Late Paleozoic remagnetization of Precambrian crystalline rocks along the Precambrian/Carboniferous nonconformity, Rocky Mountains: a relationship among deformation, remagnetization, and fluid migration

Abstract

Paleomagnetic and rock magnetic data and petrographic observations demonstrate that Archean to late middle Proterozoic rocks (three localities in metamorphic rocks, two in granitic plutons) in parts of the Rocky Mountains, along and below a regional nonconformity with Carboniferous strata, have acquired a secondary magnetization. This magnetization is exclusively of reverse polarity and, based on its unique direction, was presumably acquired during the Permo-Carboniferous Reverse Superchron. At most sites, the remanence is carried exclusively by hematite, although it also resides in magnetite at some localities. Based on comparison between paleomagnetic poles derived from locality mean secondary magnetizations (e.g. Decl.=137.2°, Incl.=−18.2° α95=4.1°, k=221.4, N=7 of seven samples, Archean gneiss, northern Laramie Range; Decl.=148.7°, Incl.=−15.0°, α95=4.6°, k=77.0, N=14 of 23 samples, Sherman Granite, southern Laramie Range; Decl.=174.3°, Incl.=−18.9°, α95=6.6°, k=48.6, N=11 of 13 samples, metavolcanic rocks, Sandia Mountains) and late Paleozoic reference poles for North America, this secondary magnetization does not appear to have been acquired as a direct consequence of subareal erosion prior to deposition of younger sedimentary strata. At two localities, the age of the remanence is demonstrably younger than the oldest overlying strata. Locally, the nonconformity between basement rocks and late Paleozoic strata, as well as steep shear zones within the basement rocks, may have been efficient channelways for brines flushed out of basins created during Ancestral Rocky Mountains deformation. On a continental scale, migration of fluids and attending remagnetization is consistent with epeirogenic uplift of the Pangean supercontinent and a relative lowering of ground water levels. The fact that these basement rocks do not show evidence of subsequent remagnetization during latest Cretaceous to early Tertiary (Laramide) contraction in this area suggests that the characteristics of the late Paleozoic remagnetization phenomenon were unique in time.