Monashee Mountains Research Articles

Linking metamorphic textures to U–Pb monazite in-situ geochronology to determine the age and nature of aluminosilicate-forming reactions in the northern Monashee Mountains, British Columbia

The Monashee Mountains of the Canadian Cordillera are thought to expose a classic Barrovian-facies series of isograds. The timing of aluminosilicate growth in the region was determined for four pelitic schist samples by combining textural relationships with monazite compositional zoning and monazite U–Pb geochronology conducted directly on thin-sections by the laser ablation method. Three distinct phases of kyanite growth are recorded in the kyanite zone: at c. 153Ma, between 122 and 94Ma and between 76 and 58Ma. For each phase, monazite and garnet grew synchronously with kyanite, probably by a reaction involving the breakdown of staurolite. In contrast, sillimanite growth by muscovite dehydration melting occurred at or before c. 104Ma in the sillimanite zone, and retrograde sillimanite grew in schists previously metamorphosed at the kyanite grade during the first two phases by the influx of hot, acidic fluids during top-to-the-east shearing at ca. 71Ma. These results indicate that rocks metamorphosed at different places and different times in the orogen were juxtaposed prior to being overprinted at the sillimanite grade in the Late Cretaceous–Early Paleocene during the influx of hot fluids in a structurally coherent body deforming by easterly directed shearing. This study also provides new insight into monazite petrogenesis and suggests that, at least in some circumstances, monazite formation is linked to the staurolite-out reaction that produces kyanite.

Deciphering cryptic P–T-d–t histories in the western Thor-Odin dome, Monashee Mountains, Canadian Cordillera: A key to unravelling pre-Cordilleran tectonic signatures

A detailed structural and metamorphic study along a transect from the core of the Thor-Odin dome to its western margin (South-eastern Canadian Cordillera) has allowed the reconstruction of the P–T-d–t history estimating PT changes, during successive fabric development. The dome comprises a recumbently-folded and transposed Proterozoic high-grade basement and Late Proterozoic-Palaeozoic metasedimentary cover, characterised by a regional foliation (S T). An inferred, nearly-isothermal decompressional path, from a maximum depth of ca. 40 km, transgresses the “maximum relaxed geotherm”, consistent with exhumation during lithospheric extension. Contrasting pre-S T PT conditions indicate that different units followed different PT paths before S T development. Traces of these early and variable structural and metamorphic re-equilibration steps are preserved in meter-scale boudins and granular-scale domains poorly affected by transposition. The various paths represent events, from Palaeoproterozoic to Proterozoic Rodinia break up, to Jurassic-Cretaceous convergence and final exhumation in the Eocene. Much of the deformational and metamorphic history may have been accumulated before onset of S T. However, the early structural history is overprinted, and mineral assemblages and their relative ages are mostly obscured by S T development. A glimpse of the earlier geologic history is possible only by combining micro-structural analysis and PT estimates.

Topographic influences on recent changes of very small glaciers in the Monashee Mountains, British Columbia, Canada

AbstractAn analysis of the local topographic setting of very small (<0.4 km2) glaciers within a small region of the Monashee Mountains, British Columbia, was conducted to investigate its influence on recent changes in the extent of these glaciers. Net changes in glacier extent were determined from a detailed manual comparison of remotely sensed imagery acquired in 1951, 2001 and 2004. Most of the 86 glaciers included in the study showed no observable net change in area over the study period, while six glaciers retreated, four disappeared entirely and only one advanced. Indices derived to characterize elements of the local topographic setting that might affect the local mass balance suggest that most of the glaciers are situated in locations that favor ice preservation by enhancing mass input and/or reducing ablation rates. Glaciers situated in less favorable settings generally either decreased in area or disappeared. The results suggest that most of the glaciers studied have retreated as far as they are likely to under the climatic conditions of the late 20th century.

Potentials of Natural Tree Regeneration after Clearcutting in Subalpine Forests

Abstract Regeneration of interior mountain forests still is not adequately understood, although these forests are subject to intensified use over the last decades. We examined factors influencing the success of natural tree regeneration after harvesting in the Engelmann spruce–subalpine fir zone of the Monashee Mountains, British Columbia, Canada. Distance from the forest edge was an important factor for regeneration; at distances exceeding 70 m from the forest edge only 50% of plots showed sufficient natural regeneration to meet stocking targets compared with 90% of plots closer to forest edges. Seedling density and growth were superior in the more protected southern portions of clearcuts. Seedling growth was less in plots containing high cover of downed woody debris. There was no relationship between understory plant diversity or composition and tree seedling regeneration. However, cover of fireweed (Epilobium angustifolium) had a significant negative relationship with density but not growth of tree seedlings, particularly for lodgepole pine (Pinus contorta var. latifolia). Cover of fireweed decline substantially within the first 10 years after clearcutting. We conclude that natural regeneration of trees has potential to help achieve stocking targets and also to maintain natural diversity of tree species if spatial constraints, especially thresholds in clearcut size, are considered.

