Manuel Calvo-Rathert

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.

Paleomagnetism of the Pleistocene Tequila Volcanic Field (Western Mexico)

This paper presents new paleomagnetic results from 24 independent cooling units in Tequila area (western Trans-Mexican Volcanic Belt). These units were recently dated by means of state-of-the-art 40Ar-39Ar method (Lewis-Kenedy et al., 2005) and span from 1130 to 150 ka. The characteristic paleodirections are successfully isolated for 20 cooling units. The mean paleodirection, discarding intermediate polarity sites, is I = 29.6°, D = 359.2°, k = 26, α95 = 7.1°, n = 17, which corresponds to the mean paleomagnetic pole position Plat = 85.8°, Plong = 84.3°, K = 27.5, A95 = 6.9°. These directions are practically undistinguishable from the expected Plestocene paleodirections, as derived from reference poles for the North American polar wander curve and in agreement with previously reported directions from western Trans-Mexican Volcanic Belt. This suggests that no major tectonic deformation occurred in studied area since early-middle Plestocene to present. The paleosecular variation is estimated trough the study of the scatter of virtual geomagnetic poles giving SF = 15.4 with SU = 19.9 and SL = 12.5 (upper and lower limits respectively). These values are consistent with those predicted by the latitude-dependent variation model of McFadden et al. (1991) for the last 5 Myr. The interesting feature of the paleomagnetic record obtained here is the occurrence of an intermediate polarity at 671 ± 13 ka which may correspond the worldwide observed Delta excursion at about 680–690 ka. This gives the volcanic evidence of this event. Two independent lava flows dated as 362 ±13 and 354 ±5 ka respectively, yield transitional paleodirections as well, probably corresponding to the Levantine excursion.

A paleointensity study on middle Miocene to Pliocene volcanic rocks from south-eastern Spain

Thirteen middle-Miocene to Pliocene volcanic sites, belonging to four different types of volcanism (calc-alkaline volcanism, potassic calc-alkaline and shoshonitic volcanism, lamproitic volcanism and basaltic alkaline volcanism), have been sampled in south-eastern Spain for paleointensity determinations. Rock-magnetic studies yield low-Ti titanomagnetite as the main carrier of remanence, showing also the presence of titanohematites in several cases. Analysis of hysteresis parameters indicates a PSD domain structure. Paleointensity determinations were performed with the Coe (1967) method. Of the 90 paleointensity determinations carried out, 29 provide successful determinations which fulfil selection criteria; most of these were in samples of lamproitic volcanism. Only four lamproitic sites out of the 13 studied ones yield reliable results. All have a similar age of approximately 7 M.y. Virtual dipole moments (VDM) of three of these display values between approximately 4·1022 A m2 and 8·1022 A m2, thus providing new paleointensity data for a time interval (between 4 and 8 M.a.), with a lack of Thellier-type paleointensity determinations. The fourth site shows a much lower paleointensity of 1.6·1022 A m2, and may correspond to a polarity transition, in accordance with its paleo-directional results.

Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico)

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40Ar/39Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude ϕ = 81.1°, longitude λ = 94.3°) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed polarity observed in the other site (220 ± 36 ka) very likely provides new evidence for the Pringle Falls excursion or the event recorded in the Mamaku ignimbrite.

A comparison of Thellier‐type and multispecimen paleointensity determinations on Pleistocene and historical lava flows from Lanzarote (Canary Islands, Spain)

project CGL2012-32149 (Ministerio de Econom ia y Competitividad, Spain), project 320/2011 (Ministerio de Medio Ambiente y Medio Rural y Marino, Spain) and the European Regional Development Fund (ERDF).

A paleointensity study of Cretaceous volcanic rocks from the Western Cordillera, Colombia

The Cretaceous Normal Superchron is a period of great interest to investigate global scale variations of the geomagnetic field. Long periods of single polarity are still a matter of debate: up to now there are two contradicting theories, which try to relate geomagnetic field intensity and reversal rate. We aim to shed light on the geomagnetic field strength during the Cretaceous Normal Superchron because data are still scarce and of dissimilar quality. To obtain reliable, absolute paleointensity determinations we investigate volcanic rocks from the Western Cordillera of Colombia. Several age determinations allow relating the samples to an age of about 92.5 Ma. To characterize the samples, we investigate rock magnetic properties and determine the characteristic remanent magnetization behavior. To determine paleointensities, we use a multimethod approach: first, we apply the classic Thellier-Coe protocol, and then, the relatively new multispecimen method. Rock magnetic measurements indicate magnetite as the main ferrimagnetic mineral, a stable magnetization revealed by reversible and nearly reversible thermomagnetic curves, and grain sizes that are either in the pseudosingle domain range or a mixture of single and multidomain grains. Alternating field and thermal demagnetization are rather complex, although we observe a few vector diagrams with a single, essentially uni-vectorial component with a small viscous overprint. Paleointensity determination with the Thellier-Coe protocol was unsuccessful, while with the multispecimen protocol we obtained four successful determinations out of 20. The failure of the Thellier-Coe protocol can be attributed to multidomain grains, which were observed during demagnetization and in rock magnetic experiments, and to the inhomogeneity of the volcanic rocks. Our multispecimen paleointensity determinations support low field strength at around 90 Ma during the Cretaceous Normal Superchron.

