Luis M. Alva-Valdivia

Magnetostratigraphy of the Middle Miocene continental sedimentary sequences of the Mae Moh Basin in northern Thailand: evidence for counterclockwise block rotation

Abstract A magnetostratigraphic study has been performed on the Mae Moh basin, well known for its lignite mining in the Lampang province. Paleomagnetic samples were collected from 66 stratigraphic levels. Rock magnetic investigations indicate the presence of low coercivity minerals. Specimens subjected to progressive thermal (or alternating field) demagnetization procedures show that nearly all of them exhibit a low temperature magnetization component, and a high temperature characteristic component (ChRM), with either positive or negative virtual geomagnetic pole latitudes and opposite polarity, considered as the characteristic Miocene magnetization. The overall mean paleomagnetic direction (incl.=22.2°, decl.=358.3°, κ =15, α 95 =4) documents a counterclockwise vertical axis rotation of about 13°±1.32 with respect to the expected Miocene direction derived from the Eurasian polar wander curve (incl.=42°, decl.=11°). Our paleomagnetic results are not consistent with the previously reported paleomagnetic data. The rotation observed in the Mae Moh basin can be the response to local tectonics. A section sampled for magnetostratigraphy reveals a polarity sequence of nine magnetozones that can reliably be correlated to the geomagnetic polarity time scale. According to biochronological constraints, the magnetostratigraphic results from the mammal-bearing succession correlate with chron C5ABn–C5An2n, between 13.5 and 12.1 Ma. According to the mean sedimentation rate of about 17.5 cm/ka, ages of 12.5 and 12.8 Ma are proposed for the fossiliferous levels (J5 and K1, K2 lignite zones) were the mammal remains were found.

Magnetic properties and archeointensity determination on Pre-Columbian pottery from Chiapas, Mesoamerica

As part of the effort to establish an archeointensity variation curve for Mesoamerica, 13 archeologically well-identified pottery samples belonging to the Ocozocoautla site (Chiapas) were studied. Analyzed samples consist of ‘ofrenda type’ pottery fragments found in several caves. Three archeological intervals are involved: 450-100 B.C., 200–550 A.D. and 550–900 A.D. The Thellier method in its modified form was applied to small fragments previously embedded in salt pellets. Raw intensity values were further corrected for cooling rate effects. The common time-consuming TRM anisotropy correction protocol was substituted by an alternative approach during the paleointensity experiments. Forty-two specimens, belonging to six samples, yielded high-quality Thellier determinations. The NRM fraction f used for paleointensity determination ranges between 0.42 to 0.99, and the quality factor q (Coe et al., 1978) varies from 4 to 59, being normally greater than 5. These results correspond to data of good quality. The mean archeointensity values per pottery fragments range from 14.6±1.5 to 59.5±13.8 μT, while the corresponding virtual axial dipole moments range from 2.5±0.3 to 10.0±2.4 × 1022 A m2. These new data, although not numerous, are of high quality and definitively contribute to the Mesoamerican, still insipient, archeointensity database.

Further constraints for the Plio-Pleistocene geomagnetic field strength: New results from the Los Tuxtlas volcanic field (Mexico)

A rock-magnetic, paleomagnetic and paleointensity study was carried out on 13 Plio-Pleistocene volcanic flows from the Los Tuxtlas volcanic field (Trans Mexican Volcanic Belt) in order to obtain some decisive constraints for the geomagnetic field strength during the Plio-Pleistocene time. The age of the volcanic units, which yielded reliable paleointensity estimates, lies between 2.2 and 0.8 Ma according to the available K/Ar radiometric data. Thermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occurred during the initial flow cooling. Unblocking temperature spectra and relatively high coercivity point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Six flows yield reverse polarity magnetization, five flows are normally magnetized, and one flow shows intermediate polarity magnetization. Evidence of a strong lightning-produced magnetization overprint was detected for one site. The mean pole position obtained in this study is Plat = 83.7°, Plong = 178.1°, K = 36, A95 = 8.1°, N = 10 and the corresponding mean paleodirection is I = 31.3°, D = 352°, k = 37, α95 = 8.2°, which is not significantly different from the expected direction estimated from the North American apparent polar wander path. Thirty-nine samples were pre-selected for Thellier palaeointensity experiments because of their stable remanent magnetization and relatively weak-within-site dispersion. Only 21 samples, coming from four individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 6.4 to 9.1 × 1022 Am2. Combining the coeval Mexican data with the available comparable quality Pliocene paleointensity results yield a mean VDM of 6.4 × 1022 Am2, which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and are of dissimilar quality. Additional high-quality absolute intensity determinations are needed to better constraint the geomagnetic field strength during the Plio-Pleistocene time.

