A new Late Cretaceous paleomagnetic pole from the Adel Mountains, west central Montana

Abstract

North America's apparent polar wander path has been poorly defined between the mid‐Cretaceous and Paleocene reference pole positions. Existing data allowed 13° of apparent polar motion over about 22 m.y. (87–65 Ma) roughly coinciding with the beginning of Laramide deformation (∼80 Ma). We report on a paleomagnetic study of the Adel Mountain Volcanic rocks to refine the North American apparent polar wander path for this interval. The shonkinite rocks of the Adel Mountain Volcanic field are on the eastern edge of the Cretaceous‐Paleocene fold and thrust belt; some of these structures disturb the western edge of the volcanic pile. We obtained two new K‐Ar dates from the Adel rocks. One date, on biotite ( from a shonkinite dike that crosscuts most of the volcanic rocks, is 71.2±2.7 Ma. The other, a whole rock date from a flow deep in the volcanic pile, is 81.1±3.5 Ma. We collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows, dikes, and laccoliths. Positive fold and conglomerate tests, along with alternating field and thermal demagnetization, indicate that our characteristic remanent directions are primary magnetizations acquired before Late Cretaceous to Paleocene thrust belt deformation. Averaging the virtual geomagnetic poles from 26 reliable sites, all of normal polarity, yields a paleopole at 82.2°N, 209.9°E (α95 = 6.80°, k = 18.38). This pole is concordant with the Paleocene reference pole (82.0°N, 170.2°E, α95 = 3.5°, k = 18.6 (Diehl et al., 1983)) and is 11.6° from the Globerman and Irving (1988) mid‐Cretaceous pole at 71°N, 196°E. The youngest information in the Cretaceous stillstand pole is from the Niobrara Formation (Shive and Frerichs, 1974) at about 85–89 Ma. If we take the average age of the Adel Mountain Volcanics to be 76 Ma, then ∼12° of apparent polar motion occurred between 87 Ma and 76 Ma. Thus, rapid apparent polar motion correlates well with the onset of Laramide deformation.

The big flush: paleomagnetic signature of a 70 Ma regional hydrothermal event in displaced rocks of the northern Canadian Cordillera

“…To make paleogeographic interpretations, we must compare this pole to the reference pole for cratonic North America. Following a relatively long period with little or no apparent polar wander , the Upper Cretaceous is marked by a quick but poorly constrained jump of about 11°in pole position (for discussion see Gunderson and Sheriff 1991;and Diehl 1991). Following Wynne et al (1992, their Marquis and Globerman (1988), due in part to the difference in the reference field calculations and in part to the slightly shallower combined mean inclination.…”

Late Cretaceous True Polar Wander: Not So Fast

“…In addition, some of the North American data used have declination uncertainties (16) that are an important consideration in the comparison presented by Sager and Koppers (1). The North American data cited (1), however, lack the temporal resolution to test the hypothesis.…”

Magnetic fabric, paleomagnetic, and 40Ar/39Ar geochronologic data bearing on the emplacement of the Late Cretaceous Philipsburg Batholith, SW Montana fold-and-thrust belt

“…To assess the possibility of whole-scale, postemplacement deformation involving rotation or tilt (or a combination of both) of the Philipsburg Batholith, the grand mean was compared to expected paleofi eld directions (Table 6) calculated from different paleomagnetic pole compilations for the Late Cretaceous (Besse and Courtillot, 2003;Gunderson and Sheriff, 1991;Van der Voo, 1993) for North America (Fig. 21).…”

“…Contours of foliation dips show an overall pattern of shallow values in the Dora Thorn Pluton, except near the estimated subsurface location of the Georgetown-Princeton thrust and near the eastern margin of the intrusion. Near intrusion margins, foliation dips N 015.1°3 52.1°3 51.1°P hilipsburg Batholith grand mean Besse and Courtillot (2003) Gunderson and Sheriff (1991) Van der Voo (1993) range from shallow to steep, yet all foliation strikes are parallel to margins. Subhorizontal AMS lineations are roughly parallel to foliation strike and to batholith margins, and they are interpreted as stretching lineations due to magma fl ow impinging on wall-rock contacts.…”