Deformation of continental crust along a transform boundary, Coast Mountains, British Columbia

Abstract

New structural, paleomagnetic, and apatite (U‐Th)/He results from the continental margin inboard of the Queen Charlotte fault (∼54°N) delineate patterns of brittle faulting linked to transform development since ∼50 Ma. In the core of the orogen, ∼250 km from the transform, north striking, dip‐slip brittle faults and vertical axis rotation of large crustal domains occurred after ∼50 Ma and before intrusion of mafic dikes at 20 Ma. By 20 Ma, dextral faulting was active in the core of the orogen, but extension had migrated toward the transform, continuing there until <9 Ma. Local tilting in the core of the orogen is associated with glacially driven, post‐4 Ma exhumation. Integration with previous results shows that post‐50 Ma dextral and normal faulting affected a region ∼250 km inboard of the transform and ∼300 km along strike. Initially widespread, the zone of active extension narrowed and migrated toward the transform ∼25 Ma after initiation of the transform, while dextral faulting continued throughout the region. Differential amounts of post‐50 Ma extension created oroclines at the southern and northern boundaries of the deformed region. This region approximately corresponds to continental crust that was highly extended just prior to transform initiation. Variation in Neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid‐Cretaceous plutons. Similarities to the Gulf of California suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops.

Tectonics and Structure of the Queen Charlotte Fault Zone, Haida Gwaii, and Large Thrust Earthquakes

“…It is a major transcurrent fault structure that may have carried terranes to the northwest (e.g., Evenchick et al, 1999), but there is no indication of significant motion since the Eocene (e.g., Rusmore et al, 2001). Also, this fault appears to have been deformed from a likely original nearly linear trend by midTertiary extension (e.g., Rusmore et al, 2010). Although no recent motion has been detected (summary by Hippchen, 2011), this is a difficult area in which to resolve active faulting, and no detailed study has been carried out specifically directed at recent motion.…”

“…This plate explains inferred northward terrane transport along the southeastern Alaska-British Columbia margin between 70 and 50 Ma (e.g., Pavlis et al, 2004;Christeson et al, 2010;McCrory and Wilson, 2013). Rusmore et al (2010) described the 250 km wide region along the margin and showed evidence for a complex history that includes normal and dextral faulting since about 50 Ma. They concluded that the zone of active extension narrowed and migrated toward the margin transform through time, leading to formation of the Queen Charlotte basin about 20-30 Ma after the initiation of the transform margin.…”

“…The extension may have re- sulted from a period of 5°-10°very oblique extension relative to the current margin trend and caused the extensive Massett volcanic and intrusive rocks found mainly on Haida Gwaii (e.g., Hickson, 1991;Souther and Jessop, 1991;Hamilton and Dostal, 1993;Hyndman and Hamilton, 1993;Rusmore et al, 2010) and crustal thinning in the Queen Charlotte basin ( Fig. 4) (Spence and Asudeh, 1993;Lowe and Dehler, 1995).…”

“…Slowly northwesterly moving mainland coastal block (modified afterRusmore et al, 2010) inferred from GPS data. The prominent Coast Shear zone may define the landward boundary of the moving block, but no recent motion has been identified on it.…”

Paleogeography of the Insular and Intermontane terranes reconsidered: Evidence from the southern Coast Mountains Batholith, British Columbia

“…Arc magmatism was driven by www.gsapubs.org | Volume 5 | Number 5 | LITHOSPHERE subduction of oceanic plates from the Jurassic until Early Tertiary time (Armstrong, 1988;Engebretson et al, 1985;Lonsdale, 1988;Monger et al, 1982;Stock and Molnar, 1988;van der Heyden, 1992), when contact with the Pacifi c plate began to convert the margin from convergent to dextral transform (Engebretson et al, 1985;Haeussler et al, 2003;Hyndman and Hamilton, 1991;Madsen et al, 2006;Stock and Molnar, 1988). Signifi cant Early Tertiary crustal extension disrupted the central part of the orogen, adjacent to the transform boundary, causing opening of the Queen Charlotte basin and tilting of crustal blocks within the batholith (Butler et al, 2001a;Butzer et al, 2004;Dehler et al, 1997;Rohr and Dietrich, 1991;Rohr and Currie, 1997;Rusmore et al, 2010). A convergent margin persisted along the southern part of the orogen and produces an active volcanic arc in the Pacifi c Northwest.…”