A Deep-seated Gravitational Slope Deformation in the northern Monashee Complex, Monashee Mountains, British Columbia, Canada

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1. INTRODUCTION 
 
 
1.1 Goal of the work 
 
 
 
The primary goal of this work is to establish and apply a conceptual framework for the study of a 
Deep-seated Gravitational Slope Deformation (DsGSD) in the Blais Creek Area (Monashee 
Mountains, British Columbia, Canada). This goal has been reached by performing field 
investigations, including detailed geological and geomorphological mapping of the Blais Creek 
Area and integrating the results of previous stratigraphic and structural studies along the west 
flanks of the Frenchman Cap Dome (northern Monashee Mountains, British Columbia, Canada; 
Journeay’s Phd Thesis). A secondary goal is to reconstruct the evolutionary stages of slope 
deformation and to evaluate its controlling factors by integrating orthophotos, stereo models and 
3D models of the DsGSD with field and literature data concerning tectonic and glacial history of 
the Seymour Valley. The last goal of this work is to evaluate general geomechanical properties 
of the deforming rock mass for use in possible future numerical models in order to investigate 
the failure mechanism at Blais Creek and to define a geomechanical characterization of different 
portions of the DsGSD.
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1.2 Orogenesis of the Canadian Cordillera 
 
 
 
The Canadian Cordillera is a relatively youthful orogenic belt with respect to other Canadian 
mountain belts (figures 1.1, 1.2, 1.3; Monger, 2002). However, its origin extends back some 750 
million years to the episode of rifting that marked the first stages in the break-up and dispersal of 
“Rodinia”, a Neoproterozoic supercontinent that had existed since about 1000 Ma. The late 
Neoproterozoic-earliest Cambrian break-up (between 750-540 Ma) led to the opening of a new 
ocean basin that was the distant ancestor of the present Pacific Ocean basin and to the formation 
of a continent-ocean boundary that is preserved today within the eastern Canadian Cordillera 
(Monger, 2002). The protracted evolution of the Canadian Cordillera is dominated by 
interactions between the margin of the old, stable North American continent and the oceanic 
lithosphere located outboard of it. The initial intra-plate continent-ocean boundary was 
analogous to that of the present boundary between eastern North America and the Atlantic Ocean 
basin. It persisted until the Middle Devonian ( ~ 390 Ma), when a convergent, inter-plate 
boundary formed, along which magmatic arcs were generated within the edge of the North 
American Plate by subduction of oceanic lithosphere beneath it. Arc magmatism has persisted to 
a greater or lesser extent until the present but has varied in character. In the late Paleozoic and 
early Mesozoic (between ~ 355 and 185 Ma), the convergent plate boundary apparently lay well 
offshore, and involved chains of island arcs separated from the old continental margin by back 
arc basins. Starting in the Jurassic ( ~ 185 Ma), the North American continent converged with the 
offshore subduction zones (Monger, 2002). The rocks of the back arc basins and of the offshore 
arcs were accreted to the original continental margin, and new continental arcs were built on 
both accreted material and parts of the ancient continental margin. By the Late Cretaceous ( ~ 90 
Ma), a new continental margin was located near its present position, about 500 km oceanwards 
of the position of the original margin (Monger, 2002). The record of the million year long 
evolution is preserved both in rocks of the Cordillera and in those of the Western Canada 
Sedimentary Basin on its eastern margin and the plains, which evolved hand-in-hand with the 
Cordillera (Monger, 2002; Price, 1994).
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Figure 1.1. The Canadian Cordillera showing: A. Location of the five morphological belts (Monger, 2002); the south red 
line show the approximate position of the southern Canadian Cordilleran Lithoprobe transect (fig. 16) B. Simplified 
metamorphic map of the Canadian Cordillera, showing the close correspondence between the distribution of higher grade 
metamorphic rock facies and granitic rocks and Omineca and Coast Belts (Monger, 2002). The map legend below is a 
pressure-temperature diagram whose colours correspond with those on the map; metamorphic facies are: Sg 
subgreenschist; G greenschist; A amphibolite, and B blueschist (blue dots on map);box labelled Gr denotes grnitic rock 
(Monger, 2002). 
 
 
 
Figure 1.2.  Simplified lithosperic structure along the southern (S) Cordilleran Lithoprobe transect (Monger, 2002; 
Clowes and Hammer, 2000). The heavy green line is the crust-mantle boundary (Moho). AW accreted wedge; CD 
Cadwallader terrane; MT-SH undivided Methow and Shuksan terranes; QN Quesnel Terrane. Vertical exaggeration  
2.7:1.
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Figure 1.3: Evolution of the Canadian Cordillera in south British Columbia including the tectonic wedging and crustal thickening. 
 