Magnetic signature of the 22 June 1932 tsunami deposits (Jalisco, Mexican Pacific coast)

Recent studies have demonstrated that rock-magnetic analysis may provide additional information to distinguish and characterize extreme marine inundation events such as tsunamis. Rock-magnetic proxies reinforce and improve the environmental evidences supplied by other methods, adding some decisive clues for the interpretation of the origin and genesis of the sedimentary deposits. Here we report rock-magnetic, XRD and SEM microscopy results obtained in the Palo Verde estuary (Colima Pacific coast, Mexico) in order to enhance the tools for identification and reconstruction of two tsunami-induced deposits. The sedimentary sequence includes two sand units, a tsunami deposit (PV1) associated with the 22 June 1932 tsunami and a deeper sandy layer (PV2) related to a possible palaeotsunami that occurred around 1300 CE. Both sandy units are topped by finer grained units. Magnetic properties exhibit a significant correlation with the stratigraphy. High susceptibility (χ) and high saturation isothermal remanence (SIRM) values typical of high concentrations of (titano)magnetite are a distinctive feature of the most recent sandy tsunamigenic unit PV1 and the overlaying soil. The lower sandy tsunamigenic unit PV2 shows significantly lower χ and SIRM values, indicating lower concentration of (titano)magnetite in this unit and the overlaying clayey-silt unit. The latter also shows a higher coercivity component associated to (titano)hematite. Magnetic grain-size differences are also observed between PV1 and PV2 suggesting differences in hydraulic conditions at the time of deposition. The bulk mineralogical composition and sediment texture of these units also supports the hypothesis of different provenances for each tsunamigenic unit as inferred from magnetic properties.

Palaeomagnetic results from the Chiapanecan Volcanic Arc, Chiapas, Southern Mexico: geomagnetic and geodynamic significance

This article presents the first palaeomagnetic results from 13 independent cooling units in the Chiapanecan Volcanic Arc (ChVA). Six sites were directly dated by Ar–Ar or K–Ar methods: their dates range from 2.14 to 0.23 Ma. We isolated the characteristic palaeodirections for all 13 lavas. Eleven non-transitional directions yield a mean direction with inclination, I = 30.7°, declination, D = 4.1°, and precision parameters k = 63 and α95 = 5.8°. The corresponding mean palaeopole position is Plat = 83.3°, Plong = 203.8°, K = 227, A 95 = 5.1°. The mean inclination is in good agreement with the expected value for the last 5 million years, as derived from the synthetic North American polar wander path [Besse and Courtillot 2002, Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years: Journal of Geophysical Research, v. 107, no. B11, p. 2300], but a measured rotation of the palaeodeclination of about 8° with respect to the expected direction suggests the possibility of a clockwise rotation of the studied ChVA units. We have estimated the characteristics of palaeosecular variation through study of the scatter of virtual geomagnetic poles, obtaining a palaeosecular variation parameter S b = 14.5° with upper limit S U = 19.6° and lower limit S L = 11.7°, in reasonable agreement with the fit of model G [McFadden et al., 1988, Dipole/quadrupole family modeling of paleosecular variation: Journal of Geophysical Research, v. 93, no. B10, p. 11583–11588; 1991, Reversals of the Earths magnetic field and temporal variations of the dynamo families: Journal of Geophysical Research, v. 96, no. B3, p. 3923–3933] to the Johnson et al. [2008, Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows: Geochemistry, Geophysics, Geosystems, v. 9, no. 4, ID Q04032, doi:10.1029/2007GC001696] databases for the last 5 million years. In those cases in which age determinations are available, the polarity obtained for the studied flows is consistent with their stratigraphic positions, except for the Huitepec site, which probably reflects the transitional geomagnetic regime prior to the Matuyama–Brunhes geomagnetic reversal.