Three-dimensional seismic crustal structure of the Monashee hinterland core complex on the Lithoprobe Southern Canadian Cordillera Transect

Three-dimensional seismic coverage in an approximately 12 km × 12 km area of the southern Monashee metamorphic complex in the south-central portion of the Canadian Cordillera reveals a complex geometry to the Mesozoic–early Tertiary contractional Monashee décollement. Data were acquired as part of the Lithoprobe Southern Canadian Cordillera Transect where two approximately perpendicular lines intersected on the south flank of the Monashee mountains in the hinterland of the Cordillera. Stacks of traces within 100 m × 100 m bins are nominally 6-fold, but range from zero to 108-fold due to the crooked nature of the lines. Both migrated and unmigrated data have been examined for interpretation, but the highly variable data quality and discontinuous reflectivity cause excessive added noise during the migration process.

Petrogenesis of two-mica granites, northern Monashee Mountains, British Columbia: Evidence from oxygen, hydrogen, and strontium isotopic compositions

Syn- to postkinematic two-mica granite dikes, sills, and sheets intrude late Precambrian sillimanite and K-feldspar-sillimanite zone metasedimentary and metavolcanic rocks in the northern Monashee Mountains of British Columbia. U-Pb ages indicate that two periods of granite emplacement occurred ∼40 m.y. apart, at 100 Ma, concurrent with peak regional metamorphism, and at 63 Ma, which marked the beginning of a period of accelerated cooling in the area. Rare earth element contents, high initial 87 Sr/ 86 Sr ratios, inheritance of Precambrian zircons, and the restriction of most of the peraluminous granites to the highest-grade metamorphic zones are all compatible with derivation of the granitic magmas by partial melting of associated metasedimentary rocks. However, stable isotope studies of whole-rock samples and minerals indicate that the source of the peraluminous granitic magmas could not be simple whole-sale melting of spatially associated pelitic rocks, but likely was Precambrian lower continental crust that must have included metapelites. The disparate ages of the granites in conjunction with what is known about the tectonic evolution of the area indicate that the granites are tectonically displaced from their source areas. Isotopic equilibration temperatures of coexisting muscovite and biotite in the granites are near 400 °C and are lower than temperatures estimated for the same mineral pairs in the metapelitic rocks. This difference in equilibration temperatures is likely the result of differential rates of exchange related to cooling in the presence of fluids in the granites. Although the δD values of muscovites from both the granites and the metapelites indicate equilibration with fluids having a relatively restricted range of δD values near -50, the calculated δ 18 O values of fluids in equilibrium with the granites are ∼2 per mille lower than those in equilibrium with the associated metapelitic rocks. In contrast to the hydrogen isotope systematics in the muscovites, δD values of biotite in the granites and metapelites range from -173 to -91, implying that the biotites were affected by early Tertiary meteoric water when this area was at high latitudes and elevations, concurrent with an episode of tectonic denudation that occurred at ca. 45 Ma.

Shear frame stability parameters for large-scale avalanche forecasting

During the winters of 1990, 1991 and 1992, a field study of stability parameters for forecasting slab avalanches was conducted in the Cariboo and Monashee mountains of western Canada. In a level study plot at 1900 m and on nearby slopes, the shear strength of the weak snowpack layer judged most likely to cause slab avalanches was measured with a 0.025 m2 shear frame and a force gauge. Based on the ratio of shear strength to stress due to the snow load overlying the weak layer, a simple stability parameter and a more theoretically based stability index which corrects the strength for normal load were calculated. These stability parameters are compared with avalanche activity reported for the same day within approximately 30 km of the study plot. Each stability parameter is assessed on the basis of the number of days that it successfully predicted one or more potentially harmful avalanches and the number of days that it successfully predicted no potentially harmful avalanches. Both parameters predicted correctly on at least 75% of the 70 days they were evaluated. The simpler empirical stability parameter worked as well as the one that corrects strength for normal load. For large-scale forecasting of dry-snow slab avalanches, shear frame stability parameters appear to be a useful addition to meteorological data, snowpack observations and slope tests.

