Paleomagnetism and Tectonics in Latin America II
Presiding: A Gogichaishvili, National University of Mexico; J Urrutia-Fucugauchi, Universidad Nacional Autonoma de Mexico
GP34A-01 INVITED 15:30h
Paleomagnetic Constraints on the Neoproterozoic Evolution of West Gondwana
In order to constrain the paleogeographic evolution of West Gondwana, an updated paleomagnetic database for Africa and South America is presented. The most striking feature that sorts out from the compilation is the contrasting evolution of most western Gondwana blocks with respect to Amazonia and the other Rodinian affiliates surrounding Laurentia. In our configuration they are separated from Laurentia and surrounding blocks by the Brasiliano Ocean which starts to close at around 940 Ma. The assembly of most of these blocks seems to have been accomplished by 630 Ma as indicated by the coincidence of their paleomagnetic poles, and the collisional ages recorded throughout the western Gondwana. On the other hand, paleomagnetic evidence is compatible with a Rodinian affiliation of Amazonia. In contrast to the other considered cratons, Amazonia seems to have joined the western Gondwana comparatively late, in the Early Cambrian, after rifting away from Laurentia at the end of Proterozoic times. In such scenario, the western Gondwana was formed through at least two distinct orogenic episodes comprising the assembly of central blocks around the Congo-Sao Francisco craton at ~630 Ma, which have collided later with Amazonia, Rio Apa, and Pampean blocks at ~520 Ma.
GP34A-02 INVITED 15:45h
Paleomagnetism from the Chicxulub Impact Crater: Past, Present and Prospectives
Magnetic surveys over the Chicxulub Impact Crater had been carried out for the last 30 years, however it was until the Chicxulub Scientific Drilling Project started, that studies on the impact breccias and post-impact rock magnetic properties were performed. In this contribution, we report the results of our investigations on the magnetic properties from samples of breccia and post-impact lithologies recovered by Chicxulub Scientific Drilling Project, between 1996 and 2002. In general, NRM intensity and magnetic susceptibility present wide ranges, with values positively correlated, suggesting varying magnetic mineral contents and textures in the melt-rich breccias sequence. Vectorial composition and magnetic stability of NRM were investigated by stepwise alternating field and thermal demagnetization, which show characteristic magnetizations with linear components going through the origin of vector plots. NRM intensity ranges from 0.02 mA/m up to 37510 mA/m, suggesting varying magnetic mineral contents and textures in the melt-rich breccias sequence. Both upward and downward inclinations are observed, and we interpret the reverse magnetization as the primary component. The multivectorial composition of remanence may be related to the heterogeneous nature of the breccias and effects of hydrothermal activity. Clasts and matrix forming the breccias appear to have been subjected to a wide range of temperature/pressure conditions and show distinct rock-magnetic properties. We examined clasts of melt, granitic clasts and matrix from different intervals of breccias, to estimate grain-size and, along with thermomagnetic measurements, to identify the magnetic mineralogy. From the melt clasts we found "wasp-waisted" hysteresis cycles and we interpret the magnetic carrier as magnetite with a low content of titanium, with a low paramagnetic contribution and with grain-size within the range of pseudo-single domain. We found that the magnetic carrier in the granitic clasts is composed mainly of an iron mineral that present a strong paramagnetic component and grain-size within the multi-domain range.
GP34A-03 16:00h
New Contributions to the Geomagnetic Instability Time Scale: Paleomagnetic study of Tequila and Ceboruco-San Pedro-Amado Nervo Volcanic Fields (Trans Mexican Volcanic Belt)
The Trans-Mexican Volcanic Belt (TMVB) is one of the largest continental volcanic arcs of the North American plate. It spans about 1000 km from the Pacific to the Gulf of Mexico. Despite the abundance of thick lava sequences with quite high extrusion rates, the TMVB have been relatively little studied from a paleomagnetic point of view. Previous studies were aimed for tectonic evolution of the region rather than documenting fluctuations of Earth's magnetic field in terms of both directions and intensity. We report a detailed paleomagnetic and rock-magnetic study of Tequila and Ceboruco-San Pedro-Amado Nervo volcanic fields. 350 oriented samples belonging to 31 independent cooling units were collected. All these sites were previously dated by means of the state-of-the-art 40Ar-39Ar geochronological method and span from 1.1 Ma to 2 Ky. Rock-magnetic experiments which included continuous susceptibility, isothermal remanence acquisition and hysteresis measurements point to simple magnetic mineralogy. In most of cases, the remanence is carried by Ti-poor titanomagnetite of pseudo-single-domain magnetic structure. The paleodirections of the flow dated as 819±25 ka correspond to a VGP latitude of 18° N. This anomalous field behaviour apparently recorded prior to the Matuyama-Brunhes reversal may coincide with the geomagnetic event, defined as M-B precursor. Two independent lava flows, dated as 623±91 and 614±16 ka respectively, yield reverse paleodirections and one lava flow dated as 690±29 yields transitional paleodirections. It is possible that these lavas erupted during the worldwide observable Big Lost or Delta events.
