow us to characterize each material’s behavior in response to variable water content, SLR, and normal stress. We conclude that analog materials are valuable to simulate the behavior of the seismogenic brittle lithosphere. From such experiments, we can gain insight into stick-slip mechanisms relevant to earthquakes.