The Pootlass High Strain Zone (PHSZ) is a corridor of brittle and ductile deformation, at least 2 km wide and up to 30 km long, located in the Coast Belt just northeast of the boundary between the Intermontane and Insular superterranes, which is demarked by the Coast Shear Zone at this latitude (Rusmore et al., 2001). In this paper, we report recent observations from field mapping, and new geochronological and petrological data, from which we place the PHSZ into a regional tectonic framework. The PHSZ is comprised of volcanic and sedimentary rocks of the lower to mid-Jurassic Hazelton Formation, and Jurassic to Eocene plutonic rocks varying in composition from granodiorite to diorite and tonalite. The field area has undergone at least three phases of deformation; a folding event that resulted in tight, southwest-verging folds correlated with the regional Late Cretaceous Coast Belt thrust and fold system, intense ductile, sinistral shear, and brittle dextral shear. The PHSZ is defined by a steep foliation, with a very well developed subhorizontal stretching lineation, and southwest-verging folds with the fold axes parallel to the stretching lineation. Many of the intrusive rocks occur as syn-kinematic mafic and felsic sheeted intrusions that are tightly folded and display sinistral kinematics. L-tectonites within the deformed plutonic rocks attest to the weakness of the rocks during deformation and support syn-kinematic magmatism. Zones of highest strain are found within the mafic and felsic sheeted intrusions. Predominantly sinistral ductile shear in the PHSZ is overprinted by brittle, dextral faults. Along strike to the northwest, the high strain zones displays both dextral and sinistral ductile shear components. Metasedimentary rocks in the PHSZ reached a maximum of amphibolite facies metamorphism as indicated by the abundance of hornblende. Greenschist facies metamorphism is dominant however, with rocks composed mainly of quartz, chlorite, muscovite and garnet. Ar/Ar, analyses of hornblende and biotite from a folded and sinistrally sheared mafic sheeted intrusion within the PHSZ provide cooling ages of 62.39 ± 0.40 Ma and 63.04± 0.49 Ma, respectively. U/Pb analysis of zircons from syn-kinematic, folded and sinistrally sheared felsic sills within the PHSZ returned crystallization ages of 113.8 ± 2.0 Ma and 160.9 ± 4.4 Ma. Additional petrological, microstructural and geochronological studies are in progress. The PHSZ may be kinematically related to similar transpressive structures in the western Canadian Cordillera; namely the Grenville, Kitkatla and Principe-Laredo sinistral shear zones located to the northwest, that exhibit steep to vertical gneissic foliation in plutonic rock and intense isoclinal folding in supracrustal rocks of lower greenschist to amphibolite metamorphic grade, with estimated temperatures of deformation between 300 and 500°C (Chardon et al., 1999). These shear zones are interpreted to have been active from 110 to 87 Ma, during the intrusion of voluminous batholiths (Chardon et al., 1999).Chardon et al., 1999; Tectonics, Volume 18, No. 2, p. 278-292. Rusmore, M.E. et al., 2001; GSA Bulletin, Volume 113, No. 8, p. 961-975.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 8104 Continental margins: convergent