Ministry of Employment and Investment CHAPTE,R3 STRUCTU'W AND METAMORPIlI!SM"- OVERVIEW rassic or Early Cretaceous deformationis also doclimented locally within the area, an by angular unconformity separat- The structure of the Taseko Bridge River areais domi- - ing the lower and upper unitsthe of Relay Mountain Group nated by a systemof northwest to north-trending faults that east of Lake. However, the extent of Jura!:sic and reflect a complex history of mid-Cretaceous to Tertiary con- Lorna Early Cretaceous deformation remains poorly undmtood. tractional, strikeslip and extensional deformation.The old- Although the Middle Jurassic has been pslulateli as the est map-scale structures include systems of mainly time of initial thrust-amalgamation ofCadwallader Terrane southwest-vergent thrust faults that have been mapped with the Bridge River andShulaps complexes (Pottf r, 1986; within several discontinuousbelts extending from the south- Rusmore et al., 1988). the present study documens struc- east comer of the map area toBig Creek. These faults are, tures of this age only in Bridgethe RiverComplex, and pro- at least in part, Cretaceous in age and are inferred to be vides no clear evidence for thrust imbrication and .lplift of broadly synchronouswith deposition of synorogenicclastic adjacent terranes until the mid-Cretaceous. rocks of the mid-Cretaceous Taylor Creek Group and Sil- verquick formation; angular unconformities beneath the Within the Taseko - Bridge River map area, i~enetra- Hauterivian unit of the Relay Mountain Group, the Albian tively deformed metamorphic rocks are restricted to the Taylor Creek Group, and the Upper Cretaceous SilverquickShulaps Range and contiguous Mission Ridge,whxe they and Powell Creek formations may relate to this protracted are represented mainly by the Bridge River schists (Potter, interval of deformation. Contractional deformation contin- 1983, 1986; Coleman and Parrish, 1991) of the Shulaps - ued into the Late Cretaceous andgenerated a later suite of Mission Ridge metamorphic belt. Potter (1983) attributed southwest-vergent oblique-sinistral reverse faults, includ- the metamorphism and relatedductile deformation toover- ing the Eldorado fault system,as well as northeast-vergent thrusting of the Bridge River Complex bythe Shulaps U1- thrust faults and folds that are recognized at several places tramafic Complex in Early to Middle Jurassic time. within the map area. Subsequent dating of penetratively deformed Eocene dikes Later deformation was dominated by dextral strike-slip within the Bridge Riverschists led him to revise th,is inter- faulting, which occurred in latest Cretaceous through Eo- pretation somewhat, andsuggest a Mesozoic phash of am- cene time. Dextral faults are the most prominent and con- phibolite to greenschist-facies metamorphism as!:ociated tinuous structures in the map area, and include the Castle with Shulaps thrusting, followed bya phase of Eocene de- Pass fault and the Yalakom- Marshall Creek- R.elay Creek formation and greenschist-facies metamorphism (Potter, fault system. Extensional faults, such as the Mission Ridge 1986). During thecourse of the present study, work byAr- fault (Coleman, 1990), are locally important and are spa- chibalderal. (l989,1990,199la,b),Calon er al. (15'90) and tially and temporally associated with dextralstrike-slip sys- Coleman (1990) has provided further constraint: on the tems. Northeast-striking faults with minor offsets are not structural and metamorphic evolutionof the Shulaps- Mis- abundant,but are conspicuous as they transectthe northwest sion Ridge metamorphic belt.These studies suggest that the structural grain of the area; they are among the youngest metamorphic rocks in the Shulaps Range record at least structures in the map area.. threedistinct events: Permianocean-floormetamorphismof the Shulaps ophiolite complex; metamorphism related to The Cretaceousand Tertiary structures which dominate Cretaceous dike intrusion during thelate stages of imhrica- the map pattern of the region are superimposed on older tion and emplacement of the Shulaps Complex akove the structures that are generally not well understood.The oldest Bralome-East Liza Complexand Cadwallader Grcup; and recognized structures are synplutonic faults and ductile Eocene metamorphism and ductile deformation that af- shears within plutonic knockersof the Shulaps serpentinite fected the Bridge River schistsbefore their upliftand juxta- mLIange, that formed duringlate Paleozoic construction of position against the Shulaps Complex across a system of Shulaps oceanic crust. Early structuresalso include outcrop- Eocene normal faults.The Eoceneevents were sync'hronous scale brittle faults that pervade the BridgeRiver Complex, with dextral strike-slip onthe