Avto Goguitchaichvili

A rock-magnetic and paleointensity study of some Mexican volcanic lava flows during the Latest Pleistocene to the Holocene

Eleven Late Quaternary lava flows were sampled in the Chichinautzin volcanic field of central Mexico to determine their magnetic characteristics and absolute paleointensity. The samples studied cover a geological time interval of approximately 0.39 My to 2000 years. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Continuous susceptibility measurements with temperature in most cases yield reasonably reversible curves with Curie points close to that of almost pure magnetite, which is compatible with low-Ti titanomagnetite resulting from oxi-exsolution. Judging from the ratios of hysteresis parameters, it seems that all samples fall within the pseudo-single domain grain size region, probably indicating a mixture of multidomain and a significant amount of single domain grains. Forty-two samples belonging to six independent cooling units yielded acceptable absolute paleointensity estimates. The NRM fractions used for paleointensity determination range from 0.34 to 0.97 and the quality factors varies between 4.5 and 97.8, being normally greater than 5. The obtained virtual dipole moment values are higher than those recently reported for the past 5 My and to the present day geomagnetic field strength. Individual paleointensity of around 2000 BP is substantially higher than the present day intensity, which is in broad agreement with worldwide archeomagnetic results.

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.

Magnetism of oriented single crystals of hemoilmenite with self‐reversed thermoremanent magnetization

Ten large, single crystals of hemoilmenite (yFeTiO3 (1-y)Fe2O3) with y close to 0.54 were extracted from the self-reversed Pinatubo dacitic pumice erupted in 1991 and oriented with respect to the c axis of the hexagonal structure by means of an X-ray diffractometer. Hysteresis measurements show that c is the hardest magnetization axis while the softest axes lie in the basal plane. The hysteresis curves observed at room temperature in this plane suggest that in addition to the multidomain (MD) regions commonly seen under the microscope using the Bitter technique, there are also crystal regions which behave like single-domain (SD) or pseudosingle-domain (PSD) particles. As could be expected from the hysteresis measurements, the directions of natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM), and thermoremanent magnetization (TRM) all lie in the basal plane, regardless of the direction of the applied field. Thus the antiferromagnetically coupled spins of hemoilmenite of intermediate composition lie in the basal plane. The partial TRMs (pTRMs) are approximately reversed; that is, their direction is always more than 90° away from the applied field direction and most often close to 180°. In fact, pTRM directions are discretely distributed along three directions in the basal plane which are 60° away from each other. This distribution indicates that the magnetic anisotropy is of magnetocrystalline origin. Both NRM and TRM are exceptionally resistant to alternating field (AF) with no decrease in remanence intensity up to 30 mT at least. This behavior, together with the fact that the Lowrie-Fuller test is SD-type, indicates that natural and laboratory TRMs are carried by non-interacting single-domain regions. We propose a self-reversal model based on negative exchange interactions in which the self-reversed TRM is carried by cation-ordered (ferrimagnetic) SD-like regions dispersed within a cation-disordered (antiferromagnetic with weak ferromagnetism) MD matrix carrying a weak, normal TRM.

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.

On the features of the geodynamo following reversals or excursions: by absolute geomagnetic paleointensity data

We carried out a Thellier paleointensity study of a ∼3.6 million year Pliocene geomagnetic excursion recorded in a lava flow succession from southern Georgia (lesser Caucasus). Previous paleomagnetic study [Phys. Earth Planet. Int. 96 (1996) 41] revealed that several consecutive lava flows record an intermediate polarity direction at the base of the section followed by a thick reverse polarity zone. Samples of 71 from 26 flows from both polarity zones were pre-selected for paleointensity experiments because of their low viscosity index, stable remanent magnetisation and close to reversible continuous thermomagnetic curves. Altogether 54 samples from 21 flows yielded reliable paleointensity estimates. The mean paleointensity of the intermediate field is 7.8 ± 2.4T (three flows). The stable polarity paleointensity is higher with a mean 24.2 ± 8.2T (15 flows), which corresponds to a mean virtual dipole moment (VDM) of 4.6 ± 1.8 × 10 22 Am 2 . This value is significantly lower than the average Pliocene geomagnetic dipole moment and post-intermediate dipole moments recorded in volcanic sequences at Hawaii (∼4 Ma) and Steens mountain (∼16.2 Ma). However, our results are quite similar to the post-intermediate field recorded in Iceland during the Gauss–Matuyama reversal. These results suggest that the regime of the geodynamo following reversals or excursions may vary significantly from one case to the next without any apparent systematic features.

