Paleomagnetism and Tectonics in Latin America III Posters
Presiding: J Urrutia-Fucugauchi, Institute of Geophysics, National University of Mexico; A Gogichaishvili, National University of Mexico; M Ernesto, Universidade de Sao Paulo; A Rapalini, Universidad de Buenos Aires
GP41A-01 0830h
Paleomagnetism of the Independencia Group, Eastern Paraguay: A New Late Permian Pole for Stable South America
The late Permian segment of the South American apparent polar wander path (APWP) is poorly defined due to scarce and unreliable paleomagnetic poles, justifying the need for further paleomagnetic efforts on suitable rocks of such age from stable areas of South America. A study was carried out on the Late Permian Independencia Group, which is discontinuously exposed along Eastern Paraguay and consists of several hundred meters of flat lying continental pink to reddish sandstones and siltstones, deposited on the western margin of the Parana basin. Thirteen sites (83 samples) were collected on the top unit of the Group (the latest Permian Tacuary Fm) from outcrops distributed over 1000 square kms. Detailed AF and thermal demagnetization permitted the isolation of within-site consistent characteristic remanence in eleven sites, being generally hematite the main carrier of the remanence. Excluding one outlier, a paleomagnetic pole was computed by averaging 10 virtual geomagnetic poles of mixed polarities (6 reversed, 4 normal), yielding a position at: 80.7 S, 7.0 E, A95: 6.6. The new pole may be considered as key for a better definition of the South American APWP by filling a gap between recently obtained and reliable Early Permian poles and an older and less reliable Early-Middle Triassic group of poles.
GP41A-02 0830h
Incidence of Metamorphism on Magnetic Mineralogy in the Sarmiento Ophiolitic Complex, Southern Chile
The Sarmiento Ophiolitic Complex (SOC), located in the southern Andes of Chile, represents the mafic portion of the Late Jurassic to Early Cretaceous oceanic floor of a back-arc basin closed and uplifted in the Mid Cretaceous. Its igneous pseudostratigraphy consists of mainly mafic pillow lavas, dikes, and gabbros. These rocks underwent a non - deformative "ocean floor metamorphism" which developed secondary mineral assemblages in a vertical steep metamorphic gradient passing from zeolite to actinolite facies, followed by a transition to fresh gabbros Syntectonic greenschist facies assemblages in mylonitic rocks bordering the thrust slices that expose the SOC, represent a different metamorphic event, that probably occured before the latest Cretaceous. A paleomagnetic study of these rocks has shown that a post-tectonic widespread remagnetization affected the whole ophiolite. Incidence of the mentioned metamophic processes on the natural remanent magnetization of the ophiolite was evaluated thru a susceptibility survey and a study of ferromagnetic (s.l ) minerals. These were identified by microscope observations under reflected and transmitted light. A very good agreement between magnetic susceptibility values and optical observations was found. Metamorphic processes have produced strong effects on ferromagnetic minerals. These metabasites are disctintly poor in titanomagnetites, except the metagabbros and dikes crosscutting gabbros, which show high susceptibilities due to magnetite from serpentinisation of olivines and uralitization of pyroxenes. Sheeted dikes and pillow lavas show low susceptibilities controlled by paramagnetic minerals with titanite as witness of a primary titanium-bearing mineral. Pure microcrystalline magnetite, main carrier of natural remanent magnetization, likely formed during the greenschist post-tectonic overprint. Sulfide mineralization in the Complex requires further studies. Finally, equilibria between Fe3+ in oxides and Fe3+ in silicates seem to have played an important role in determining the ocurrence and abundance of magnetite during the metamorphism.
GP41A-03 0830h
Paleogeography of Southwest Gondwana Boundary During the Upper Paleozoic
Current results are summarized from the southwest Gondwana boundary, from rocks from La Pampa province, Argentina. This area is the westward part of an orogenic belt, known as Sam Frau geosincline or Cordon de las Gondwánides and may represent the suture zone between the Gondwana and Patagonia terranes. This collision model is still uncertain and there are many controversies related to the age of the deformation of this belt. Paleomagnetism and the exploration of the magnetic fabric signatures (AMS) can be useful to evaluate the regional deformation and a better understanding of the assembly, deformation, and fragmentation of Gondwana / Pangea. A systematic paleomagnetic study was done in the Cerro Centinela: 36/deg S, 67/deg W, belong to the Choiyoi Group and consist of Lower Paleozoic volcanic rocks. In this paleomagnetic study is present results from 19 sites (84 specimens), sampled from the base to the top in different layers. Samples were demagnetized with thermal procedures. A high unblocking temperature component carried by hematite was defined between 580° C and 680° C, showing very good within site consistency (alpha 95<15° and k>20). Remanent magnetization ranges around 1500 mA m-1. In all samples it was possible to isolate one component with the same behavior and positive inclination. These stable remanent magnetization were group into two different Population: 1 and 2, clearly separated. Population 1 (from sites 1 to 12) in situ mean direction is: D=146°, I=63.5°, alpha 95=4°, k=104, N=12. Population 2 (from sites 13 to 19) in situ mean direction is: D=164°, I=42°, alpha 95=4°, k=197, N=7. Reversed characteristic remanence magnetization suggesting that the magnetization was acquired during the Kiaman interval, in accordance with their age. Two high quality paleomagnetic poles were computed for each population on the basis of the in situ remanence directions. The corresponding PP are for Population 1: 63° S, 353° E, Alpha 95=7 and K=44 and for Population 2: 71° S, 60° E, Alpha 95=5, K=169. These two poles are consistent with the age of the rocks and with others PP from the Gondw nides, and are reflecting a huge displacement of the continent during the Permian, before the final assembly of the Pangea.
