GP24A-01 INVITED
Paleomagnetism and Tectonic Evolution of Mexico: Precambrian to Recent
It is 20yr since publication of the last synthesis of country-wide paleomagnetic data and almost 30yr of first attempt to construct an apparent polar wander path for Mexico. During this time, data has increased ten-fold and a new synthesis may appear long overdue. Mexico constitutes the southern portion of the North American plate, and tectonic models long suggested a complex evolution involving ocean-basin closure, continental collision/break-up, terrane accretion, large-scale motions, orogenic deformation, oceanic plate reorganizations and oceanic basin development. Paleoreconstructions of Atlantic-bordering continents show major overlap of northern South America onto Mexico, highlighting the geometric problem, allocthonous nature and opening room for a diversity of models. Paleomagnetism appears well-suited to tackle these problems, and data had early been used to evaluate tectonic models. Likewise, the larger database is now used to assess models and develop alternatives. This task is greatly aided by new available geochronological, geophysical and geological data. Our new synthesis still shows data gaps, with Paleozoic and Precambrian units poorly represented. Paleomagnetic data and tectonic models require complex large-scale motions of Precambrian and Paleozoic blocks, solutions for opening of Gulf of Mexico and Caribbean Sea, and post-Triassic-Jurassic amalgamation of Mexico and Central America. The southern edge of the North American craton is positioned in northern Mexico, marking the limit for accreted terranes to the south. Possible southern extension of Marathon-Ouachita Paleozoic belt, obscured or truncated when it comes into Mexico, remains unstudied paleomagnetically. Left-lateral motions along faults related to opening of Gulf of Mexico are not supported by paleomagnetism. The Western Cordilleran belt appears laterally displaced in southern US and into Mexico, where the paleomagnetic signature of northward latitudinal translations and clockwise rotations pervasive in the north changes to counterclockwise rotations and smaller latitudinal translation. The Cenozoic record is somehow consistent with models of regional left-lateral shear and vertical axis rotations. Recent major deformation is related to Gulf of California opening, margin truncation, oblique subduction and translation of margin slivers.
GP24A-02 INVITED
Tectonics Along Western-Central Part of the Trans Mexican Volcanic Belt as Inferred From Palaeomagnetic Data: A Summary
The Trans-Mexican Volcanic Belt (TMVB), one of the largest continental volcanic arcs built on the North America plate, spans about 1000 km and crosses central Mexico from the Pacific Ocean to the Gulf of Mexico. The initial stage of the TMVB is marked by widespread Miocene basaltic volcanism, emplaced from the Nayarit state, in the west, to the longitude of Mexico City. This volcanism is characterized by plateau-like structures resulting from the shield volcanoes and fissure lava flows, which have an estimated aggregate volume ranging between 3200 and 6800 km3. The western-central Mexico has been affected by right-lateral transtension within the western TMVB but previous paleomagnetic studies indicate some 15-20° anticlockwise tectonic rotations for the Rio Grande de Santiago canyon and surrounding areas, in accordance with a Miocene left-lateral transtensional tectonic regime. We present a summary of paleomagnetic and rock-magnetic studies of that Miocene volcanic succession from the TMVB. A total of 114 consecutive basaltic lava flows (more than 550 oriented samples) were collected from four localities: Tepic, Guadalajara, Los Altos, and Queretaro which span from 11 to 7.5 Ma. The mean paleodirection obtained for Tepic area is I = 33.7°, D = 358.4°, k = 140, á95 = 3.0°, N = 17. These directions are in perfect agreement with the expected paleodirections for late Miocene time, as derived from reference poles given by Besse and Courtillot (1991) for North America.. The mean paleodirection obtained for Guadalajara is I = 31.1°, D = 354.6°, k = 124, 95 = 2.1°, which corresponds to the mean paleomagnetic pole position Plat = 84°, Plong = 129.8°, k = 29, 95 = 4.4°. These directions are in reasonably good agreement with the expected paleodirections for middle Miocene time. The mean paleomagnetic direction calculated for Los Altos and Queretaro is I = 32.46°, D = 341.2°, k = 7.2 and 95 = 11.6°. Thus, our results suggest that no major block rotation has occurred in the Nayarit and Guadalajara areas since about 10 Ma but 20° anticlockwise tectonic rotations for the Los Altos and Queretro areas is in accordance with the proposed left-lateral transtensional tectonic regime.