Yalakom - Marshall'Creek leading to a pronounced lenticularity of lithologic units. - Relay Creekfault system. Much of this deformation,as well as that recorded in pene- tratively deformed blueschist-facies rocks, is attributed to Most rocks outside the ShulapsRange are unmetamor- deformation within an accretion-subduction complex. This phosed or at very low metamorphic grade; higher-grade deformation was apparently operative, perhaps episodi- rocks occur locally nearsome of thelarger Cretaceous-Ter- cally, from theMiddle Triassic to at least late Middle Juras- tiary plutons. In general, the Bridge River Compkx, Cad- sic time, after which the Bridge River Complex was wallader Group and Bralorne-East Liza Complex depositionally overlain by clastic sedimentary rocks of the commonly contain metamorphic minerals indicative of Relay Mountain Group and Cayoosh assemblage. LateJu- prehnite-pumpellyite-facies metamorphism, whereas other Bulletin IO0 91 units, comprising mainlyJurassic and Cretaceous sedimen- curred during generation of oceanic crust in a divergmt- tary rocks, are essentially unmetamorphosed. Within the plate setting. This deformation was part in ductile in nature, former units, metamorphic minerals are most common in and included local development of penetrative schisto:;ity mafic volcanic and intrusive rocks, where they occur in and mineral elongationlineations in gabbro knockers.Mota- veins and amygdules, and locally as partial replacements ofmorphic recrystallization ranged from growth of hornbleilde original groundmass and phenocryst phases.The metamor- rims around porphyroclastic pyroxene grains, to compjete phic minerals commonly observedinclude chlorite, epidote, replacement ofmafic grains by foliated and lineated amphi- pumpellyite, prehnite, calcite, stilpnomelane, albite and bole. The ductile deformation zonesare locally crosscul, by quartz. The assemblage chlorite-epidote-actinolite was re- Ieucogabbro and gabbroicto dioritic dikes, indicating chat corded in Bridge River metabasalt at one locality west of they were broadly contemporaneous with plutonismduring lower Sebring Creek, and in greenstone of the Bralome - construction of Shulapsoceanic crust. East Liza Complex that outcrops between the two strands Synplutonic metamorphism within the Shulaps U1- of the Marshall Creekfault directly north of Marshall Lake. tramafic Complexis assigned an Early Permian age, based Actinolite is also reported from the volcanic ofunit the Cad- on AI-AI dating oftwo separate amphiboliteknockers fiom wallader Group in the Eldorado Creek area, where it rims the serpentinite mClange. Similar knockers are widespraad clinopyroxene crystals and also occurs as small needles in throughout the m6lange unit, where they commonlyrmge the basaltgroundmass (Rnsmore, 1985). Greenishbiotite is from less thana metre to several tens of metres in size.T'ley present in some of the actinolite-bearing rocks (Rusmore, typically comprisea foliated and/or lineated intergrowth of 1985, 1987).and was also noted with calcite and epidote in hornblende and plagioclase, but locally grade into fint: to a sample of Bridge River greenstone from the north shore medinm-grained diabasic-textured rocks in which amphi- of Downton Lake. Clastic rocks of the Hurley Formation bole only partially replaces pyroxene. Their composition and Bridge River Complex commonly contain calcite and and textural variations are similar to those of synplutcnic chlorite, often in association witheither sericite or epidote. ductile deformation domains withinlarger gabbroic knock- One Hurley sample from the Camelsfoot Range contains ers (Calon et al., 1990). chlorite, calcite and pumpellyite; and another, chlorite, epi- dote and greenish biotite. Sericite is a common component One dated knockeroccurs on the northwestern margin of Bridge River chert and cherty argillite, whichalso may of the Shulaps Complex, about 1.5 kilometres south of contain chlorite or stilpnomelane. Noaxe Lake. This tabular massis about 6metres wide,;.md comprises coarse-grained, massive to weakly lineated,.lo- The low metamorphic grade of rocks outside the Shu- cally brecciated amphibolite.The sample fromthis knocker laps - Mission Ridge metamorphic belt is corroborated by contains well-preserved brown hornblende, sanssnriti;ced the colour alteration indices (Epsteinet al., 1977) of cono- donts extracted from the Cadwallader Group and Bridge plagioclase and epidote,cut by closely spacedquartz veins. Step-heating of a hornblende separate from this rock River Complex (Appendix 1). The colour alteration index (,b- chibald et al., 1991a; Appendix 7, Sample TI,-8843) commonly rangesfrom 2 to 4 for samples collected north of yielded an integrateddate of 253.7+14.7 Ma and a plateau Carpenter Lake, although