Roberto S. Molina-Garza

Paleomagnetism of the Moenkopi and Chinle Formations in central New Mexico: Implications for the North American Apparent Polar Wander Path and Triassic magnetostratigraphy

In central New Mexico, red sedimentary rocks unconformably overlying Permian carbonates of the San Andres Formation have been correlated with the Early-Middle Triassic Moenkopi and Late Triassic Chinle Formations of the Colorado Plateau. Paleomagnetic samples from Triassic sections exposed on basement cored uplifts along both the east and west side of the Rio Grande rift near Albuquerque yield, upon thermal and chemical demagnetization, well-defined, high unblocking temperature, dual-polarity magnetizations carried by hematite. The characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding-parallel magnetization polarity zonation. The Moenkopi and lowermost Chinle formations produced paleomagnetic poles respectively at 57.6°N-100.3 °E (N=36 sites, K=74.1, A95=2.8°) and 60.8°N-88.9°E (N=17 sites, K=130.3, A95= 3.1°). These data plus previously published and additional results from the underlying Permian strata suggest that portions of central New Mexico have experienced a small clockwise rotation (i.e., less than 10°) similar to that of the Colorado Plateau with respect to the North American craton. The paleomagnetic directions of the Chinle Formation and related strata in eastern New Mexico document about 12° (great circle distance) of rapid apparent polar wander during mid-Carnian to late Norian times along a track which contains other cratonic poles of similar age. We present a preliminary magnetic polarity time scale for the Triassic that incorporates the present New Mexico data and previously published data, mostly form continental red bed sequences. This magnetic polarity scale provides a basic framework which can be tested with future data from Triassic sections where additional biostratigraphic control exists.

Secular variation in Mexico during the last 40,000 years

Paleomagnetic data are available for Mexico from various archives, covering irregularly the last 40,000 years. Data obtained from lake sediments are affected by ambiguous conversion of age to depth, and the paleomagnetic data themselves are not of high quality, as samples were only demagnetized at one field intensity. These lake sediment data are therefore considered as less reliable for construction of a secular variation curve, although they may give some qualitative information for the last 25,000 years. Archeological samples provide data for the period, 100–1200 a.d. Their 14 C ages are some times not well constrained, with unrecoverable analytical and statistical procedures, and often ages were only inferred from the stratigraphic position and the paleomagnetic record. The paleomagnetic record of these materials is of high quality with very low internal dispersion, although no magnetic cleaning techniques have been applied. Variations of inclination and declination with time are hampered by large age uncertainties and thus of limited value for the construction of secular variation curves. Paleomagnetic data from an U–Th dated stalagmite provide reasonable directions for most of the last 1200 years, being of lower quality in those parts with very low remanence intensity. The U–Th age determinations were interpreted in terms of a constant growth-rate, which might be only valid as a first-order approximation. Recent volcanic rocks from the Trans-Mexican volcanic belt provide 19 paleomagnetic directions of reasonable to high quality, and often also absolute paleointensities. Ages have been determined from 14 C analysis of charcoal or paleosols found below the lavas, and extend over the last ∼40,000 years. Therefore, volcanic rocks provide the most adequate archive for the construction of a secular variation curve, but much more volcanic structures need to be dated to obtain a semi-continuous secular variation curve. Comparison of the data from different archives is possible mainly for the last 2500 years and despite the aforementioned problems similar variations of declination and inclination are observed. This holds as well when comparing them to data from Holocene volcanic rocks from western North America.

Geology of the coastal Chiapas (Mexico) Miocene plutons and the Tonalá shear zone: Syntectonic emplacement and rapid exhumation during sinistral transpression

Late Miocene plutons in coastal Chiapas, Mexico, represent the roots of an extinct magmatic arc. Miocene granitoids of calc-alkaline composition and arc chemistry intruded into and were deformed within the Tonala mylonite belt in the middle to upper crust. The mylonite belt is a crustal-scale shear zone extending along the western margin of the Chiapas Massif for ∼150 km. Deformation is characterized by a dominantly subhorizontal lineation and subvertical foliation along a strikingly linear zone that trends ∼310°. Mylonitic fabrics contain ambiguous but dominantly sinistral shear indicators. Intrusions are interpreted as syntectonic on the basis of similar U-Pb zircon crystallization age estimates (ca. 10 Ma) and the cooling age estimates obtained on neoformed micas in the mylonite. The plutons are elongated, their long axis is parallel to shear zone, and some plutons show markedly asymmetric outcrop patterns, with sheared tails that trail behind the intrusions and that are consistent with sinistral displacement. Parts of plutons were mylonitized by continuous deformation in the Tonala shear zone, locally developing intricate pseudotachylyte and cataclasite veins slightly oblique to the mylonite foliation. Outside of the shear zone, plutons preserve magmatic fabrics. These observations are consistent with features common to syntectonic granites interpreted to have been emplaced along strike-slip shear zones in a transpressional setting. We interpret the Tonala mylonites as representing a relict transform boundary that was slightly oblique to the Polochic-Motagua fault system, which accommodated over 100 km of sinistral displacement between the Chortis block (on the Caribbean plate) and Chiapas (on the North America plate) in late Miocene time.