Paleomagnetic data from the Trans-Mexican Volcanic Belt: implications for tectonics and volcanic stratigraphy

We report a paleomagnetic and rock-magnetic study of Miocene volcanic rocks from the Trans-Mexican Volcanic Belt. A total of 32 sites (238 oriented samples) were collected from three localities: Queretaro, Guadalajara and Los Altos de Jalisco basaltic plateaux, which span from 11 to 7.5 Ma. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Microscopic observation of polished sections shows that the main magnetic mineral is Ti-poor titanomagnetite associated with exsolved ilmenite. Continuous susceptibility measurements with temperature yield in most cases reasonably reversible curves with Curie points close to that of magnetite. Judging from the ratios of hysteresis parameters, it seems that all samples fall in the pseudo-single domain (PSD) grain size region, probably indicating a mixture of multidomain (MD) and a significant amount of single domain (SD) grains. Based on our paleomagnetic and available radiometric data, it seems that the volcanic units have been emplaced during a relatively short time span of 1 to 2 My at each locality. The mean paleomagnetic directions obtained from each locality differ significantly from that expected for the Middle Miocene. The mean paleomagnetic direction calculated from 28 sites discarding those of intermediate polarity is I = 32.46°, D = 341.2°, k = 7.2 and α95 = 11.6°. Comparison with the expected direction indicates some 20° anticlockwise tectonic rotations for the studied area, in accordance with the proposed left-lateral transtensional tectonic regime already proposed for this period.

Rock-Magnetic and Oxide Microscopic Studies of the El Laco Iron Ore Deposits, Chilean Andes, and Implications for Magnetic Anomaly Modeling

Microscopic and rock-magnetic studies of the ores and host rocks of the El Laco iron oxide deposits permit us to characterize the magnetic mineralogy and the processes affecting natural remanent magnetization (NRM) during emplacement and evolution of the deposits. Particular attention was devoted to identifying the magnetic mineral composition (magnetite and/or titanomagnetite, and hematite and/or titanohematite, and titanomaghemite) and grain size variations of both ores and host rock. Rock-magnetic data are used to clarify magnetic domain states and remanence acquisition processes, and to assess their significance as a source of magnetic anomalies. Microscopy under reflected light demonstrates that magnetic carriers are mainly magnetite, with significant amounts of ilmenite-hematite minerals. Magmatic titanomagnetites in the andesitic rocks show trellis textures, compatible with high-temperature oxy-exsolution processes. Supergene reactions in ore deposits under eruption conditions are indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 μm. Hysteresis measurements point to pseudo-single-domain states. Thermal spectra, continuous temperature-dependent susceptibility measurements, and isothermal remanent magnetization (IRM) acquisition suggest predominance of spinels (titanomagnetite or titanomaghemite) with low-Ti contents as magnetic carriers. Although the presence of (titano)hematites is indicated by hysteresis and IRM studies, their contribution to the total remanence seems to be minor. The Fe-oxides in the ore are typically poor in Ti, whereas in the rocks they are Ti-bearing. For the modeling of the magnetic anomalies, we used data on bulk susceptibility and NRM intensity and direction in order to constrain the relative contributions of induced and remanent magnetization components, and to obtain improved control regarding depth and geometry of source bodies. The deep magnetic source corresponds to an ENE-striking tabular body, steeply inclined 65° to the north.