GP24A-03
Lower Tertiary Remagnetization in Mesozoic Rocks from Southern Sierra Madre Oriental of Mexico. Tectonic Implications.
32 Mesozoic sedimentary sites (about 350 samples) have been sampled in southern Sierra Madre Oriental (SMO), México. Stepwise demagnetization analyses have been carried out in Jurassic to Cretaceous carbonates and Late Triassic to Early Jurassic mudstones and sandstones. Both groups of lithologies showed a common Characteristic Remanence Magnetization (ChRM), always of reversed polarity despite the different lithologies and ages, which is considered a synchronous remagnetization. These 32 sites were grouped in 10 localities in order to perform statistical fold tests with their paleomagnetic directions for tectonic purposes. Fold tests results show pre-, syn- and mostly post-tectonic ChRM acquisition, implying that the remagnetization is simultaneous with the Early Tertiary shortening period of the southern SMO Laramide Orogeny. In some Cretaceous limestone sites, a pre-folding high temperature component with mixed polarities has been also observed showing directions close to the post-folding remagnetization. Very similar ChRM paleolatitudes were found in between localities, and they are also in agreement with the expected paleoinclination obtained from the 50 Ma paleomagnetic poles of North America's Apparent Polar Wander Path (Besse & Courtillot, 2002). Paleomagnetic poles of the studied localities lies in an equal paleolatitud minor circle evidencing the existence of great post-Paleocene anticlockwise differential vertical axis rotations, which increases eastwardly up to 40°. In addition, rock magnetic analyses have been performed in order to study the origin of the remagnetization.
GP24A-04 INVITED
New Geophysical L Study Northwestern Antarctic Peninsula: Geodinamic Implicances
The current tectonic arrangement of Tierra del Fuego and Antarctica is the product of an extensive and complex evolution that involved the most southern part in South America (SA) from the Early Mesozoic when a subduction area settles down along the Pacific border of Gondwana (GDW) including the then continuous SA and Antarctic Peninsula (AP) (Dalziel and Elliot, 1973; Dalziel, 1982). The paleogeographic position of the AP immediately before the separation of the GDW has been a topic of controversy. The presently available information for the beginning of GDW fragmentation is contradictory. In the last years a new accretion model has been proposed for the Mesozoic development of the AP. This model requires of more data to sustain the extension of the old magmatic arc and GDW paleomargin. A geophysical campaign has been developed along Danco Coast - between Py Point to the south, and Vivot Cape to the north- with the aim of acquire new data to define the geometry, lateral relationships and tectonic development of the geologic units of the Mesozoic magmatic arc exposed in the western border of the AP. Rock outcrops mostly belong to plutonic bodies of granitic through tonalitic composition, and rhyolites, andesites and basaltic rocks. The geologic survey included the lithologic and structural sampling, a total of 216 rock cores were obtained for AMS and paleomagnetic studies. The acquired data have been GPS referred. The magnetic survey was carried out with two magnetometers of proton free- precession: a Scintrex ENVI GRAD (used as permanent base) and a Geometrics G856 magnetometer employed for sampling stations (which measured the intensity of the total magnetic field and worked synchronized with the base magnetometer in order to make the corresponding corrections). A total of 1670 registrations were acquired in the 334 GPS fixed onland stations. A Zodiac MK5 was employed for the offshore magnetic survey. 5162 stations constitute the magnetic grid in offshore areas, organized along orthogonal sections with a total length of around 380 km. The magnetic anomaly map produced with the acquired data shows five N-S-oriented belts of maximums up to 3000 nT separated by minimum of -500nT. The preliminary interpretation of the data might allow to infer the offshore prolongation of volcanic levels such as those observed in onland areas. Results of the magnetic survey along with the paleomagnetic and petrologic information presently in process will provide new insights of the western border of AP and its geodynamic relation with other segments of the old GDW margin.
