NEW AND OLD PALEOMAGNETIC RESULTS FROM MESOZOIC SEDIMENTARY ROCKS OF NEW MEXICO: COMPARISONS AND IMPLICATIONS FOR JURASSIC APWP FOR NORTH AMERICA

One of Roberto Molina-Garza’s lasting accomplishments are his paleomagnetic studies of Mesozoic sedimentary rocks, including the often-complex magnetizations in hematite-bearing sediments, and their implications for North America APWP and tectonic models. We highlight new results from Upper Triassic to Middle Jurassic rocks from New Mexico (Mirzaei et al., 2021), and prior studies (Steiner, 2003; Muttoni and Kent, 2019). We will test recent models of North America APWP by Kent and Irving, 2010; Kent et al, 2015; and Muttoni and Kent (2019) who propose a very rapid northward motion for North America during the Jurassic (160 to 145 Ma).

Mirzaei et al. (2021) defined 25 polarity zones in Upper Triassic (Garita Creek, Trujillo, and Redonda Formations) to Middle Jurassic (Todilto, Entrada, Summerville, and Morrison Formations) strata sampled in five locations in eastern New Mexico. The characteristic magnetizations were carried by hematite, and isolated during thermal demagnetization with several steps between 630 and 690 ⁰C. The mean directions of the Upper Triassic units yield paleopoles of 55.3⁰ N, 076.7⁰ E, A95 = 4.0⁰, n=120. Because the inclination from these units was essentially horizontal (I = 0.4⁰), effects of inclination error are negligible. Results from the Summerville Formation from Mirzaei et al., 2021 were combined with those from Steiner (2003), who reported results from some of these same sections- finding a mean (from 96 sampled horizons) pole of 56.1⁰ N, 152.3⁰ E, A95 = 6.6⁰, n=96. For these directions, an Elongation/Inclination (E/I) analysis was performed to evaluate and correct for inclination error- this corrected the mean inclination from I = 36 to I = 57.6, resulting a corrected paleopole of 61.8⁰ N, 183.3⁰ E, A95 = 6.3⁰, n=96.

Comparisons between the inclination-error corrected poles of the Summerville Fm (163 Ma) and coeval poles from kimberlite units, and also from results imported into NA coordinates from Europe (Muttoni and Kent, 2019) find that inclination-error cannot bring poles from the Summerville Fm into any agreement with the higher-latitude Jurassic APWP of Muttoni and Kent (2019). This may suggest that ages for these units may need revision, or may diminish support for the “monster shift” of the NA APWP, and suggests further work is needed.