A: Early Jurassic (~185 Ma) island arc and its early Mesozoic back-arc basin, (mainly) on top of Slide Mountain terrane; onset of convergence of 
North America with trench to west and collapse of back-arc basin. B: Late Early Jurassic (~180 Ma) collapsed basin thrust over old continental 
margin; flattening of subduction zone and initiation of continental margin arc. C: Early Middle Jurassic (~170 Ma) southwest verging deformation 
occurred as Kootenay terrane was detached from North America and wedged under the old continental margin deposits; North American 
lithosphere wedged under Quesnel terrane; entrained and consumed in the subduction zone; subduction zone flattened and magmatic arc migrated 
eastward into the zone of southwest verging deformation (Monger, 2002).
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1.3 Regional tectonic setting 
 
 
 
The magmatism and regional metamorphism, which are the hallmarks of the Omineca Belt, are 
superimposed on the suture zone (interface) between the North American rocks of the 
Cordilleran miogeocline and the accreted rocks of the Intermontane superterrane (Price and 
Monger, 2003; figure 1.4). 
 
The Monashee Terrane is one of several structural and metamorphic culminations that lie within 
the Omineca Belt, a major structural and metamorphic zone that straddles the paleocontinental 
margin of North America and allochthonous terranes that were accreted during Early Jurassic to 
mid-Cretaceous time (Scammel and Brown, 1989 and Monger et al., 1982). The terrane, has 
been interpreted as an antiformal duplex of basement-cored horses bounded above by the 
Monashee decollement, a north-easterly verging compressional mylonitic shear zone that served 
as the root to the Rocky Mountain thin-skinned fold and thrust belt to the east (Scammel and 
Brown, 1989; Brown et al., 1986).  
 
Several regional fault zones are visible along the Monashee Terrane. Each involves a profound 
change in the level of exposure within the orogen (Price and Monger, 2003). Some of these 
faults bound two broad structural domains: the Selkirk fan structure, which straddles the 
boundary between the Foreland and Omineca belts, and the Shuswap metamorphic complex, 
which lies within the Omineca belt (Price and Monger, 2003; figure 1.5).  
 
The Blais Creek Area is between two extensional faults: the Columbia River Fault and the 
Okanagan-Eagle River Fault.  
 
The Early to Middle Eocene Columbia River fault zone is a crustal scale, east-dipping, 
extensional detachment fault that follows the northwest-trending valley of the Columbia River 
near Revelstoke, B.C. (Price and Monger, 2003). It separates middle to lower crustal rocks of the 
Shuswap metamorphic core complex, which have been ezhumed from a depth of about 25 km, 
from upper crustal low-grade metasedimentary rocks of the Selkirk Mountains (Price and 
Monger, 2003). Displacement on the Columbia River fault has exhumed not only the deepest 
levels of the allochthonous wedge of detached and displaced North American basement rocks,
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which are known as the Monashee Complex (Price and Monger, 2003). The Monashee Complex 
is also exposed in a window eroded through the Monashee decollement, which is marked by 
thick zone of mylonitic rocks above parautochtonous basement rocks (Price and Monger, 2003).  
 
The Okanagan – Eagle River fault zone is an Early and Middle Eocene, west-north-west verging, 
crustal scale, extensional detachment fault that has exhumed the west side of the Shuswap 
metamorphic complex (Price and Monger, 2003; Johnson and Brown, 1996). The rocks in the 
hanging wall include variably metamorphosed Lower Paleozoic and upper Neoproterozoic 
supracrustal rocks of the pericratonic Kootenay terrane, intruded by Late Devonian-Early 
Missipian granodioritic gneiss (Price and Monger, 2003; Okulitchet al., 1975), and also late 
Paleozoic to Jurassic rocks of Intermontane terrane. As with the rocks in the western Selkirk 
Mountains, they have been deformed by tight, syn-metamorphic folds and penetrative foliation 
of Middle Jurassic to early Cretaceous age (Price and Monger, 2003; Johnson and Brown, 1996). 
The higher grade metamorphic rocks in the footwall are of uncertain stratigraphic correlation, 
but appear to comprise parts of the off-scraped North American supracrustal cover that have 
been metamorphosed and penetratively deformed at mid-crustal depths (Price and Monger, 
2003).  
 
