Paleomagnetism of Eocene rocks of the Kelowna and Castlegar areas, British Columbia: studies in determining paleohorizontal

Abstract

The middle Eocene Marron volcanics (mean age 52 ± 2 Ma) of the Kelowna outlier form the upper part of the hanging wall of the westerly dipping Okanagan Valley fault (OVF) in south-central British Columbia. They overlie Quesnellia. The OVF is currently interpreted as the westernmost member of a network of low-angle extension faults in the southern Omenica belt. The OVF was active in the middle Eocene at much the same time that the Marron volcanics were cooling. Relative to present horizontal, the magnetizations are widely scattered (Fisher's precision parameter k = 8) and after correction for bedding attitudes, there is no significant improvement (k = 9). Evidently, some magnetizations were acquired before (referred to as category 1) and others after (category 2) tilting; that is, the horizontal plane at the time of magnetization sometimes did and sometimes did not coincide with the bedding plane. Partial unfolding experiments, carried out on the two categories separately, yield a precision comparable to that expected for paleosecular variation, and a mean direction (D, I) of 352°, 70° (24 sites spanning 2000 m, 275 specimens, k = 23, α95 = 6°, paleopole 86°N, 230°E, A95 = 10°). The Marron is predominantly normally magnetized. Rock units slightly older and others slightly younger are reversely magnetized. The transition from reversed to normal polarity occurs in basal beds of the Marron Formation. The overall mean direction of the Marron and stratigraphically adjacent units is 352°, 69 °(28 sites, 300 specimens, spaning 4000 m, k = 21, α95 = 6°), yielding a paleopole at 85°N, 197°E (A95 = 10°), which is in excellent agreement with that for middle Eocene rocks of cratonic North America. Hence this part of Quesnellia had reached its present position relative to North America by middle Eocene time, and there has been no significant rotation of it. In contrast, the mean direction (020°, 72°, k = 9, α95 = 11°) after correction for bedding (calculated assuming the magnetization to be entirely pretilting) implies a clockwise rotation of 28°. We believe that this is incorrect; the apparent rotation, we argue, is caused by wrongly assuming that the bedding plane always coincides with the paleohorizontal at the time magnetism is acquired.Further tests have been carried out on intrusive and metamorphic core-complex rocks in the region of Eocene crustal extension 100 km to the east of Kelowna. These rocks are coeval with the Marron, and are located in both the hanging walls and footwalls of the Slocan Lake normal extension fault, which dips 30° eastward. Paleodirections are very different from those at Kelowna (four bodies, mean direction (D, I) 60°, 52°, k = 66, α95 = 6°), and we argue that this divergence is caused by tilting 37 ± 10° to the west antithetical to the Slocan Lake fault. We suggest that paleomagnetism provides a means by which tilts in such plutonic and metamorphic terrains can be determined. We suggest further that such tilts may have been responsible for some of the aberrant magnetizations observed in plutonic rocks of the Coast Plutonic Complex being much more widespread in the cordillera than previously envisioned.