Tectonic setting of the northern Okanagan Valley at Mara Lake, British Columbia
Creators: name: Nielsen, Kent Christopher
Publisher: name: University of British Columbia
Year: 2010

description: Mara Lake, British Columbia, straddles the boundary between the Monashee Group on the east and the Mount Ida Group on the west. Both groups of rock have experienced four phases of deformation. Phases one and two are tight and recumbent, trending to the north and to the west respectively. Phases three and four are open to close and upright, trending northwest and northeast respectively. Second phase deformation includes large scale tectonic slides which separate limbs of major folds. These slide surfaces are folded by third and fourth phase structures and outline domal outcrop patterns. Peak metamorphism accompanied and followed phase two. Metamorphic grade is related to position within the second phase structure, increasing downward from greenschist to amphibolite facies. Greenschist conditions accompanied phase three while hydrothermal alteration characterizes phase four. Brittle fracturing and local faulting along a northeasterly trend followed phase four. Abrupt changes in metamorphic grade found at the northern end of Mara Lake are related to these late faults. Correlation of lithologies across the southern end of Mara Lake and the similar structural sequences indicate that no stratigraphic or structural distinction is necessary between the Mount Ida Group and the Monashee Group. On a regional scale similar structural sequences are observed in other areas of the Shuswap Metamorphic Complex. Microscopic deformation features are common in many mineral phases in the Mara Lake area. Amphibole rarely shows evidence of plastic deformation. To examine this apparent high strength characteristic, fifty samples of hornblendite (AM-2) were deformed in a large, solid-medium Griggs-type apparatus at 700° to 1000°C at strain rates from 10⁻⁴/sec to 10⁻⁶/sec and at 10 kb confining pressure. Talc, pyrophyl lite, and platinum jacketing were used to yary water content. From 700° to 850°C both mechanical twins (101) and translation glide (100) were observed. Twin development appears to be favored over glide at higher confining pressures, lower temperatures, and higher strain rate. Above 850°C subgrain development and recrystallization occur just prior to melting. A flow law, Є = ~ 1.5 x 10⁻¹ exp (-38/RT)σ[sup 4.8] describes steady state deformation from 750° to 910°C under wet conditions. Decreasing water and temperature are accompanied by increasing n values and perhaps increasing activation energy. At 750°C under dry conditions an exponential relationship, Є = 53 exp (.23 σ) best fits the data. From 910° to 950°C the amphibole structure "hardens" such that strain rate remains constant for a given load. This hardening is interpreted to be related to oxidation and distortion within the lattice. Uncertainty regarding the activation energy precludes effective extrapolation of the data to "geologic" strain rates. A tentative comparison of amphibole and quartz data reveals an order of magnitude difference in flow stress, suggesting that quartz will yield plastically before amphibole.