Counterclockwise Rotation of the Michoacan Block: Implications for the Tectonics of Western Mexico

Subduction of the Farallon plate beneath North America resulted in formation of the Rivera and Cocos oceanic plates, the extensive magmatic arcs of the Sierra Madre Occidental (SMO), and the Trans-Mexican Volcanic Belt (TMVB). Southern Mexico consists of crustal blocks separated by a regional extensional structural system; the latter, called the Guadalajara triple junction, is defined by the Tepic-Zacoalco (TZR), Colima (CR), and Chapala (CHR) rifts. TZR and CHR separate the SMO from the Jalisco and Michoacan blocks, whereas CR is the boundary between the Jalisco and Michoacan blocks. In this study, we carried out combined radiometric and paleomagnetic analyses in the Michoacan block. Radiometric dates of 31.60 to 8.39 Ma confirm both the southern extension of the Sierra Madre Occidental and the early mafic TMVB succession into the Michoacan block. The Oligocene age agrees well with the radiometric dating reported for the southern SMO and the Tertiary volcanic fields of the Sierra Madre del Sur. Paleomagnetic data indicate a counterclockwise rotation of ∼24° about a vertical axis for the Michoacan block. Several plate models suggest either dextral or sinistral oblique convergence of the Cocos plate relative to North America. Our new results help to constrain these different models. These data demostrate that deformation in the Michoacan block is as old as late Miocene, and is related to sinistral oblique convergence of the Cocos plate relative to North America—inducing the southeast relative motion of the Michoacan block. The structural trends along both CHR and CR are thereby explained. On the other hand, right-lateral transtension along the TZR is related to the westward motion of the Jalisco block because of oblique convergence of the Rivera plate.

Berthierine and chamosite hydrothermal : genetic guides in the Peña Colorada magnetite-bearing ore deposit, Mexico

We report the first finding of berthierine and chamosite in Mexico. They occur in the iron-ore deposit of Peña Colorada, Colima. Their genetic characteristics show two different mineralization events associated mainly to the magnetite ore. Berthierine is an Fe-rich and Mg-low 1:1 layer phyllosilicate of hydrothermal sedimentary origin. Its structure is 7 Å, dhkl [10 0] basal spacing and low degree structural ordering. The phyllosilicate has been identified by a lack of 14 Å basal reflection on X-ray diffraction (XRD) patterns. These data were supported by High Resolution Transmision Electron Microscopy (HRTEM) images that show thick packets of berthierine in well defined parallel plates. From the analysis of Fast Fourier Transform (FFT), we found around [1 0 0] reflections of berhierine 7.12 Å and corresponding angles of hexagonal crystalline structure. Berthierine has a microcrystalline structure, dark green color, and high refraction index (1.64 to 1.65). Birefringence is low, near 0.007 to null and it is associated to nanoparticles (<15 nm) and microparticles of magnetite (<25 μm), fine grain siderite, and organic matter. Its texture is intergranular-interstratified with colloform banding. The chamosite Mg-rich is of hydrothermal epigenetic origin affected by low-degree metamorphism. It is an Fe-rich 2:1 layer silicate, with basal space of 14 Å, dhkl [0 0 1]. The chamosite occurs as lamellar in sizes ranging from 50 to 150 μm. It has intense green color and refraction index from 1.64 to 1.65. The birefringence is near 0.008, with biaxial (-) orientation and a 2V small. It is associated mainly to sericite, epidote, clay, feldspar, and magnetite. Chamosite is emplaced in open spaces filling and linings. Mössbauer spectra of berthierine and chamosite are similar. They show the typical spectra of paramagnetic substances, with two well defined unfoldings corresponding to the oxidation state of Fe+2 and Fe+3. Chemical composition of both minerals was obtained by an electron probe X-ray micro-analyzer (EPMA). The radio Fe+Mg+Mn vs Si and Al show similar chemical compositions and different XRD patterns in the crystalline structure provoked by the environmental conditions of emplacement. A hydrothermal environment was predominant, occurring before, during, and after the magnetite mineralization. The identification of magnetite nanoparticles supports the hypothesis of a marine environment, specifically exhalative sedimentary (SEDEX) for the berthierine.