Palaeomagnetic assessment of plutons from the southern Peninsular Ranges batholith and the Jurassic Vizcaíno igneous suites, Baja California, México

ABSTRACT We report geological and palaeomagnetic data from five discrete plutons in the southern part of the Peninsular Ranges batholith (PRB) and one pluton that is part of the Jurassic plutonic suite in the Vizcaíno peninsula. The PRB plutons are Cretaceous and belong to the Alisitos island arc. The Jurassic pluton intrudes a Triassic-Jurassic ophiolite. Our study was designed to evaluate the palaeomagnetic homogeneity of the batholith from the Sierra San Pedro Mártir, at ~31°N, to about ~28.3°N. The Punta Prieta, Nuevo Rosarito, San Jerónimo, and La Rinconada plutons in the western zone of the PRB are characterized by magnetizations residing in magnetite. The Compostela pluton is emplaced in a transition zone and has a magnetization that resides in haematite. The five Cretaceous plutons yield a combined palaeopole at 80.3°N, 162.1°E, A95 = 9.8°, N = 5 that after correcting for the opening of the Gulf of California rotates to 77.6°N, 173.6°E, the rotated pole being in angular distance of only 4.4° from the North America reference pole. The Jurassic San Roque pluton yields a mean 0.6°N, 306.1°E, A95 = 9.2°, N = 10, which is discordant, showing a clockwise rotation of about 131° ± 16° and flattening of 9.5° ± 12.9° with respect to the 150 Ma cratonic reference palaeopole. The results suggest that the intrusion of the undeformed Cretaceous Punta Prieta to Compostela plutons (128.1 ± 1.4 and 100.5 ± 2.7 Ma, respectively) restricts tectonic accretion of the Jurassic-Early Cretaceous sequences to the North America margin to the time before mid-Cretaceous magmatism (~100 Ma) in the PRB near present latitude 28°N. Mesozoic and Cenozoic strike-slip faulting along the Vizcaíno margin can account for the 131° clockwise rotation of the San Roque pluton. Our results do not support significant latitudinal movement between Vizcaíno, the PRB, and mainland Mexico with the exception of the Neogene San Andreas Fault-related right lateral movement.

The Colorado Plateau Coring Project (CPCP): 100 Million Years of Earth System History

1. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA 2. Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA 3. Department ofEarth and Planetary Sciences, University ofNew Mexico, Albuquerque, New Mexico 87131 , USA 4. Martin-Luther-Universitiit Halle-Wittenberg, Institut fiir Geowissenschaften, 06120 Halle (Saale), Germany 5. Department of Geology, Northern Arizona University, Flagstaff Arizona 86011, USA 6. Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 7. Utah Museum of Natural History, 1390 E. Presidents Circle, Salt Lake City, Utah 84112, USA 8. Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands 9. Centro de Geosciencias, Campus Juriquilla UNAM, Queretaro, 76230 Mexico 10. Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, USA 11. Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China

Paleomagnetism and magnetic fabrics of Mio-Pliocene hypabyssal rocks of the Combia event, Colombia: tectonic implications

Mio-Pliocene hypabyssal rocks of the Combia event in the Amagá basin (NW Andes-Colombia), contain a deformational record of the activity of the Cauca-Romeral fault system, and the interaction of terranes within the Choco and northern Andean blocks. Previous paleomagnetic studies interpreted coherent counterclockwise rotations and noncoherent modes of rotation about horizontal axes for the Combia intrusives. However, rotations were determined from in-situ paleomagnetic directions and the existing data set is small. In order to better understand the deformational features of these rocks, we collected new paleomagnetic, structural, petrographic and magnetic fabric data from well exposed hypabyssal rocks of the Combia event. The magnetizations of these rocks are controlled by a low-coercivity ferromagnetic phase. Samples respond well to alternatingfield demagnetization isolating a magnetization component of moderate coercivity. These rocks do not have ductile deformation features. Anisotropy of magnetic susceptibility and morphotectonic analysis indicate that rotation about horizontal axes is consistently to the south-east, suggesting the need to apply a structural correction to the paleomagnetic data. The relationships between magnetic foliations and host-rock bedding planes indicate tectonic activity initiated before ~10 Ma. We present a mean paleomagnetic direction (declination D = 342.8°, inclination I = 12.1°, 95% confidence interval α95 = 12.5°, precision parameter k = 8.6, number of specimens n = 18) that incorporates structural corrections. The dispersion S = 27° of site means cannot be explained by secular variation alone, but it indicates a counterclockwise rotation of 14.8° ± 12.7° relative to stable South America. Paleomagnetic data within a block bounded by the Sabanalarga and Cascajosa faults forms a more coherent data set (D = 336.5°, I = 17.4°, α95 = 11.7°, k = 12.5, n = 14), which differs from sites west of the Sabanalarga fault and shows a rotation about a vertical axis of 20.2° ± 10.7°. Deformation in the Amagá basin may be tentatively explained by the obduction of the Cañas Gordas terrane over the northwestern margin of the northern Andean block. However, it can also be related to the local effects of the Cauca-Romeral fault system.

The Southern Cordillera and Beyond

Prepared in conjunction with the 2012 GSA Cordilleran Section Meeting, Queretaro, Mexico, this volumes eight field guides showcase three aspects of the geology of the southern end of the North America cordillera: Mid-Tertiary and Quaternary volcanology, environmental geology, and Mesozoic tectonics. Field Guide 25 explores the Cenozoic stratigraphy of Sierra de Guanajuato, one of the most important Mexican mining districts, and addresses a controversial topic, the accretion of the Guerrero terrane and its possible role in the Late Cretaceous—Early Tertiary orogeny. Three guides related to the Trans-Mexican Volcanic Belt, an active magmatic arc related to subduction of the Rivera and Cocos plates, include new data about the recent volcanic history, physical volcanology, and volcanic hazards in Mexicos most densely populated area. Bringing the geosciences into societal problems, one guide presents data on ground deformation related to water extraction in urbanized areas of the Mexico City basin, and another explores the ghost town of the Mineral de Pozos mining district and the effect of mine tailings on groundwater.