On the Reliability of Paleomagnetic Poles Obtained From Permo-Carboniferous Rocks From Oaxaca Mexico, Belize and Guatemala: Insights From Rock Magnetic Studies

In the reconstruction of past movements of tectonic plates, the determination of reliable paleomagnetic poles is of utmost importance. To achieve accurate results, a full knowledge of the rock magnetic properties of the samples, is required particularly for Curie points and for grain-size analyses of the minerals carriers of the magnetization in addition to thermal and af demagnetization experiments. In this study we present the comparative results of 20 sites drilled at three different Paleozoic areas: The Permian rocks of the Juchatengo area in Oaxaca, Mexico; the Carboniferous (336 Ma) Mountain Pine Ridge Granite, the Triassic (237 Ma) Hummingbird Granite in Belize, and the Permian Chochal Limestone in Guatemala. A minimum of 8 cores were drilled and sun-oriented at each site. The samples of all 20 sites were subjected to AF demagnetization in sixteen steps from NRM to 1,000 Oe. Next, the samples were thermally cleaned in fifteen increasing temperature steps from NRM up to 675 C. Principal component analysis was applied to each one of the samples in order to obtain their respective mean directions. SIRM, hysteresis loops, and coercivity experiments performed on samples from the sites in question indicate that about 90 percent of the samples were characterized by Multi-Domain (MD) grain sizes and the rest were PSD. Curie point determinations results ranged from relatively low temperatures of 190 up to 660 C, indicating the presence of Ti-rich, Ti- poor magnetites as well as hematite. The Juchatengo complex of southern Mexico, a green rock tholeiitic sequence emplaced beetwen the Oaxaquia and Xolapa terranes is made up from bottom to top, of gabbroic and plagiogranitic dikes and stocks (289 +/- 6 Ma, K-Ar hornblende); basalts and pyroclastic rocks and a series of deep water marine sediments. The deposition and deformation of the complex occurred in the Early Permian followed by the emplacement of Permian-Triassic calc-alkaline plutons (K-Ar hornblende ages from 286 +/- 6 Ma to 238 +/- 5 Ma). At this area we have sampled nine different sites. Thus far, reliable poles were obtained from 3 sites and the results obtained are: Juchatengo sites JU: PLON= 128.2; PLAT= 82.4; A95= 5.7. LS: PLON= 182.5; PLAT= 61.2; A95= 13.8 and ES: PLON= 171.9; PLAT= 74.6; A95=17.5. This inclination corresponds to a latitude of (220) slightly higher than expected for the sampling site at 160 north. However it falls within the analytical error. At Belize, eight sites were sampled in the Maya Mountains region: 3 sites in the Carboniferous (336 Ma) Mountain Pine Ridge Granite; 3 other sites in the Triassic (237 Ma) Hummingbird Granite, and 2 other sites in the sandy facies of the Santa Rosa Formation of Pennsylvanian age. Only three sites gave some reliable results as follows: Belize site 1 at Mountain Pine Ridge Granite: PLON= 171.9 PLAT= -22.2; A95= 12.8 and Belize site 2 also at Mountain Pine Ridge Granite: PLON= 279.5; PLAT= -75.3; A95= 11.3. Belize site 3 at Hummingbird Granite yielded: PLON= 106.8; PLAT= 32.1; A95= 7.7. At Guatemala, out of three sampling sites in the Permian Chochal Formation north of the Motagua Fault Zone, two sites yielded useful results with a calculated pole at: Guatemala site 9: PLON= 162.5; PLAT= -66.2; A95= 11.6. Guatemala site 11: PLONG= 130.1; PLAT= 57.8; A95= 13.7. The 8 reliable sites that yielded tectonically coherent results are characterized by samples that showed reversible Curie curves that contained almost pure magnetite and are PSD in size.