The Omineca and Coast Plutonic Belts are linear zones of polyphase deformation, regional 
metamorphism and granitic magmatism that are believed to have formed by extreme shortening 
and thickening of the continental margin during the impingement and accretion of these two 
composite terranes (Journeay, 1986; Monger et al., 1982; Brown, 1981, Read and Brown, 1981; 
Brown et al., 1983; Price, 1986; Brown et al., 1986).  
 
Shortening and tectonic thickening within the southern Omineca Belt, following the obduction of 
accreted terranes in late-Early Jurassic time, is believed to have been accommodated by folding 
and thrust faulting within the miogeclinal cover, and by displacements along major zones of 
decollement faulting and crustal shear at deeper structural levels (Journeay, 1986; Murphy, 1985; 
Price, 1986; Brown et al., 1986). The stacking order and deformation histories of major thrust 
sheets which make up the southern Omineca Belt indicate at least one and perhaps two 
sequences of piggyback or break-foreward thrusting, in which tectonically thickened portions of 
the evolving Hinterland (Shuswap Metamorphic Complex) were successively detached and 
displaced eastward along foreland-stepping zones of crustal shear (Journeay, 1986; Brown et al., 
1986). In order of emplacement, these imbricate thrust sheets are: the Quesnell Lake Sheet,
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which includes obducted tectonic slivers of Quesnellia and the Eastern Assemblage (Journeay, 
1986; Rees, 1981), the Selkirk Allochton, a thickened wedge-shaped mass of deformed 
miogeoclinal rocks (Journeay, 1986; Brown, 1980, 1981; Read and Brown, 1981) and the 
Monashee Complex, an exhumed antiformal stack of basement-cored thrust and fold nappes 
(Journeay, 1986; Read and Brown, 1981; Brown and Read, 1983). The Monashee Complex is a 
folded mylonitic shear zone which separates Aphebian basement gneiss and overlying 
metasedimentary cover rocks of the Monashee Complex in its footwall from high-grade 
miogeoclinal rocks of the Selkirk Allochton in its hanging wall (Journeay, 1986; Brown, 1980, 
1981; Read and Brown, 1981; figure 1.6). It is interpreted to have formed as a floor thrust during 
initial shortening and eastward overthrusting of the Selkirk Allochton in Middle Jurassic time 
(Journeay, 1986; Read and Brown, 1981; Brown and Read, 1983), and to have later acted as a 
roof thrust during deformation and imbriction of the underlying Monashee Complex (Journeay, 
1983, 1986; Brown and Read, 1983). The Monashee decollement separates the Monashee 
Complex from overlying highly sheared metasedimentary rocks and amphibolites.
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Figure 1.4. Geological map of the southern Omineca Belt and that part of the southern Foreland Belt in British Columbia (Mathews and 
Monger, 2005)
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Figure 1.5. Regional tectonic map of the southern portion of the Omineca Crystalline Belt (Brown and Journeay, 1987) . 
Tectonic belts of Canadian Cordillera: 1 = Foreland fold and thrust Belt; 2 = Omineca Crystalline Belt; 3 = Intermontane 
Belt;     4 = Coast Plutonic Complex (Brown and Journeay, 1987). 
Bounding faults: QLF = Quesnell Lake fault; MD = Monashee Decollement; OF = Okanagan fault; ERF = Eagle River 
fault;   SLF = Slokan Lake Fault; NF = Newport fault; GRF = Grandby River fault; KRF = Kettle River fault; SCF = 
Standfast Creek fault; CRF = Columbia River fault; VSZ = Valkyr River fault (Brown and Journeay, 1987). 
Map units: 1 = Late Proterozoic and Mesozoic age units of uncertain provenance and accreted terranes; 2 = Proterozoic 
and lower Paleozoic continental margin sequences; 3 = Proterozoic Belt-Purcell Group; 4 = Proterozoic platform 
sequence of Monashee Complex; 5 = Aphebian basement terrane of Monashee Complex; 6 = undifferentiated Paleozoic 
plutonic rocks; 7 = granitic intrusions, predominantly Middle Jurassic and Lower Cretaceous; 8 = deformed Upper 
Cretaceous to Eocene granitic intrusions; 9 = undifferentiated Cenozoic plutonic and related volcanic rocks (Brown and 
Journeay, 1987). 
MC = Monashee Complex; VC = Valhalla Complex; OC = Okanagan Complex; KC = Kettle Complex; OD = Okanagan 
Dome; KD = Kettle Dome; FCD = Frenchman Cap Dome; TOD = Thor-Odin Dome; RMT = Rocky Mountain Trench 
(Brown and Journeay, 1987)