Paleomagnetism and rock magnetism of Quaternary volcanic rocks and Late Paleozoic strata, VC‐1 core hole, Valles Caldera, New Mexico, with emphasis on remagnetization of Late Paleozoic strata

Abstract

Paleomagnetic and rock magnetic data obtained from azimuthally unoriented core samples, collected at approximately 1‐ to 3‐m intervals, of Continental Scientific Drilling Program core hole VC‐1 have prompted reinterpretations of the Quaternary volcanic stratigraphy intersected by the bore and have aided in evaluating the thermal regime within late Paleozoic strata attending fluid circulation and mineral deposition during and after development of the Toledo and Valles calderas. The results from Quaternary units (Banco Bonito Obsidian: I = +35.4°, a95 = 2.8° (inclination only determinations), n = 33; Battleship Rock Tuff: D = 359.6°, I = +42.4°, a95 = 2.8°, n = 5 site means (surface sites); VC‐1 Rhyolite: I = +39.2°, a95 = 12.8°, n = 7; Upper VC‐1 Tuff: I = +37.2°, a95 = 10.7°, n = 13; Middle VC‐1 Tuff: I = +42.1°, a95 = 2.1°, n = 39; South Mountain Rhyolite: D = 350.9°, I = +49.9°, a95 = 3.4°, n = 10 (one surface site)) are consistent with isotopic age data, indicating that the entire moat volcanic sequence intersected is less than 650 kyr. Monitoring of natural remanent magnetization (NRM) intensity, NRM directions, directions of magnetizations isolated during progressive demagnetization, median destructive forces, and rock magnetization parameters has identified systematic variations within the thick Banco Bonito Obsidian and VC‐1 Tuff units. The Permian Abo Formation, Pennsylvanian to earliest Permian Madera Limestone, and Pennsylvanian Sandia Formation typically contain a moderate positive inclination magnetization component (Abo Formation: I = +52.2°, a95 = 7.4°, n = 16; Madera Limestone: I = +58.4°, a95 = 2.8°, n = 105; Sandia Formation: I = +53.9°, a95 = 4.8°, n = 21); when residing in magnetite, it is usually unblocked in the laboratory by 350°C; when carried by hematite it is unblocked by 550°C. A moderate negative inclination (e.g., Madera and Abo strata: D = 173.1°, I = −46.6°, a95 = 5.5°; n = 47 samples; assuming a north seeking positive inclination RM providing a useful means of core orientation), oppositely directed magnetization is occasionally isolated and removed at higher laboratory unblocking temperatures (up to 550°C in magnetite‐dominated and 600°C in hematite‐dominated lithologies). Where present, magnetizations of shallower inclination (e.g., Madera and Abo strata: D = 160.1°, I = −10.9°, a95 = 4.4°, n = 28) are removed over higher ranges of unblocking temperatures, and these are probably of late Paleozoic age. The moderate negative and positive inclination magnetizations are in all likelihood viscous partial thermoremanent magnetizations (TRMs) (VPTRMs). These were activated at moderate (∼300°C) temperatures between 1.45 and 0.97 Ma, attending and following the main stages of Toledo/Valles caldera development during the Matuyama, and at near‐present downhole temperatures (∼100°–180+ °C) during the Brunhes and possibly within the last 10 kyr, respectively. Importantly, the preservation of negative inclination RM components in magnetite‐dominated lithologies of the Madera Limestone dictates that temperatures during the Brunhes chron (past 730 kyr) did not exceed approximately 300°–350°C. The inferred temperatures for thermal activation of the VPTRMs, based upon the Middleton and Schmidt thermal activation curves for both magnetite and hematite derived following Walton, agree with independent estimates of thermal activity, using fluid inclusion and K‐Ar isotopic age determination data, during the Quaternary. The results further illustrate the potential usefulness of thermal activation theories for inferring temperatures of past geologic events, especially when they can be applied to magnetizations of similar age residing in both magnetite and hematite, and of paleomagnetic and rock magnetic studies in general in continuous coreholes in complex geologic environments.