Paleomagnetism of late Paleozoic rocks in the northern Cache Creek terrane near Atlin, British Columbia

Abstract

The Cache Creek terrane is exposed along the length of the Canadian Cordillera and is composed of oceanic strata that are probably, at least in part, exotic to North America. In the northern portion of the Cache Creek terrane near Atlin, British Columbia, paleomagnetic samples were collected from layered Paleozoic rocks at 22 sites (≥ 6 samples/site) on Alfred Butte. Principal component analysis of detailed thermal demagnetization data allowed clear isolation of a characteristic remanent magnetization (ChRM) from 17 of these sites. Blocking temperatures to 680 °C indicate that this magnetization is carried by hematite, and site-mean ChRM directions are determined with α95 < 10° for the majority of sites. On Sentinel Mountain, samples were collected from 16 sites in layered Paleozoic volcanic and chert rocks and from a diabase sill. Thermal demagnetization revealed a ChRM in the chert and volcanic rocks with blocking temperatures to 680 °C, whereas alternating-field demagnetization to 40 mT successfully isolated ChRM in the diabase sill. ChRM directions from four sites involved in a mesoscopic S-fold at Alfred Butte fail the fold test, indicating that the ChRM is a postfolding secondary remagnetization. Tests for relative age of structural tilting and remagnetization are ambiguous, with attendant uncertainties in tectonic interpretations. However, rock-magnetic and geologic constraints argue for a chemical remagnetization of these Paleozoic rocks in Late Triassic to Middle Jurassic time, possibly associated with structural juxtaposition of the Cache Creek and Stikine terranes along the Nahlin fault zone. Although certainly nonunique and speculative, the simplest tectonic interpretation of these paleomagnetic data involves postfolding and posttilting remagnetization during the Early Jurassic in a paleolatitudinal position that approximately agrees with predicted North American paleolatitudes for this time.