GP24A-05 INVITED
ENVIRONMENTAL MAGNETISM STUDIES IN SOUTH AMERICAN SOILS AND PALEOSOLS DEVELOPED IN LOESS AND LOESSIC SEDIMENTS
A summary of results of magnetic measurements of loess and loessic sediments-paleosol sequences from South America is presented. Most of them were carried out in pampean plain (Argentina). The main objective in the selection of the studied Argentinian units was to cover the widest possible temporal interval. Pure loess deposition was interrupted at the end of the Pleistocene, given place to the deposition of reworked loess mainly due to the fluvial action. Unlike in China, the paleosols are not characterized by a systematic magnetic enhancement. The magnetic mineralogy of Pampean Plain loess reflects its volcanic origin, marked by the abundance of volcanic glass. The magnetic signature of these loesses is dominated by low-Ti titanomagnetites. Finally, according to a comparison of an index of potential water storage (PWS) with the magnetic signal of soils and paleosols from Pampean plain and others regions of the world, the existence of climatic thresholds that affect the forma¬tion, preservation or depletion of ferrimagnetic minerals could be suggested. Soils characterized by a positive PWS (as those studied in pampean plain) are wetted during an important part of the year, creating an appropriate environment that favors the depletion of ferrimagnetic minerals due to mainly reductive dissolution. Such soils are characterized by a depletion of detrital ferrimagnetic minerals.
GP24A-06 INVITED
Pollution Studies on Soils and Stream Sediments From Argentina Using Magnetic Techniques
The number of studies of contamination and magnetic proxies for environmental pollution in developed countries increased gradually since the 80s. In South American countries pollution studies are reduced in number; particularly in Argentina, studies on magnetic proxies are scarce and have been carried out since few years ago by the author and collaborators. This review deals with studies of magnetic proxies for pollution conducted in soils, lagoon and stream sediments from three districts (La Plata, Chascomús and Tandil) of the most densely populated province, Buenos Aires. Stream and lagoon sediments were studied in La Plata and Chascomús. In La Plata a higher concentration of magnetic minerals and a clear trend to finer grain size is observed in the top of the sequence of the most polluted streams. These results suggest an extra contribution of anthropogenic materials, possibly related to significant contribution of pollutants. The heavy metals concentration and some magnetic parameters show a good correlation. Longitudinal and vertical behaviour of the most polluted stream sediments was studied. According to the magnetic parameters and the content of heavy metals two distinctive groups are identified. The vertical distribution of sediments reveals a recent anthropogenic influence, possibly corresponding to the last 20-40 years. The magnetic parameters dependent on magnetic size grain and characteristics are more significant than those dependent on magnetic concentration for description of longitudinal and vertical distribution of present heavy metals. In Chascomús lagoon sediments, magnetic parameters and chemical determinations were correlated; the best results were obtained for parameters depending on magnetic characteristics, which show distinctive points and wide areas affected by pollution. This magnetic inference is supported by high contents of heavy metals, especially lead and zinc. Influence of pollution was investigated in Tandil and La Plata soils, revealing magnetic enhancement and presence of pollutants only in Tandil, where the main magnetic carriers are SD/PSD ferrimagnetic oxides in industrial areas (concentration about 0.1%) and SD magnetite with contribution of SP grains in natural areas (concentration about 0.01%). On the other hand, a magnetic and chemical distinction between unpolluted and polluted soils was not obtained in La Plata. PSD magnetite is the main magnetic carrier according to the analyses of different magnetic parameters. The low values of concentration-related magnetic parameters suggest lack of pollution in these soils, despite some neighboring streams are contaminated. Probably it is so, because the main pollution input is via liquid effluents or the distances to the sources are too far for atmospheric contamination.
