HR: 1340h
AN: GP43A-0848    [Abstracts]
TI: Pangean Reconstruction of the Yucatan Block and its Permian, Triassic, and Jurassic Geologic and Tectonic History
AU: * Steiner, M
EM: magnetic@uwyo.edu
AF: Univ Wyoming, Dept Geol/Geophys, Laramie, WY 82070
AB: Paleomagnetic studies of Paleozoic sedimentary and plutonic rocks demonstrate that the Yucatan Block lay inverted off the NW coast of South America (SA) in Pangea, and rotated in a series of clockwise motions as SA and NA separated. Mid-Permian Yucatan was a fragment of the western margin of Pangea, just south of the equator. By 230 Ma, Yucatan rotated $\sim20\deg$ clockwise and moved to the equator. Clockwise rotation continued through the Jurassic: $\sim54\deg$ between 230 Ma and about Oxfordian, and another $\sim32\deg$ between $\sim$Oxfordian and Tithonian, when the approximate present orientation was achieved. Passage of the Yucatan Block into the gap created by the separation of North and South America requires left-lateral motion relative to North America, perhaps by translation along the Mojave Sonora megashear. One remanence, carried by magnetite in red bed strata from the margins of the marine Santa Rosa basin (Maya Mtns), decays exceedingly linearly to the origin of orthogonal-axes plots. Biostratigraphy indicates a Late Pennsylvanian to Middle Permian age, and the presence of dual polarities in a 110 m, four polarity, magnetostratigraphic sequence demonstrates a post-Late Paleozoic Reversed Superchron (i.e., Middle Permian) age. Plutonic rocks exhibit a second, also dual-polarity, remanence, which corresponds to a paleopole $60\deg$ clockwise of the mid-Permian pole. The spatial relationship between the Permian and pluton paleopoles is very similar to that of the North American Permian and Late Triassic poles. Moreover, exceedingly uniform K/Ar ages of 231$\pm7$ Ma characterize all Maya Mtns plutons and the southern volcanic complex, indicating a 230 Ma resetting of the K/Ar radiometric systems of plutons dated by U/Pb as Late Silurian (Steiner and Walker, 1996). Metamorphic aureoles developed in the Pennsylvanian-Permian Santa Rosa strata bordering the Silurian plutonic complexes suggest the occurrence of a post-intrusion hydrothermal event. The 230 Ma reset K/Ar systems, metamorphic aureoles in strata younger than the plutons, and a magnetization resembling a Late Triassic remanence all suggest that a 230 Ma hydrothermal event remagnetized and reset the igneous rocks, probably during the initial break-up of Pangea. The Maya Mtns pluton paleopole is statistically identical to that of the Chiapas Massif. Identical dual polarity magnetization populations characterizing both of these Yucatan plutonic complexes suggest that both were remagnetized around 230 Ma. Importantly, identical remanences in these widely separated plutonic complexes indicate that the Yucatan Block (including the Chiapas Massif) has been a structural entity since at least 230 Ma.
DE: 8135 Hydrothermal systems (8424)
DE: 8150 Plate boundary--general (3040)
DE: 8157 Plate motions--past (3040)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting