Sarah R. Brown

Pleistocene reversal of the Fraser River, British Columbia

The Fraser River in British Columbia, Canada, is the longest non-dammed river on the west coast of North America and supplies 20 × 10 6 t/yr of sediment to the Pacific Ocean. Abundant geomorphological evidence indicates that the Fraser River reversed its course to southward flow in the recent geological past. Investigation of two volcanic dams at Dog Creek demonstrates northward flow of the Fraser until at least 1.06 Ma, before reversal and erosion of the 270-km-long Fraser Canyon. We propose that the submarine Nitinat Fan off the coast of British Columbia records the reversal and sudden input of coarse continental-derived sediment ca. 0.76 Ma. This study confirms reversal of the Fraser River and places a firm constraint on the maximum age of that reversal. Reversal likely followed stream capture in response to enhanced glaciofluvial erosion and uplift of the Coast Mountains.

New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone

The Okanagan Valley shear zone delineates the SW margin of the Shuswap metamorphic complex, the largest core complex within the North American Cordillera. The Okanagan Valley shear zone is a major Eocene extensional fault zone that facilitated exhumation of the southern Shuswap metamorphic complex during the orogenic collapse of the SE Canadian Cordillera when convergence at the western margin of North America switched from transpression to transtension. This study documents the petrology, structure, and age of the Okanagan gneiss, the main lithology within the footwall of the Okanagan Valley shear zone, and constrains its history from protolith to exhumed shear zone. The Okanagan gneiss is an ∼1.5-km-thick, west-dipping panel composed of intercalated orthogneiss and paragneiss in which intense ductile deformation of the Okanagan Valley shear zone is recorded. New U-Pb zircon ages from the gneiss and crosscutting intrusions constrain the development of the Okanagan gneiss to the Eocene, contemporaneous with widespread extension, intense deformation, high-grade metamorphism, and anatexis in the southern Canadian Cordillera. Thermobarometric data from the paragneiss domain indicate Eocene exhumation from between 17 and 23 km depth, which implies 64–89 km of WNW-directed horizontal extension based on an original shear zone angle of ∼15°. Neither the Okanagan gneiss nor its protolith represents exhumed Proterozoic North American cratonic basement as previously postulated. New U-Pb data demonstrate that the protolith for the gneiss is Phanerozoic, consisting of Mesozoic intrusions emplaced within a late Paleozoic–Mesozoic layered sequence of sedimentary rocks.

Sounds Good: Simulation and Evaluation of Audio Communication for Multi-Robot Exploration

In order to guide the design of a new multi-robot system, we seek to evaluate two different designs of audio direction sensor. We have implemented a simple but useful audio propagation simulator as an extension to the stage multi-robot simulator. We use the simulator to evaluate the use of audio signals to improve the performance of a team of robots at a prototypical search task. The results indicate that, for this task, (i) audio can significantly improve team performance, and (ii) binary discrimination of the direction of a sound source (left/right) performs no worse than high-resolution direction information. This result suggests that a simple two-microphone audio system would be useful for our real robots, without advanced signal processing to find sound direction

Corrugated architecture of the Okanagan Valley shear zone and the Shuswap metamorphic complex, Canadian Cordillera

The distribution of tectonic superstructure across the Shuswap metamorphic complex of southern British Columbia is explained by east-west– trending corrugations of the Okanagan Valley shear zone detachment. Geological mapping along the southern Okanagan Valley shear zone has identified 100-m-scale to kilometer-scale corrugations parallel to the extension direction, where synformal troughs hosting upper-plate units are juxtaposed between antiformal ridges of crystalline lower-plate rocks. Analysis of available structural data and published geological maps of the Okanagan Valley shear zone confirms the presence of ≤40-km-wavelength corrugations, which strongly influence the surface trace of the detachment system, forming spatially extensive salients and reentrants. The largest reentrant is a semicontinuous belt of late Paleozoic to Mesozoic upper-plate rocks that link stratigraphy on either side of the Shuswap metamorphic complex. Previously, these belts were considered by some to be autochthonous, implying minimal motion on the Okanagan Valley shear zone (≤12 km); conversely, our results suggest that they are allochthonous (with as much as 30–90 km displacement). Corrugations extend the Okanagan Valley shear zone much farther east than previously recognized and allow for hitherto separate gneiss domes and detachments to be reconstructed together to form a single, areally extensive Okanagan Valley shear zone across the Shuswap metamorphic complex. If this correlation is correct, the Okanagan Valley shear zone may have enveloped the entire Shuswap metamorphic complex as far east as the east-vergent Columbia River–Slocan Lake fault zones.