Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

Abstract

Following growth of the Canadian Cordillera during the Mesozoic, the southern Cordillera was subject to extension during the Paleocene and Eocene that correlated with widespread volcanic activity in south-central British Columbia, including across much of the Nechako-Chilcotin plateau. In 2008, Geoscience BC acquired 330 km of deep vibroseis reflection profiles on the plateau, mostly over the Stikinia arc terrane, but also over its eastern contact with the oceanic Cache Creek terrane. All seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the Moho, which is defined by the downward termination of reflectivity at 11-12 s. In the uppermost crust, extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth. Extension in the deeper upper and middle crust, which was partly controlled by antiforms likely related to earlier shortening, was accommodated on a network of anastomosing shear zones that sole out into the top of the reflective lower crust. The lower crustal reflections correlate with seismic P-wave velocities of 6.45-6.98 km/s, indicating that the reflective lower crust has a more mafic composition than the middle crust. As in other extensional settings, we suggest that this pervasive fabric of reflectors arises from the intrusion of mantle-derived basaltic magma into zones of ductile shearing, and that differentiation of these melts resulted in the widespread Paleocene to Eocene volcanism. Reflector dips indicate that extension was approximately east-west, consistent with north-northwest-trending horsts separated by basins filled with Paleocene to Eocene volcanic and volcaniclastic rocks.Résumé : Après la croissance de la Cordillère canadienne au Mésozoïque, la Cordillère sud a subi une extension durant le Paléocène et l'Éocène; cette extension était corrélée avec une activité volcanique étendue dans le centre-sud de la ColombieBritannique, incluant à travers une grande partie du plateau Nechako-Chilcotin. En 2008, Geoscience BC a acquis 330 km de profiles vibrosismiques en profondeur sur le plateau, surtout au-dessus du terrane d'arc de Stikinia mais aussi par-dessus son contact est avec le terrane océanique de Cache Creek. Les sept lignes de sismique réflexion révèlent toutes une croûte inférieure fortement réflective qui s'étend depuis 7-9 s jusqu'au Moho, ce qui est défini par la terminaison vers le bas de la réflectivité à 11-12 s. Dans la croûte sommitale, l'extension s'est produite par le morcellement en bloc par des failles; les failles formant une semelle dans les détachements subhorizontaux ou avec un léger pendage, au-dessus de la profondeur de 10 km. L'extension dans les croûtes supérieure et médiane, plus profondes, partiellement contrôlée par des antiformes fort probablement reliés à un rétrécissement antérieur, était facilitée par un réseau de zones de cisaillement anastomosées qui forment une semelle sur le dessus de la couche inférieure réfléchissante. Les réflexions de la ...

Distribution of Paleogene and Cretaceous rocks around the Nazko River belt of central British Columbia from 3-D long-offset first-arrival seismic tomography

“…The boundary between the two terranes is not precisely known because younger volcanic and glacial deposits cover the contact (Riddell 2011). Further north the Cache Creek terrane is interpreted to have been thrust over the Stikine terrane along an east-dipping fault (Struik and MacIntyre 2001), which is consistent with the identification of the terrane boundary on line 13 of the Geoscience BC survey (Calvert and Talinga 2014). The eastern boundary of the Cache Creek terrane with the Quesnel arc terrane corresponds approximately to the location of Tertiary Pinchi fault (Fig.…”

“…In the b-16-J well, these rocks are 500 m thick, but >3500 m accumulated locally in a rapidly deepening basin that was intersected by well b-22-k (Riddell 2011). Crustal extension across this region beginning in the Paleocene produced basins into which substantial thicknesses of volcaniclastic sediments were similarly deposited Calvert and Talinga 2014). If originally deposited, the preservation of any Cretaceous sedimentary strata beneath these Tertiary rocks would depend on the level of erosion prior to Tertiary crustal extension.…”

“…The southern Cordillera was subject to extension in the Paleocene to Middle Eocene (Monger and Price 2002), creating a number of subbasins into which sediments were deposited prior to and during extensive regional volcanism (Grainger et al 2001;Bordet et al 2014). This normal faulting may preserve Cretaceous reservoir rocks within graben structures beneath Eocene volcanic rocks (Hannigan et al 1994;Calvert and Talinga 2014).…”

“…Horizontal seismic reflectors are observed above the western basement ridge within small <800 m deep depressions, and a synformal sequence of reflections is also present above the ridge's western flank (Calvert and Talinga 2014). The seismic velocities that correspond to these seismic reflectors are 3.6-4.5 km/s, but it is unclear whether they represent volcanic or sedimentary rocks.…”

Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation?

“…Geophysical profiles through this region image buried crust. These profiles include a 260-km-long NE-SW seismic refraction line through eastern Washington (Catchings and Mooney, 1988), a receiver function study in the High Lava Plains of southeastern Oregon and surrounding areas (Eager et al, 2011), and a network of vibroseis (seismic vibrator) reflection profiles in south-central British Columbia (Calvert and Talinga, 2014). The locations of these lines are shown on Figure 3.…”