Mickey C. Van Fossen

Paleomagnetism of 122 Ma Plutons in New England and the Mid-Cretaceous Paleomagnetic Field in North America: True Polar Wander or Large-Scale Differential Mantle Motion?

A paleomagnetic study of Cretaceous White Mountains plutonic complexes in New Hampshire and Vermont yields high unblocking temperature, dual polarity magnetizations in different types of igneous rocks. The resulting pole position for three plutons (71.9°N, 187.4°E, A95 = 6.9°, age = 122.5 Ma) agrees with previously published mid-Cretaceous poles for North America, which together give a mid-Cretaceous standstill reference pole slightly revised from Globerman and Irving [1988] at 71.2°N, 194.1°E (A95 = 3.7°, N = 5 studies). We argue on the basis of the wide geographic distribution of these studies, the variety in tectonic settings and rock types, positive reversal tests, and an overall reversal pattern consistent with geomagnetic polarity time scales, that this mean pole represents the North American mid-Cretaceous reference field for nominally 36 m.y. (124 to 88 Ma). The standstill pole limits to within ±4°, the motion of the North American plate relative to the Earths spin axis. During the same mid-Cretaceous interval, the New England hotspot track (124 Ma Monteregian Hills, 122.5-Ma Cretaceous White Mountains, and 103- to 84-Ma New England seamounts) requires 11°±4° of north-poleward motion of North America, in direct conflict with the paleomagnetic standstill. A similar (∼13°) discrepancy is independently demonstrated between the spin axis and the Tristan da Cunha hotspot track on the African plate during the mid-Cretaceous interval. The hotspot/spin axis discrepancies ended by ∼90 Ma when it is shown that both Atlantic hotspots agree with North American and African dipole paleolatitudes and present-day locations. Nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis, leaving as possible interpretations true polar wander and large-scale (but differential) mantle motion. The southerly motion of the mid-Cretaceous Louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics. The motions of the three widely separated mid-Cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid-Cretaceous “superplume.”

Miocene stable isotopic stratigraphy and magnetostratigraphy of Buff Bay, Jamaica

Previously reported biostratigraphic relationships from middle-upper Miocene sections exposed near Buff Bay, Jamaica (18°N, tropical bioprovince), differ from the subtropical North Atlantic (Sites 563 and 558). Time scales for this interval rely on correlations established at these subtropical sites, and the differences with the tropical section have implications to global correlations. Planktonic foraminiferal Zones N13 and N15 are thick at Buff Bay but are virtually absent at Sites 563 and 558; nannofossil Zone NN9 is associated with Zone N15 and uppermost Zone N14 at Buff Bay but is associated with Zone N16 at the other sites. Magnetostratigraphic data presented here further complicate the interpretation: Zone NN9 is associated with a thick normal magnetozone at Sites 563 and 558; at Buff Bay, it is associated with a thick reversed magnetozone. Although a secondary magnetization at Buff Bay makes it difficult to identify confidently Miocene normal magnetozones, the thick reversed magnetozone most likely represents the paleomagnetic field and correlates with Chron C5r. The magnetobiostratigraphic relationships require either diachrony of taxa or two mutually exclusive hiatuses in Jamaica and the North Atlantic. We address this problem by analyzing benthic foraminiferal δ 18 O and δ 13 C from the Buff Bay section. These isotopic data allow us to evaluate three hypotheses that reconcile the magneto-, bio-, and isotopic stratigraphic data and conclude that the first and last occurrences of five taxa were diachronous by ∼0.3-0.5 m.y. between tropical and subtropical locations. This requires revised age estimates for late middle to early late Miocene biostratigraphic datum levels. We suggest that the ranges of several taxa are useful for endemic tropical or subtropical zonations, but correlations between the low and midlatitudes were affected by an increase in latitudinal thermal gradients during the late middle Miocene. However, we admit that further studies are needed before this issue is resolved.