GP24A-07
Age of the Xalnene Ash, Central Mexico: A Rock Magnetic Perspective
Features interpreted as human footprints ~40 ka old have been reported from the Toluquilla quarry near Valsequillo Reservoir, 15 km south of the city of Puebla in central Mexico (Gonzalez et al., 2006). The indentations were found in an indurated basaltic lapilli tuff informally known as the Xalnene Ash. The ~40 ka age of the tuff was based on optically stimulated luminescence (OSL) analysis of baked quartz-rich materials incorporated into the ash during eruption. If true, this ~40 ka age suggests that humans migrated into the Americas at a far earlier time than is currently believed. In contrast to the conclusions of Gonzalez et al. (2006), the age of the Xalnene ash was determined as 1.30 ± 0.03 Ma (2σ) based on highly reproducible 40Ar/39Ar analyses of homogenous basaltic lapilli (Renne et al., 2005). This age is further supported by paleomagnetic data, which shows a reversed polarity for the Xalnene Ash, consistent with deposition during chron C1r.2r (1.77 to 1.07 Ma). The magnetic mineralogy within the Xalnene Ash is not straightforward, and is a reflection of its volcanic origin modified by its subsequent hydrous alteration via prolonged exposure to varying water levels in Lake Valsequillo. Submicron titanomagnetite grains occur within the cores of the lapilli and are characterized by Curie temperatures between 400-500 °C corresponding to unoxidized compositions ranging from Fe2.82Ti0.18O4 to Fe2.68Ti0.32O4. Two components of magnetization were revealed during progressive alternating field (AF) demagnetization of oriented, individual lapilli: a low coercivity, reversed component that is typically removed after 15-20 mT, and a randomly oriented, but well-defined, higher coercivity component. The directions associated with the low coercivity component are consistent with the direction of the bulk rock characteristic remanent magnetization. This low coercivity component may have been acquired immediately after deposition during the final stages of cooling, or during subsequent reheating relating to the deposition of the overlying ash layer. We interpret the high coercivity component to be related to pre- depositional cooling of the lapilli. Hydrous alteration along the margins of the lapilli has formed clay rinds containing a multitude of iron oxides, including ferrihydrite, goethite, and hematite. Although the duration of this magnetic mineral growth is unknown, these minerals record a secondary component of magnetization that is persistent in bulk sample AF demagnetization experiments. The thin ash beds that comprise the Xalnene Ash show varying degrees of induration and hydrous alteration, indicating that the relative magnetic contribution of the hydrous alteration products will vary from layer to layer. Thus, extreme care needs to be taken when interpreting paleodirectional data obtained from the Xalnene Ash.
GP24A-08
On the age of human footprints in Central Mexico: paleomagnetic constraints
Until recently, we thought we knew where ancestral Native Americans came from (central Siberia, then across Beringia), and when they arrived (about 13 000 cal BP, after a trek between the recently separated Canadian ice sheets after Fiedel, 2006). Recent theories of American origins postulate multiple pre-Clovis migrations including Transpacific or coastal voyages by Australians, Melanesians, or Ainu, and even a Transatlantic migrations by Caucasoid Solutreans from Iberia. Thus, timing, route and origin of the first colonization to the Americas remains one of the most important topics in human evolution. Human and animal footprints have been found within the upper bedding surfaces of the Xalnene volcanic ash layer that outcrops in the Valsequillo Basin, south of Puebla, Mexico (Gonzalez et al., 2005). The ash has been dated to at least 40 ka BP. Paleomagnetic investigation yielded an intermediate magnetic polarity for Xalnene ash deposits while nearby Toluquilla volcano is reversely magnetized. Moreover, the absolute geomagnetic paleointensity derived from the volcanic lava flow is significantly reduced with respect to the present day geomagnetic field strength. It is quite possible that the ash as well as volcanic lava flow formed during worldwide observed Laschamp geomagnetic event.