Jurassic and Cretaceous plutonic and structural styles of the Eagle Plutonic Complex, southwestern British Columbia, and their regional significance

Abstract

The Eagle Plutonic Complex is an elongate north-northwest-trending body of deformed Middle to Late Jurassic and middle Cretaceous rocks which underlies the southwestern margin of the Intermontane terrane. New mapping of the complex and its country rocks, in concert with geochronometry, has defined episodes of contractional, ductile deformation in the Middle to Late Jurassic and middle Cretaceous, as well as brittle deformation in Tertiary time. Synkinematic Middle to Late Jurassic Eagle tonalite at the eastern margin of the Eagle Complex intrudes mylonitic Nicola Group rocks and structurally overlies them along a southwest-dipping belt of high strain (Eagle shear zone) with a structural thickness of > 1 km and a strike length of > 100 km. In the central and western Eagle Complex, Eagle tonalite grades into tonalite orthogneiss (Eagle gneiss), and both are crosscut by mid-Cretaceous, muscovite-bearing plutons of the Fallslake Plutonic Suite. Fallslake Suite rocks are themselves ductilely deformed along the Pasayten fault, which bounds the Eagle Complex on the west and was active mainly in the mid-Cretaceous (ductile deformation with sinistral, east-side-up, reverse displacement). The Jurassic and Cretaceous episodes of deformation may reflect the respective initial and final stages of the accretion of the Insular terrane to the North American margin. West of the Pasayten fault, Middle to Late Jurassic and older(?) rocks of the Zoa Complex are structurally overlain, in part, by deformed Middle Eocene and middle Cretaceous sedimentary rocks. In the north, the Middle Eocene rocks are intruded on their west side by the Middle Eocene Needle Peak pluton.

East-derived strata in the Methow basin record rapid mid-Cretaceous uplift of the southern Coast Mountains batholith

“…Our interpretation of an eastern Coast Mountains batholithwestern Okanogan Range source for the east-derived strata of the Methow basin is consistent with previous studies that document rapid Albian uplift and unroofing of plutons east of the Methow basin (Grieg 1992;Hurlow 1993;Garver and Brandon 1994;Kiessling 1998). The Pasayten fault likely served as the locus for this early Albian uplift of eastern sources; in Washington State, the Pasayten fault zone includes a 1 km thick mylonitic shear zone that records sinistral transcurrent displacement with a component of down-to-the-west slip (Hurlow 1993).…”

“…The Methow basin formed on the southern Methow block, which would have been in a forearc position relative to the JurassicCretaceous magmatic arc of the southern Canadian Cordillera and northern Washington State (Monger et al 1994;DeGraaff-Surpless et al 2003;Gehrels et al 2009); however, the exact position of Methow basin relative to this long, north-south-trending magmatic arc has clearly been modified by younger brittle, dextral strike-slip motion along the Pasayten fault (e.g., Grieg 1992;Hurlow 1993). Methow sediment sources must have been almost entirely limited to sources just east of the middle Cretaceous Pasayten fault, and included only a few grains from other potential sources further east, including eastern Okanogan intrusive rocks.…”

“…However, sinistral oblique slip on the Pasayten fault was coeval with contraction along the Methow River thrust zone to the south, which Hurlow (1993) inferred was due to counterclockwise rotation of the southern Methow block. Alternatively, sinistral oblique motion on the Pasayten fault has been interpreted as transpressional along a steeply dipping reverse fault (Grieg 1992). Grieg et al (1992) suggest that the ductile strain in the 110.5 ± 2 Ma Fallslake plutonic suite of the Eagle Plutonic complex in southern British Columbia developed during sinistral, east-side-up reverse displacement, with northeast-dipping foliation indicating a northeast-dipping orientation for the Pasayten fault during transpressional ductile shear.…”

Late Jurassic flare‐up of the Coast Mountains arc system, NW Canada, and dynamic linkages across the northern Cordilleran orogen

“…See text for interpretation. Arc and arc proximal: 1—Saint Elias Mountains, Yukon (this study); 2—Eastern Coast Mountains, British Columbia [ Gehrels et al ., ]; 3—Western Coast Mountains [ Gehrels et al ., ]; 4—Blue Mountains, Oregon [ LaMaskin et al ., , and references therein]; 5—Sierra Nevada batholith, California [ Ducea , ]; 6—Alexander‐Wrangellia‐Peninsular composite terrane forearc and arc, Alaska [ MacKevett , ; Trop and Ridgway , ]; 7—Coast belt, British Columbia [ Greig , ]; 8—Franciscan Complex, California [ Wakabayashi and Dumitru , ]; 9—Sierra Nevada arc and host rocks, California [ Cao et al ., ]; 10—Chugach terrane (McHugh complex) strata, Alaska [ Amato and Pavlis , ]; 11—Gravina belt strata, southeastern Alaska and British Columbia [ Yokelson et al ., ]; Retroarc: 12—Omineca belt, Yukon [ Allan et al ., ; Bailey , ; Staples et al ., ]; 13—Intermontane belt, Yukon and British Columbia [ Evenchick et al ., ; White et al ., ]; 14—Omineca and Foreland belts, Alberta and British Columbia [ Colpron et al ., , Evenchick et al ., ; Pană and van der Pluijm , , and references therein]; 15—Luning‐Fencemaker belt, Nevada [ Wyld , ; DeCelles , ]; 16—Sevier hinterland, Idaho, Utah, and California [ Hoisch et al ., ; Cruz‐Uribe et al ., ]; 17—Whitehorse trough, Yukon [ Colpron et al ., ]; 18—Bowser basin, British Columbia [ Evenchick et al ., ]; 19—Alberta foreland basin [ Poulton , , Raines et al ., , and references therein]; and 20—Montana foreland basin [ Fuentes et al ., , and references therein]. Geological timescale of Cohen et al .…”

“…In the Coast Mountains of British Columbia, a 157–142 Ma high‐volume event with an apparent intrusive rate of >800 km 2 /Myr is recognized by intermediate to felsic rocks of the Coast Mountains batholith [ DeCelles et al ., ; Gehrels et al ., ]. The timing of high‐volume magmatism was coeval with Late Jurassic tectonism along the length of the Coast Mountains arc system (Figure ), including forearc and intraarc deformation in south central Alaska [ Trop and Ridgway , ], deformation of arc plutons in eastern Alaska and southwestern Yukon [ MacKevett , ], and regional deformation along the western and eastern boundaries of the Coast belt in British Columbia [e.g., Monger , ; Greig , ; van der Heyden , ].…”