GP41A-04 0830h
Magnetic Dating of Ancient Residential Areas of Teotihuacan
In total of 143 archeological samples were obtained from the Xalla site and 40 samples from Teopancazco, both belonging to Teotihuacan culture. Five contructive stages are recognized at Xalla. The samples consist mainly burned stuccos dated as 550 AD by C14 systematics and other unburned specimens (containing fine grained scoria) spending time interval between 100 AD and 350 AD. Vectorial composition of natural remanent magnetization were investigated using stepwise alternating field treatment. Mainly univectorial components were obtained for the majority of samples showing well clustered remanence directions. The possible dates are retrieved using a new paleosecular variation reference curve (Hueda et. Al., 2004).
GP41A-05 0830h
Rock Magnetic Properties of a Late Pleistocene Paleosol From Central Mexico
Rock-magnetic characteristics of a volcaniclastic-paleosol sequence from Tlaxcala, central Mexico, are analyzed in order to test the hypothesis of neo-formation of magnetic minerals as result of enhanced pedogenesis in A and B soil horizons, in comparison of the less affected or pedoganically unaltered parent material. Our study was carried out in a Chromic luvisol pedocomplex thought to have been formed during the last interglacial. The results show that, even if pedogenic fine grained magnetic minerals are present in the Bt horizons, these are destroyed in upper horizons, where pedogenesis has been more intense. Horizons A and E present the lowest magnetic susceptibility, however they also present the highest concentration of high coercivity hematite/goethite phases. In order to decode the paleoclimatic signal of this paleosol, rock-magnetism analysis in different pedogenetic materials as the soil itself, and cutans, cracks-filling material and humus found in soils, is in progress.
GP41A-06 0830h
First Archeomagnetic Intensity Results from Ancient Guatemala Potteries
Archeointensity results from 10 pottery fragments are reported as a first trial to create the intensity secular variation curve for the Preclassic period from 900 BC to 200 AD. Hysteresis loops, IRM-DIRM and Curie temperature analysis indicate that magnetite is the main carrier of magnetic signal in pottery, and PSD grain-size the predominant. The fragments were analyzed using the Thellier and Thellier method modified by Coe (1967). Intensity values were corrected for thermoremanent magnetization (TRM) anisotropy and cooling rate dependence of TRM acquisition. Although our results are still not numerous, they have good technical quality determination attested by the reasonably high Coe et al's quality factors. Natural remanent magnetization fraction f used for determination ranges between 0.35 to 0.92 and the quality factor q from 3 to 21 being generally greater than 5. The site mean archeointensities are ranging from 38.3 to 45.8 T. Because no archeointensity reference curve is available in Mesoamerica, Korte et al.'s (2004) global geomagnetic intensity models are used to better constraint the ages of studied pottery.
GP41A-07 0830h
Paleomagnetic Study of the Zihuatanejo-Papanoa Sector of the Pacific continental margin of Southern Mexico - Continental Margin Truncation and Migration of Magmatism
Studies of the western North American continental margin have highlighted the role of terrane accretion, northward translation of margin slivers, oceanic/continental plate interactions, plate subduction, magmatism, extension and vertical axis rotations on its tectonic evolution during the Mesozoic and Cenozoic. The western Pacific margin of Mexico has been proposed as a source region for some of the far-traveled terranes, and it may have been formed by terranes accreted during the Late Cretaceous-Early Tertiary. The Guerrero terrane forms most of the continental margin (being the largest terrane of western North America) and is characterized by island arc assemblages. This study concentrates on the Jalisco block, which forms the western part or of the Guerrero terrane and has been considered a separate tectonic entity bordered by two continental rifts. Formation of the Gulf of California in the past 4.6 Ma fragmented and rifted apart two large fragments of a larger paleo-Jalisco block.