Low-latitude paleosecular variation and the time-averaged field during the late Pliocene and Quaternary—Paleomagnetic study of the Michoacan-Guanajuato volcanic field, Central Mexico

We report paleomagnetic, rock magnetic, and paleointensity studies for 24 volcanic lava flows from the Michoacan-Guanajuato volcanic field (MGVF), erupted between 2.27 Ma to present according to available radiometric ages and historic records. The MGVF located in the central-western sector of the Plio-Quaternary Trans-Mexican volcanic belt is mainly composed by cinder cones and shield volcanoes. Rock magnetic experiments show remanence is carried in most cases by Ti-poor titanomagnetites, resulting from oxy-exsolution of original titanomagnetites during flow cooling. Unblocking temperature spectra and high coercivities point to “small” pseudo-single domain grains for the titanomagnetites. Single component, linear vector plots are obtained after alternating field and thermal demagnetization. Seven flows yield reverse polarity magnetization while sixteen flows are normally magnetized. The overall mean paleodirection obtained (with 14 flows normal and 6 flows reverse) is I=28.4°, D=357.9°, k=21, α95=7.3°, with a paleomagnetic pole position of Plat=85.7°, Plong= 104.5°, K=27, A95=6.4°. The paleodirection is undistinguishable from expected Plio-Quaternary paleodirections derived from reference poles for the North American polar wander curve, and previously reported paleodirections for central Mexico. Paleointensity experiments give high quality results for only twelve samples from two flows with mean values of 7.3 and 8.1 × 1022 Am2, which are close to present geomagnetic field intensity. The combination of new MGVF directional results with currently available paleomagnetic data from central Mexico yield angular dispersion estimates of SF = 15.4 with SU = 19.6 and SL = 12.7, which are in agreement with the latitude-dependent PSV model of McFadden et al. (1988, 1991) for the last 5 Ma, and show no significant inclination anomaly.

Magnetic fingerprint of tsunami-induced deposits in the Ixtapa-Zihuatanejo Area, Western Mexico

The Pacific coast of Mexico has repeatedly been exposed to destructive tsunamis. Recent studies have shown that rock magnetic methods can be a promising approach for identification of tsunami- or storm-induced deposits. We present new rock magnetic and anisotropy of magnetic susceptibility (AMS) results in order to distinguish tsunami deposits in the Ixtapa–Zihuatanejo area. The sampled, 80 cm-deep sequence is characterized by the presence of two anomalous sand beds within fine-grained coastal deposits. The lower bed is probably associated with the 14 March 1979 Petatlán earthquake (M W = 7.6), whereas the second one formed during the 21 September 1985 Mexico earthquake (M W = 8.1). Rock magnetic experiments discovered significant variations within the analysed sequence. Thermomagnetic curves reveal two types of behaviour: one in the upper part of the sequence, after the occurrence of the first tsunami, and the other in the lower part of the sequence, during that event and below. Analysis of hysteresis parameter ratios in a Day plot also allows us to distinguish two kinds of behaviour. The samples associated with the second tsunami plot in the pseudo-single-domain area. In contrast, specimens associated with the first tsunami and the time between both tsunamis display a very different trend, which can be ascribed to the production of a considerable amount of superparamagnetic grains, which might be due to pedogenic processes after the first tsunami. The studied profile is characterized by a sedimentary fabric with almost vertical minimum principal susceptibilities. The maximum susceptibility axis shows a declination angle D = 27°, suggesting a NNE flow direction which is the same for both tsunamis and normal currents. Standard AMS parameters display a significant enhancement within the transitional zone between both tsunamis. The study of rock magnetic parameters may represent a useful tool for the identification and understanding of tsunami deposits.