Pangean reconstruction of the Yucatan Block: Its Permian, Triassic, and Jurassic geologic and tectonic history
Maureen B. Steiner, 2005. "Pangean reconstruction of the Yucatan Block: Its Permian, Triassic, and Jurassic geologic and tectonic history", The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, Thomas H. Anderson, Jonathan A. Nourse, James W. McKee, Maureen B. Steiner
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Paleomagnetic studies of Paleozoic sedimentary and plutonic rocks demonstrate that the Yucatan Block did not lie between the North and South American plates in the Pangean assembly during the Permian. In the Middle Permian, the Yucatan Block lay in an inverted orientation on the western margin of Pangea at 6–10°S, probably forming part of the NW coast of South America. Subsequently the block rotated in a series of counterclockwise motions as the North and South American plates separated. By 230 Ma, Yucatan had rotated ∼47° counterclockwise and moved slightly northward, to the equator. Counterclockwise rotation continued through the Jurassic: ∼41° between 230 Ma and ca. Oxfordian time, and another ∼47° between the Oxfordian and Tithonian, at which time, the approximate present orientation with respect to North America was achieved. Passage of the Yucatan Block from NW South America into the gap created by the separation of North and South America is a motion consistent with the left-lateral translations along the Mojave-Sonora or similar megashear. The fact that Yucatan has exhibited a counterclockwise motion throughout its Mesozoic history suggests that the microplate may have acted in a ball-bearing fashion between the larger North and South American plates.
The Permian sedimentary remanence appears to be carried dominantly by magnetite and maghemite in red bed strata deposited on the margins of Silurian plutons within the marine Santa Rosa basin of the Maya Mountains. This remanence decays exceedingly linearly to the origin of orthogonal-axes plots. Biostratigraphy indicates a late Pennsylvanian to Middle Permian sedimentary age for the Santa Rosa strata, and the presence of both polarities of remanence in a 110 m magnetostratigraphic sequence of four polarity intervals demonstrates a post-Kiaman, i.e., Middle Permian, magnetization age. Another remanence is exhibited by Silurian plutons, also dual polarity, but corresponding to a paleopole ∼60° counterclockwise of that recorded in the Middle Permian sedimentary rocks. Exceedingly uniform K/Ar ages of 231 ± 7 Ma characterize all Maya Mountains plutons and a volcanic complex, indicating a 230 Ma resetting of the K/Ar radiometric systems in plutons dated as Late Silurian by U/Pb (Steiner and Walker, 1996). Furthermore, metamorphic aureoles are developed in the Permian Santa Rosa strata that border the Silurian plutons, suggesting that the 230 Ma resetting was a postintrusion event that involved the margins of the plutons. The spatial relationship between the pluton and sedimentary poles is reminiscent of that between the North American Late Triassic and Permian reference paleopoles. Therefore the combination of 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 the igneous rocks and reset their radiometric systems. This probably was an event related to the initial breakup of Pangea.
The Maya Mountains plutons yield a paleopole that is statistically identical to that of the Chiapas Massif to the south. Both plutonic complexes exhibit somewhat dispersed dual polarity magnetization populations, suggesting that both were remagnetized at ca. 230 Ma. Importantly, the 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.