Shear frame stability parameters for large-scale avalanche forecasting

During the winters of 1990, 1991 and 1992, a field study of stability parameters for forecasting slab avalanches was conducted in the Cariboo and Monashee mountains of western Canada. In a level study plot at 1900 m and on nearby slopes, the shear strength of the weak snowpack layer judged most likely to cause slab avalanches was measured with a 0.025 m2 shear frame and a force gauge. Based on the ratio of shear strength to stress due to the snow load overlying the weak layer, a simple stability parameter and a more theoretically based stability index which corrects the strength for normal load were calculated. These stability parameters are compared with avalanche activity reported for the same day within approximately 30 km of the study plot. Each stability parameter is assessed on the basis of the number of days that it successfully predicted one or more potentially harmful avalanches and the number of days that it successfully predicted no potentially harmful avalanches. Both parameters predicted correctly on at least 75% of the 70 days they were evaluated. The simpler empirical stability parameter worked as well as the one that corrects strength for normal load. For large-scale forecasting of dry-snow slab avalanches, shear frame stability parameters appear to be a useful addition to meteorological data, snowpack observations and slope tests.

Comment on "Northern Monashee Mountains, Omineca Crystalline Belt, British Columbia: Timing of metamorphism, anatexis, and tectonic denudation"

Comment on "Northern Monashee Mountains, Omineca Crystalline Belt, British Columbia: Timing of metamorphism, anatexis, and tectonic denudation"

Northern Monashee Mountains, Omineca Crystalline Belt, British Columbia: Timing of metamorphism, anatexis, and tectonic denudation

Research Article| February 01, 1990 Northern Monashee Mountains, Omineca Crystalline Belt, British Columbia: Timing of metamorphism, anatexis, and tectonic denudation J. H. Sevigny; J. H. Sevigny 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar R. R. Parrish; R. R. Parrish 2Lithosphere and Canadian Shield Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar R. A. Donelick; R. A. Donelick 3Department of Geology, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Search for other works by this author on: GSW Google Scholar E. D. Ghent E. D. Ghent 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J. H. Sevigny 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada R. R. Parrish 2Lithosphere and Canadian Shield Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada R. A. Donelick 3Department of Geology, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 E. D. Ghent 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (2): 103–106. https://doi.org/10.1130/0091-7613(1990)018<0103:NMMOCB>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation J. H. Sevigny, R. R. Parrish, R. A. Donelick, E. D. Ghent; Northern Monashee Mountains, Omineca Crystalline Belt, British Columbia: Timing of metamorphism, anatexis, and tectonic denudation. Geology 1990;; 18 (2): 103–106. doi: https://doi.org/10.1130/0091-7613(1990)018<0103:NMMOCB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The northern Monashee Mountains lie at the northern termination of a north-south zone of Eocene extension in the southern Omineca Crystalline Belt of British Columbia. Isotopic, structural, and petrogenetic studies in the northern Monashee Mountains document middle to Late Cretaceous regional metamorphism, two episodes of crustal anatexis, and a period of rapid cooling coincident with middle Eocene extension. Partial melting of geochemically and isotopically similar pelitic sources produced peraluminous granites during and postdating regional metamorphism at 100 Ma and 63 Ma, respectively. Rb-Sr muscovite-whole-rock isochrons from the younger granites give an age of 62.5 ±2.0 Ma. Rb-Sr isochron and U-Pb mineral ages are similar, indicating that the 63 Ma granites were emplaced into a terrane that had cooled to 500 °C or less by 63 Ma. Hornblende, muscovite, and biotite K-Ar and apatite fission-track mineral cooling ages from the northern Monashee Mountains are are 76.3 ±5.8 (n = 7), 51.4 ±3.5 (n = 6), 55.1 ± 2.7 (n = 10), and 45.0 ±2.9 (n = 3) Ma, respectively (mean age and standard deviation). Accelerated cooling, beginning with the emplacement of 63 Ma granites and ending at 45 Ma, occurred in response to ramping along a crustal-scale thrust fault followed by offset on the middle Eocene offset on the middle Eocene North Thompson normal fault. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrogenesis of Peraluminous Granites, Monashee Mountains, Southeastern Canadian Cordillera

Petrogenesis of Peraluminous Granites, Monashee Mountains, Southeastern Canadian Cordillera

Geometric relationship between the Scrip Nappe and metamorphic isograds in the northern Adams River area, Monashee Mountains, British Columbia

In the northern Adams River area of the Monashee Mountains, British Columbia, the axial trace of the Scrip Nappe, a megascopic southwest-verging recumbent F1 anticline, can be traced upward through all stratigraphic units of the Late Proterozoic Horsethief Creek Group westward to where it is truncated by the Eocene North Thompson Fault. The Scrip Nappe has been coaxially folded by a northeast-verging, isoclinal F2 synform with a southwest-dipping axial surface and an amplitude of about 6 km. A metamorphic culmination, defined by the surface trace of the muscovite-out isograd and consisting of a series of northwest–southeast-striking isogradic antiforms and synforms, postdates Early Cretaceous F2 folding and predates Eocene normal faulting at 51–45 Ma.

Tectonic interpretation of west-verging folds in the Selkirk Allochthon of the southern Canadian Cordillera

The Selkirk Allochthon, a composite tectonic slice composed of North American paleocontinental-margin deposits and more distal, possibly marginal-basin "suspect terrane," was displaced eastward toward the craton in the Late Jurassic and Late Cretaceous.The Carnes Nappe, a major west-verging recumbent anticline within the Selkirk Allochthon, is considered the southern continuation of Scrip Nappe, which in the Monashee Mountains has an inverted limb length of 50 km. The west-verging nappe and associated structures are interpreted as having originated in the Early to Middle Jurassic during accretion of western allochthonous terranes and prior to eastward displacement of the Selkirk Allochthon.The reversal from westward vergence away from the North American craton to eastward vergence is considered as marking a fundamental change in the evolution of the orogenic belt and may reflect a transition from underthrusting of western allochthonous terranes on blind-shear zones to east-directed breakthrough thrusts.

Conductive structures under the Canadian Rocky Mountains

The paper reports results from an array of 33 three-component magnetometers that recorded time-varying fields in 1981 over an area of some 56 000 km2 in the Canadian Cordillera. The array was centred at Tête Jaune Cache in the Rocky Mountain Trench, where a large magnetovariation anomaly had been located in an earlier array study. It was bisected by the trench and extended to the northeast across the Rocky Mountains to the Alberta Foothills and to the southwest across the Cariboo and Monashee mountains. Magnetograms and Fourier transform maps covering the period range 10–91 min show strong attenuation of the vertical component, Z, southwest of the Rocky Mountain Trench, with very large Z amplitudes in the Main Ranges of the Rockies. The horizontal components show an elongated anomaly along the Rocky Mountains Main Ranges and Trench, with three-dimensional features superimposed. The conductive structures include a highly conductive layer, probably in the lower crust, southwest of the trench and a conductive ridge rising into the upper crust near the edge of that layer. Current models have been fitted to observed vertical -and horizontal-component anomalies and show that both layer and ridge are necessary for a fit and that the ridge is 50–80 km wide. Single-station transfer functions at periods of 10 and 22 min have been calculated from a number of variation events of various polarizations, to reduce any displacement of the anomalies by auroral-zone source currents. Artificial-event analysis, with these transfer functions, shows that the conductive ridge lies under the Main Ranges of the Rockies and not under the trench. Its great width indicates a structure of major tectonic significance, which will be considered in another paper.

Stratigraphy and deformational history of the Scrip Nappe, Monashee Mountains, British Columbia

The Scrip Nappe, a large recumbent anticline that occupies the northern Selkirk and northern Monashee Mountains, has an inverted lower limb, some 50 km in length across strike, and comprises stratigraphic divisions of the Hadrynian Horsethief Creek Group, which can be traced southward with decreasing metamorphic grade through the Selkirk Mountains to the northern Purcell Mountains. The Scrip Nappe has a southwesterly vergence and it formed that way during the first folding phase of the Mesozoic Columbian Orogeny. Metamorphism no greater than biotite zone accompanied that first deformation. The nappe was subsequently refolded into tight northeast verging folds. Metamorphism rose to upper amphibolite facies late in the second deformation phase. After the metamorphic climax, northeast verging buckle folds and associated crenulation cleavage formed locally during a third folding episode. The entire nappe complex was then carried northeastward, on the Purcell thrust, over the folds and thrusts of the western Rocky Mountains.

