Structural Geology and Metamorphic Petrology

Crustal thickness, its thermal state, and the regional strain field orientation are critical factors in continental arc formation and evolution. These parameters will be constrained by combining structural and metamorphic studies with geochronology and igneous geochemistry. In combination, these studies will result in an understanding of the relation between pluton emplacement, shear zones, and burial and exhumation of mid-crustal rocks.

Understanding the changes in crustal thickness has critical implications for the formation of an ultramafic root beneath the batholith. If the arc crust was relatively thin during batholith formation, then much of the residues produced beneath the batholith would be relatively buoyant granulite facies rocks. If, however, the arc crust was overthickened during batholith formation, dense eclogite facies residues would be expected to from, setting the stage for foundering of the root. Combined with careful geochronologic work, structural and metamorphic studies provide the best constraints on past crustal thickness and how crustal thickness varied through time. We will integrate structural studies of crustal shear zones and deformation events in metamorphic rocks and supracrustal units with P-T-t studies of metamorphic rocks that host the batholith and the batholith itself. Together, these studies will constraint where and when ultramafic residues may have been formed.

Processes of crustal formation and distillation are strongly affected by the thermal state of the crust. Metamorphic dehydration of supracrustal rocks may produce dense and highly refractory lithologies typical of the lower and middle crust. To understand the thermal state and the potential for production of dense refractory rocks, we will determine the metamorphic reactions that led to the production of peak metamorphic assemblages and partial melting. This information will (1) place constraints on what lithologies may be encountered at depth and help predict what restite lithologies might be present in the lower crust, and (2) determine the composition of melt produced in reactions. Mixing of this melt with other magmas could have produced the hybrid magmas of the plutons. These data are also critical for constraining the WARRP profiles. Such studies were done in the northern CGC between 52 and 54 o N in the 70’s and 80’s, and in the Nisling terrane along the proposed northern transect (Gareau, 1991 a and b; Gareau and Woodsworth, 2000). New techniques, however, allow a much more comprehensive study to be done on the southern CGC and Nisling terrane.

Tracking changes in the orientation of the regional strain field will help define the conditions of the crust during formation and evolution of the batholith. Removal a dense root should be linked with significant changes from regional contraction to extension along the length of the batholith. Such events are well documented in the northern part of the CGC, but whether they continue to the south is unknown. To address this question, we will document the timing, direction, and magnitude of displacement on regional shear zones located east of the CSZ and on the boundary of the CGC. The metamorphic and structural evolution of the southernmost exposures of the CGC will be compared to its northern parts, and the interrelation of shear zones, plutonism and formation of CGC will be explored. Once their age and mode of formation is well understood, the shear zones and metamorphic foliations will provide records of finite strain patterns through time.

Further information on strain patterns will be derived from plutonic bodies, which can reveal strain directions present during their intrusion. Dikes and sills commonly open perpendicular to the incremental extension direction, so statistical analysis of dike orientations and observations of crosscutting relationships of dike networks will provide constraints on the local extension directions during plutonism (eg. Kirby et al, 1995). Dating of key dikes will help pin down the timing of any changes in strain field orientation.

The ductile strain associated with plutons will be used to investigate the orientation of finite shortening and extension directions present around and within plutons during batholith formation. Because different mechanisms that operate during batholith formation will give rise to substantially different stress and strain fields, it is critical to evaluate how the plutons were intruded within the CPC. For example, Chardon, (2003) used magmatic foliations in the plutons and the deflections of foliations adjacent and farther away from the pluton to place constraints on the mechanisms of pluton emplacement around the 90 Ma Ecstall pluton north of Douglas Channel. From this type of information, a picture of the regional strain field may be constructed, and information on the tectonic setting may be inferred. For instance, plutons intruded into ‘normal’ convergent orogenic settings will commonly record horizontal shortening and vertical extension (eg. Ingram and Hutton, 1994; Miller and Patterson, 2001). Those intruded during regional extension will typically record vertical shortening and horizontal extension (eg. Andronicos et al., in press; Vigneresse , 1999).

Part of the geologic background required to answer the structural and metamorphic questions has been established by prior studies along the transect lines (Gareau, 1991 a and b; Gareau and Woodsworth, 2000; Gehrels and Boghassian, 2000; Rusmore et al., 2000, 2001). Despite this work, major gaps in critical geologic background knowledge exist in two parts of the proposed study area: (1) between the northern and southern transects, from the termination of the CGC south toward Bella Coola; and (2) west of the Coast shear zone, especially on the southern transect. Andronicos, Rusmore, Hollister, and Woodsworth will complete the structural and metamorphic studies in these two areas.

In the region of the southern termination of the CGC, we will document the evolution of the CGC, especially its relation to latest Cretaceous - early Tertiary plutons and its ultimate cooling. This area provides the most southerly glimpse of the latest Cretaceous and early Tertiary mid-crust. Particular attention will be paid to the CGC boundaries and the transition from these highly metamorphosed rocks to low-grade rocks of the southern transect. Regional mapping shows large north-northwest trending faults in this area (Woodsworth 1980; Mahoney et al., 2002). We seek to determine what role, if any, these faults played in the exhumation of the CGC, and to evaluate their relation to the Early Tertiary magmatic pulse.

The second area of proposed new field geologic studies, west of the CSZ, provides us with the opportunity to characterize the mid-crust prior to the magmatic flare-up. Previous studies south of the southern transect show that the deformation and prograde metamorphism are older than 85 Ma (Gehrels and Boghassian, 2000; Rusmore et al., 2000), so work in this area will focus on the kinematics of Late Cretaceous pluton emplacement, and the cooling history of the metamorphic and plutonic rocks. Studies of these rocks will provide us some of the best information on the thermal and mechanical structure of the crust prior to the Late Cretaceous Early Tertiary magmatic flare up.

Overall, our results will allow us to track the evolution of the arc from inception through demise. This information will be the foundation for interpretation of the seismic experiments and, ultimately, the nature of the processes involved in the production of continental crust. We will coordinate our results with Lori Kennedy of UBC, who has ongoing research projects in the Bella Coola area.