Abstract of paper presented at American Geophysical Union 1986 Spring Meeting, Baltimore, Maryland.
Trans. American Geophysical Union (Eos), v. 67, p. 269-270.

Normal- and reversed-polarity synfolding CRM along the Brooks Range mountain front, Northern Alaska

D. R. Van Alstine, Applied Paleomagnetics, Inc.

During the summer of 1984, a paleomagnetic study was conducted on 596 samples collected at 17 localities distributed over nearly the entire 1,000 km width of the Brooks Range mountain front, from the Lisburne Peninsula on the southwest to the Sadlerochit Mountains on the northeast. The sampled rocks represent predominantly marine (neritic) carbonates ranging in age from late Mississippian to early Cretaceous. These strata experienced only mild heating during the Brooks Range orogeny, based on CAI between 1.0 and 3.0.

Progressive demagnetization (thermal, AF, and chemical) and IRM acquisition reveal that the characteristic magnetization at all localities is CRM residing in diagenetic magnetite and hematite, probably produced by oxidation of early diagenetic pyrite. In structurally deformed sections along the mountain front, the CRM is clearly synfolding, commonly appearing to have been acquired midway through the deformation. The prevalence of 50% synfolding CRM, in northern Alaska, in the Appalachians, and in the Overthrust Belt, probably reflects a near-equal balance between diagenetic grains that grew at the onset and at the termination of the orogeny. This relationship suggests that tectonic tilting controls the initiation and cessation of the fluid migration responsible for the chemical remagnetization.

Along the mountain front, exclusively normal-polarity CRM was found from the Lisburne Peninsula through the central Brooks Range. Exclusively reversed-polarity CRM was found in the Sadlerochit Mountains and in drillcores from depths of at least 9,000 feet beneath Prudhoe Bay. The normal-polarity CRM could date from the Cretaceous Long Normal Interval, or, equally likely, from a time of normal-polarity-bias between 80 and 63 m.y.B.P. The reversed-polarity CRM probably dates from the Paleocene to middle Eocene (63 to 43 m.y.B.P.) interval of strong reversed polarity bias. The younger age of the reversed-polarity CRM in northeastern Alaska is consistent with geologic evidence for southwest to northeast younging of deformation; tilting did not begin in the Sadlerochit Mts. until the Paleogene.