Cooling rate corrected paleointensities from the Xitle lava flow: Evaluation of within-site scatter for single spot-reading cooling units

The historic Xitle lava, which covers a great part of southern Mexico City, is one of the most studied (magnetically) volcanic unit worldwide. Studies include detailed paleomagnetic and paleointensity investigations, which have documented an enigmatic within-flow variation of absolute paleointensity first recognized in the decade of 1960s. However, attempts to find possible explanations in terms of physical/magnetic parameters or geomagnetic effects have been unsatisfactory. As an effort to understand the over- and underestimating of geomagnetic paleointensity (PI) within the Xitle lava flow, we investigated the relation of the cooling rate upon the acquisition of thermoremanent magnetization (TRM). Contrary to archaeomagnetic investigations, most paleointensity experiments on volcanic rocks do not consider cooling rate effects in a systematic manner. Our results show that the scatter and overestimation of PI values obtained on the single Xitle lava flow are drastically reduced when using the cooling rate correction to raw selected data. The Thellier method combined with cooling rate experiments provide a higher precision compared to conventional Thellier and microwave techniques. The cooling rate effects upon acquisition of TRM in volcanic rocks seem to be as critical as in archaeomagnetic investigations.

Paleomagnetic poles and paleosecular variation of basalts from Paraná Magmatic Province, Brazil: geomagnetic and geodynamic implications

A paleomagnetic study was carried out along two sections (altogether 35 lava flows, 300 samples) in the central Parana Magmatic Province (PMP), Brazil. The two sections, distanced ca. 200 km apart, yield statistically indistinguishable paleomagnetic poles. The combined paleomagnetic pole with coordinates −85.7 ◦ N, 197.9 ◦ E( A95 = 2.6 ◦ , N = 35) is statistically different from previously published paleomagnetic poles for other sections of PMP. We suggest that this angular difference, as well as differences between previously published poles, is caused by undetected local tectonic rotations not easily identified in the often-poorly exposed lavas of the PMP. A joint analysis of all published PMP paleomagnetic data indicate that paleosecular variation, estimated as the angular standard deviation (SF) of the virtual geomagnetic pole distribution, does not support the suggestion of anomalously high secular variation at low latitudes in the 110–195 Ma period [J. Geophys. Res. 96 (1991) 3923]. All SF estimates are, in fact, in better accordance with latitudinal dependence estimates derived from the curve for the 0–5 Ma period. Moreover, we find that all PMP paleomagnetic poles lie ∼10 ◦ away from the pole predicted by an assumed fixed hotspot reconstruction of South America. The PMP paleomagnetic poles, therefore, call for either true polar wander or motion of Indo-Atlantic hotspots.

Comprehensive paleomagnetic study of a succession of Holocene olivine-basalt flow: Xitle Volcano (Mexico) revisited

A detailed paleomagnetic study of a young Late Holocene olivine-basalt flow from the Xitle volcano in the southern Basin of Mexico was completed to evaluate the consistency and reliability of the record derived from fresh well-preserved and exposed lava flows. One-hundred oriented standard paleomagnetic cores corresponding to 10 different lava effusion episodes were collected from unit-flow V. Thermomagnetic analyses show that bulk magnetic properties and remanence is carried in most cases by Ti-poor titanomagnetite, presumably resulting from oxy-exsolution processes during the initial flow cooling. Unblocking temperature and coercivity suggests pseudo-single domain magnetic grains for these (titano)magnetites. Thermal and alternating field demagnetizations show well-defined univectorial magnetizations. Most sites present a mean direction with small angular dispersion around the dipolar direction for central Mexico. The overall mean direction (N = 10, Dec = 1.1°, Inc = 34.1°, k = 531 and α95 = 2.1°) is characterized by small angular dispersion and inclination close to the dipolar value for the locality. Anisotropy ofmagnetic susceptibility lineation agrees with the geologically-inferred flow direction. Various samples from the 10 lava flows were selected for Thellier paleointensity experiments because of their stable remanent magnetization and relatively low within-site dispersion. According to reliability parameters, the obtained paleointensities are of reasonably good quality. Nine mean paleointensities range between 48.6 and 73.9 μT. The overall mean paleointensity of 59.9–7.7 μT is higher than the present-day field of 43 μT, consistent with the global data for this time-period. Most samples presented alteration during the cooling rate test, and no correction was made to these samples. Those samples on which cooling-rate correction was applied give a flow mean lower than the raw paleointensity data, as was expected.