The interaction of the northern Nazca and southwestern Caribbean oceanic plates with South America, and the collision of the Panama-Choco arc have significant implications on the evolution of the northern Andes. We integrate an alternative interpretation of the Nazca and Caribbean kinematics with the magmatic and deformation history in the region. The northeastward migration of the Caribbean plate caused a progressive change in the geometry of the subducting Farallon plate, causing flat-slab subduction throughout the late Eocene-late Oligocene, inhibition of magmatism and eastward migration of the Andean deformation. Meanwhile, the Paleocene-Eocene highly oblique convergence of the Caribbean plate against South America changed by the mid-Eocene, when the Caribbean plate began to migrate in an easterly direction. These events and the late Oligocene breakup of the Farallon plate, prompted a Miocene plate reorganization, with further plate fragmentation, changes in convergence obliquity, steepening of the subducting slabs and renewal of magmatism. This tectonics was complicated by the accretion of the Panama-Choco arc to South America, which was characterized by early Miocene subduction erosion of the forearc and trench advance, followed by breakoff of the subducting slab east of Panama and collisional tectonics from the middle Miocene. By 9 Ma the Coiba and Malpelo microplates were attached to the Nazca plate, resulting in an abrupt change in convergence directions, that correlates with the main pulse of Andean orogeny. During the late Pliocene, the Nazca slab broke, triggering the modern volcanism south of 5.5º N. Seismicity data and tomography support the proposed reconstruction.

The relationships between deformation and erosion in transpressive systems are still poorly understood. Here, we present a new set of analog models to investigate how the tectonic and surface processes present at transpressive plate boundaries interact to shape topography. The experimental setup comprised a 2 x 1 x 0.5 m3 plexiglass box fit with a plexiglass board cut to 20º obliquity. A motor pulled a mylar sheet beneath the board to generate a velocity discontinuity at the interface. We loaded a ~5 cm thick layer of a granular material onto the board and sheet composed of 40 wt. % silica powder, 40 wt. % glass microbeads, and 20 wt. % PVC powder (cf. CMII in Reitano et al., 2020, doi: 10.5194/esurf-8-973-2020). This setup allows deformation to nucleate at the velocity discontinuity and naturally form a transpressional wedge. The model was monitored with digital cameras and a laser scanner to conduct particle image velocimetry and digital elevation model analysis, respectively. To explore surface processes associated with mass transport and erosion, we used a sprinkler system that casts a uniform mist across the model surface. We allowed ~1 cm of relief (equivalent to ~10 cm of convergence) to form before misting began to ensure the formation of realistic drainage networks. Before misting, experiments evolved in 3 stages: 1) distributed strain, 2) strike-slip faulting along synthetic structures, and 3) uplift and formation of a wedge along bivergent thrust structures. After misting, strike-slip deformation was still fully partitioned to synthetic structures and thrust sheets propagated in the prowedge direction. As the experiment continued, sub-longitudinal drainage systems formed with their orientation controlled by synthetic structures. Strike-slip displacement along these structures interrupted transverse streams, which ultimately captured the sub longitudinal systems. On the retrowedge, a longitudinal basin formed along a coalesced extensional structure, which also was later captured by transverse channels. These and other interactions between fault structures and channel networks provide insight into erosion and mass transport in transpressional systems and the nature of the complex reorganization of stream networks in response to deformation.

The Atlas-Meseta intracontinental orographic system of Morocco experienced recent, large-scale surface uplift as documented by elevated late Miocene, shallow-water marine deposits exposed in the Middle Atlas Mountains. The Anti-Atlas Mountains do not present any stratigraphic records that document regional vertical movements, however, the presence of a high-standing, erosional surface, and the transient state of river networks, provides insights into the uplift history of the belt and the mechanisms that drove it. Here, we combine geomorphic and stream profiles analyses, celerity of knickpoints and linear inverse landscape modelling with available geological evidence, to decipher the spatial and temporal variations of surface uplift in the Anti-Atlas and the Siroua Massif. Our results highlight the presence of a transient landscape, and document a long wave-length topographic swell (~ 100 x 600 km) with a maximum surface uplift of ~1500 m in the Siroua Massif and ~1100 m in the central Anti-Atlas starting from ~10 Ma, in association with late Miocene magmatism in the Siroua and Saghro Massif and contractional deformation in the High Atlas. Uplift rates for the central Anti-Atlas range between 70 and 180 m/Myr, fall within the same order of the rates obtained from uplifted marine deposits suggesting a similar deep-seated mechanism of uplift most likely related to astenopsheric upwelling. Overall, our approach allows to quantitatively constrain the transient state of the landscape and the contribution of regional surface uplift on mountain building processes.

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.