GP34A-04 16:15h
Long-Term Variation of Geomagnetic Field Strength - Contributions from Latinamerica
Study of the long-term variation of polarity and paleodirection of the Earth's magnetic field in the Precambrian and Phanerozoic has proved valuable in geosciences. Magnetic polarity in the Mesozoic and Cenozoic is fairly well-established, with some first- order trends defined for older times. Defining of comparable long-term Earth's dipole moment variation has proved far more difficult an understanding. Among many other findings, magnetic polarity studies have defined several first-order features like the Permo-Triassic and Cretaceous superchrons, characterized by constant reverse and normal polarity, respectively, over long intervals. The proposal of even longer dipole moment characteristic features in the geologic past, like a low dipole moment during most of the Mesozoic, has important implications for the geodynamo. Since 1999, we are engaged to determine the Earth's field intensity during Miocene, late Paleozoic and Mesozoic using dated basalt units mainly from Mexico (Trans-Mexican Volcanic Belt and Baja California Peninsula) and South America (Brazil, Chile and Argentina). We will discuss three major issues currently debated: (1) paleointensities during a large period of Mesozoic (260-120 My) to examine the question of unusually low field known as "Mesozoic Dipole Low". (2) mid-Cretaceous paleointensities to examine theoretically predicted abnormally high intensity during the CNS and (3) paleointensities from 5 to 60 My old volcanic rocks.
GP34A-05 16:30h
Towards the determination of Mesoamerican chronology: Past and present of archeomagnetic studies in Mexico
Until recently, only few reliable archeomagnetic studies were available for Mesoamerica. The most detailed work was carried out by Wolfmann (1973, 1981, 1990) who reported preliminary results of the archaeomagnetic dating project of Central America, which was directed to re-asses the Mesoamerican relative and absolute chronology in a period from the year AD 0 to 1200. The value of the reference curve, however, is limited, since the specimens were not demagnetized and the magnetic mineralogy was not investigated. Moreover, in most of cases, ages were estimated only on the grounds of archeological stratigraphy. As part of their efforts to study the details of the magnetic properties of Mesoamerican rich cultural heritage, the detailed archeomagnetic works has been done at different places in Mexico and Central America. These investigations include the study of both conventional (kilns, potteries, human shelters.) and non-conventional (lime plasters, mural and cave paintings.) materials. First archeomagnetic reference curve for Mesoamerica is now available from the year AD 0 to 1200. Some preliminary archeointensity determinations were carried on selected samples yielding rather encouraging results.
GP34A-06 16:45h
On the importance of cooling rate correction in volcanic rocks: case study of a single Xitle lava flow (Central Mexico)
Many studies have been devoted to understand the significant variation of absolute Thellier paleointensity often observed within a single lava flow. Any attempts however, to find possible correlation between these variations and some physical/magnetic parameters has been unsatisfactory. Up to now, any possible explanations are mainly based on distinct oxidation states of opaque minerals throughout the profile or with the chemical alteration of the sample during the heating. Apart from archaeomagnetic investigations, the effect of cooling rate upon acquisition of thermoremanent magnetization (TRM) has not been taking into account. Systematic scatter and overestimating of PI values obtained in a single lava flow from Xitle volcano is drastically reduced when using cooling rate correction to raw selected data. Moreover, precision of individual determinations is substantially increased. Cooling rate effect upon acquisition of TRM in volcanic rocks seems to be as critical as in archaeomagnetic investigations.