Paleomagnetic consequence of thermal evolution of the North American Passive Margin: The Jurassic APWP controversy

Abstract

This study revisits the long-standing controversy in selecting Jurassic paleomagnetic data to define the North American (NA) craton APWP. Data from igneous rocks in the eastern part of NA and select global data rotated into NA coordinates define a high-latitude path. In contrast, the data from mostly sedimentary rocks in the NA Southwest interior (southwestern group) describe a lower-latitude path. Analysis suggests that previous explanations, such as inclination error in the southwestern group, cannot fully explain the disparity, and raises concerns that some magnetization of the eastern group may be partially secondary or younger, and others do not average secular variation. We present a new hypothesis that attributes the discrepancy to the post-tilt magnetization of the eastern NA continental margin caused by thermally induced non-uniform relative vertical movements during the post breakup evolution of northeast NA. This is tested by comparing mid-Jurassic paleomagnetic data of the eastern group with the coeval directions calculated from an updated late Triassic-Early Cretaceous reference NA APWP, presented here defined using a select and refined set of paleomagnetic poles mostly from southwestern region. One resolution is that a) after the breakup of NA from Africa (∼190 Ma), the thermally uplifted oceanside of the margin started to subside, while the more in-land regions experienced uplift due to landward lateral heat propagation in the lithosphere; b) during ∼190–140 Ma, the differential vertical motion tilted the edge of the margin eastward, about a horizontal axis with ∼53.5° azimuth. This eastward tilting peaked at ∼175–165 Ma and gradually decreased until ∼145–140 Ma. During this phase, the eastern group igneous rocks were emplaced and magnetized in this tilted framework; c) after ∼140–110 Ma, subsidence dominated the entire region. However, higher subsidence rates in more in-land parts tilted the marginal part along with their magnetizations acquired in the eastward-tilted position back (westward) to their current attitudes.