GP41A-08 0830h
New Early Triassic Paleomagnetic Pole for South America from the Alto Paraguay Alkaline Province
The Alto Paraguay Province consists of a series of ring complexes that parallel the Paraguay river (the border between Brazil and Paraguay) for more than 40 km.Intrusive to subintrusive rocks are prodominant, and are usually topped by lava flows and ignimbrites, although one large lava field is also present. The province is well dated by different radiometric methods with ages ranging mainly from 240 to 250 Ma. Samples were collected from different levels of seven complexes which were submitted to both alternating field and thermal demagnetizations. Normal and reversed characteristic magnetic components were isolated, and magnetites were identified as the main magnetic carriers. The resulting paleomagnetic pole is located at 319E 78S ( =6; k=23) based on 26 samples. This pole plots within the large field defined by other available poles supposed to be same age, as they lack precise dating. It matches also the corresponding mean pole for Africa, and represents a good candidate to better define the Early Triassic pole position for South America.
GP41A-09 0830h
San Gaspar ignimbrite, western Mexico: Magnetic fabric and Paleomagnetic studies - constraints on emplacement mode and source vents
Paleomagnetic and magnetic fabric data for the 4.8 Ma San Gaspar ignimbrite, one of the largest in western Mexico, are used to investigate on the source vents, emplacement mechanism and tectonics. The within-site consistency of directional features of magnetic fabric supports that the ignimbrite welded and cooled in situ and was probably emplaced in NW-SE rift zone from a single eruption center located south or east of a younger Pleistocene caldera. Paleomagnetic, rock-magnetic and ore microscopy data support the hypothesis that the extensive and widespread ignimbrite deposits in western Mexico correspond to a major explosive volcanic phase in the Pliocene Rock-magnetic properties are similar, suggesting relatively homogeneous mineralogy of the unit. Isothermal remanence and continuous susceptibility-temperature experiments point to low to medium-Ti titanomagnetite as the main ferromagnetic mineral. Hysteresis parameters of most samples fall in the pseudo-single-domain grain size region; wasp-waisted hysteresis loops were identified corresponding to high Hcr/Hc values. Stepwise thermal and alternating field demagnetization shows that secondary components are negligible, and always completely removed below 20 mT or 400° C. The characteristic component is isolated in all sites with small within-site dispersion of mean direction in most of them (alfa 95 less than 10 at 7 out of 10 sites).
GP41A-10 0830h
Magnetic Properties in Volcanoclastic Deposits in the Valle de Santiago, Guanajuato, in Central Mexico.
Valle de Santiago is a volcanic province, part of the Michoacan-Guanajuato volcanic field. The area is characterized by volcanic maars. These structures show a very well defined NW-SE direction. In this study, three sequences are described and analysed in order to determinate their magnetic properties, specially, anisotropy of magnetic susceptibility (AMS), and determinate different types of signatures in relation to nature of deposits. The sequences of maars contain a high percentage of reworked material (approximately 65-75) from previous rocks. This arrangement is directly related with a high kinetic energy during the eruption. All the deposits are unwelded, and principally are fine grained pyroclastic deposits of hydromagmatic origin. Volcanic and magnetic data suggest an alternation of fall and turbulent transportation and traction deposition of particles. The preliminary results indicate the anisotropy of magnetic susceptibility (AMS) could be related with grain size, chemical composition, and type of deposit.
GP41A-11 0830h
Magnetic polarity stratigraphy of "Mexican Solnhofen" Tlayua Formation, Tepexi de Rodriguez, State of Puebla, Mexico
The Tlayua Formation is the richest Cretaceous fossiliferous locality in Mexico. Research in the quarry has yielded thousands of fossil fish associated with other vertebrate remains. From a vertical section, a total of 95 paleomagnetic specimens from 31 horizons were measured to determine the paleomagnetic signal. Limestones are weakly magnetized and exhibit in general two component magnetizations. The magnetic properties and characteristic remanence are dominated by soft and hard coercivity magnetic minerals, most of which probably formed early in the depositional and diagenetic history. Rock-magnetic properties and a positive reversal test suggest that remanence is primary. The section displays both reverse and normal polarities with mean directions: D = 344.9°, I = 32.4°, k= 21, a95= 4.2°; and D = 149.4°, I = -36.6°, k= 17, a95= 8.7°. Comparison with the North America reference curve, indicates tectonic stability of the region since the Cretaceous. Correlation of these polarity zones with the geomagnetic polarity time scale provides a time framework for the discrepancy of the previous biochronological studies. Based on biostratigraphic age assignments using ammonites, the polarity magnetozone sequence correlates with C34n.1n-C34n.2n chrons, with an age of 100 to 105 Ma for the Tlayua Formation.