Seismicity in the Mica Reservoir (McNaughton Lake) area: 1973–1978

Impoundment of McNaughton Lake was initiated on March 29, 1973 and full load of 25 × 109 m3 and maximum depth of 191 m were first achieved in August 1976. The reservoir lies principally in the Rocky Mountain Trench, a major structural boundary between the Rocky Mountain Thrust Belt to the east and the Eastern Metamorphic Belt to the west. Although the regional seismicity is moderate, with an average of about one earthquake with Richter magnitude ML ≥ 3.0 every 2 years in the period 1963–1972, a magnitude 6 earthquake occurred adjacent to the present reservoir site in 1918.A monitoring program initiated in January 1973 has located 212 earthquakes with M ≥ 1.5; the largest ML = 4.8. Most of the seismicity was confined to four sequences. The first swarm started prior to initiation of impoundment. The next two sequences occurred in the same area 17 km into the Rocky Mountains from the central section of the reservoir. The final sequence, consisting of the ML = 4.8 earthquake and its aftershocks, had a preferred epicentre 4 km east of the Rocky Mountain Trench near the northern section of the reservoir. Comparison of the characteristics of the earthquake sequences with data from reservoirs where seismicity has been induced indicates that the observed seismicity at McNaughton Lake is unlikely to have been induced.The regional seismicity pattern from 1973 to 1978 was similar to that of the historical record: distributed earthquakes in the Rocky Mountains and in the adjacent Monashee and Cariboo Mountains of the Eastern Metamorphic Belt, no earthquakes within the Rocky Mountain Trench, and the Selkirk Mountains continuing to be almost aseismic. Both the regional seismicity and swarms yield b values near 0.65. Inclusion of the historical data with the regional seismicity gives b = 0.74 ± 0.08.

Metamorphism, structure and stratigraphy in the Southern Park Ranges, British Columbia

The Southern Park Ranges lie immediately east of the Rocky Mountain Trench, near the Big Bend of the Columbia River, B.C. They are composed of quartzites, pelites and carbonates in an uninterrupted sequence from the Proterozoic Windermere Supergroup, through the Lower Cambrian Gog Group, to the Middle Cambrian Chancellor Group.Two phases of folding and metamorphism are recognized. The first phase of folding (F1) occurred before and during the first phase of metamorphism (M1), whereas the second phase of folding (F2) was associated with a second (very minor) phase of metamorphism (M2). Both F1 and F2 structures are apparently associated with the rise of the Porcupine Creek Anticlinorium, a major fold complex trending northwest–southeast across the eastern part of the study area. Associated with F2 folds are several west-dipping thrust faults, some of which terminate upwards in the cores of F2 anticlines. All structures are cut by late-stage, west-dipping normal faults.The first phase of metamorphism (M1) resulted in a Barrovian sequence of isograds such that chlorite zone rocks occur in the east and sillimanite zone rocks occur in the west. Phase equilibria studies suggest that the metamorphic temperatures in the area ranged from less than 480 °C (east) to 570 °C (west) at a pressure of approximately 5 kbar (5 × 105 kPa).The M1 isograds in the Park Ranges outline a thermal antiform whose western limb is truncated by the Purcell Fault (a major Cordilleran structure) on the western slopes of the study area.A comparison of main metamorphic phases between the Park Ranges and the Selkirk and Monashee Mountains across the Purcell Fault, suggests that the western (Selkirk–Monashee) terrane underwent temperatures approximately 40 °C higher than the study area (for equivalent metamorphic zones) and a pressure between 2 and 3 kbar (2 × 105 and 3 × 105 kPa) higher than the study area. This pressure difference suggests that there has been a minimum of 7 km post-metamorphic vertical motion on the Purcell Fault. A difference in structural style across the Purcell Fault, due to a higher ductility in the western terrane, is interpreted as a result of the difference in depth during the deformation of the two terranes.

THE ATHABASCA VALLEY ERRATICS TRAIN, ALBERTA AND PLEISTOCENE ICE MOVEMENTS ACROSS THE CONTINENTAL DIVIDE

The Athabasca Valley Erratics Train contains a variety of low- to medium- grade metamorphic rocks, the most abundant of which is talcose schist, with lesser amounts of garnet schist and biotite–quartz schist. This erratics train occurs in and west of the Athabasca Valley west of Edson, Alberta. It is probably a late stage deposit of the same glacier that carried and deposited the Erratics Train, Foothills of Alberta. The metamorphic erratics were incorporated into a glacier that originated in the northern part of the Monashee Mountains and Premier Range of British Columbia. This ice movement is also recorded by numerous U-shaped valleys, which extend across the Continental Divide. Thus, during a brief period in late(?) Wisconsin time, the Cordilleran ice in the Rocky Mountains of the Jasper National Park area was partly derived from west of the Continental Divide and the Rocky Mountain Trench. These data agree with the inferred ice movements shown on the 1958 Glacial Map of Canada.