Harald Böhnel

Pre-Miocene palaeogeography of the Los Cabos Block, Baja California Sur: geochronological and palaeomagnetic constraints

The Los Cabos Block (LCB) is located at the southern end of the Baja California peninsula and is composed mainly of intrusive rocks that were emplaced into pre-Cretaceous heterogeneous metasediments and are partly covered by Miocene volcanic and volcaniclastic rocks. Field observations provide evidence that the undeformed, homogeneous gabbronorite of the Sierra El Novillo in the NE part of the LCB is the oldest crystalline unit, which is intruded by undeformed to slightly deformed tonalitic-quartz-dioritic granitoids. For these rocks an Rb‐Sr intrusion age of 129±15 Ma and a biotite whole-rock cooling age of 116±2 Ma were obtained. Low initial 87Sr/86Sr and high e-Nd values of about 0.7036 and + 5t o+6 respectively characterise the intrusives as primitive, mantle-derived granites. Further to the southeast and south, granitoids are more silicic, with abundant deformed sequences — orthogneisses, diatexites and migmatites, among others. From the southern part of the LCB an Rb‐Sr intrusion age for the undeformed granites of 115±4 Ma and a biotite cooling age of 90±2 Ma were determined. Initial 87Sr/86Sr was determined to be 0.7054 and e-Nd values fall between ’2 and 0. The isotopic data do not clearly allow us to characterise the deformed granitoids of the LCB as being intruded by the undeformed unit. Deformation could have been partly due to syn-intrusive tectonics. Palaeomagnetic data suggest minor, if any, northward displacement of the LCB with respect to continental Mexico, corresponding to the rifting in the Gulf of California since the late Miocene, and significant 35‐45° clockwise rotations. Possible tilting eVects, which may have occurred given the dip of Tertiary volcanics on top of the intrusives, would modify these values for rotation and to a lesser degree of northward displacement. Geochemical, isotopic and palaeomagnetic results and the cooling history of the LCB are similar to those from the Puerto Vallarta Batholith, whereas other Mexican continental margin plutons further to the SE display lower intrusion ages, more rapid cooling rates and generally more primitive magma compositions. In the light of the data presented, a common magmatic evolution of the LCB and the Puerto Vallarta Batholith is strongly suggested.

Tectonics along the Trans-Mexican volcanic belt according to palaeomagnetic data

Abstract Subduction of the Cocos plate beneath the southern Mexico continental margin takes place at the Middle America trench. Volcanism of calc-alkaline character occurs along the E-W trending Trans-Mexican volcanic belt (TMVB), which includes several active stratovolcanoes, monogenetic cinder cone fields and silicic centres. However, a direct association between the volcanism and plate subduction has been questioned for a number of reasons, including the oblique orientation of the TMVB with respect to the trench. Most palaeomagnetic directions for the Cretaceous to Quaternary within or immediately to the south of the TMVB appear discordant. The declinations are rotated anticlockwise relative to expected declinations. This angular divergence may be explained by rotations of crustal blocks by different degrees in response to large-scale left-lateral shear across central Mexico. The fault system may have acted as an efficient structural control for the rise of magmas, which may explain some of the peculiarities of the TMVB.

Secular variation in Mexico during the last 40,000 years

Paleomagnetic data are available for Mexico from various archives, covering irregularly the last 40,000 years. Data obtained from lake sediments are affected by ambiguous conversion of age to depth, and the paleomagnetic data themselves are not of high quality, as samples were only demagnetized at one field intensity. These lake sediment data are therefore considered as less reliable for construction of a secular variation curve, although they may give some qualitative information for the last 25,000 years. Archeological samples provide data for the period, 100–1200 a.d. Their 14 C ages are some times not well constrained, with unrecoverable analytical and statistical procedures, and often ages were only inferred from the stratigraphic position and the paleomagnetic record. The paleomagnetic record of these materials is of high quality with very low internal dispersion, although no magnetic cleaning techniques have been applied. Variations of inclination and declination with time are hampered by large age uncertainties and thus of limited value for the construction of secular variation curves. Paleomagnetic data from an U–Th dated stalagmite provide reasonable directions for most of the last 1200 years, being of lower quality in those parts with very low remanence intensity. The U–Th age determinations were interpreted in terms of a constant growth-rate, which might be only valid as a first-order approximation. Recent volcanic rocks from the Trans-Mexican volcanic belt provide 19 paleomagnetic directions of reasonable to high quality, and often also absolute paleointensities. Ages have been determined from 14 C analysis of charcoal or paleosols found below the lavas, and extend over the last ∼40,000 years. Therefore, volcanic rocks provide the most adequate archive for the construction of a secular variation curve, but much more volcanic structures need to be dated to obtain a semi-continuous secular variation curve. Comparison of the data from different archives is possible mainly for the last 2500 years and despite the aforementioned problems similar variations of declination and inclination are observed. This holds as well when comparing them to data from Holocene volcanic rocks from western North America.

Palaeomagnetic data from central Mexico and their use for palaeosecular variation studies

Abstract Three Brunhes-age palaeomagnetic data sets from central Mexico exhibit different dispersion of virtual geomagnetic poles. Volcanics from the eastern part of the Trans-Mexican Volcanic Belt result in a S F = 14.8 °. This value is interpreted to represent an upper limit, as these rocks probably are affected by localized tectonic rotations. For a stalagmite, S F = 9.5 °, which is interpreted as a lower limit, as the stalagmite is only 1200 years old and the data are strongly biased in time. Seventy-four volcanic units from the Chichinautzin Formation result in S F = 11.7°, with confidence limits of 10.4° and 13.3°. This dispersion is much lower than expected from palaeosecular variation models, but corresponds to data obtained for other areas in or beneath the Pacific Basin, e.g. the Hawaiian islands.

Dating of the Valsequillo volcanic deposits: Resolution of an ongoing archaeological controversy in Central Mexico

The timing and origin of the earliest human colonization of the Americas has been the subject of great debate over the last 100 years and is still a matter of heated discussion today (Renne et al., 2005; Gonzalez et al., 2006a). It is widely accepted that the Clovis culture was the first to migrate into the New World at 13.1 ka (Waters and Stafford, 2007). However, archaeological evidence, in the form of stone tools, linguistics, craniometrics and genetics all suggest that the first Americans were ethnically diverse, and a few sites dated to 15e16 ka challenge the ‘Clovis First’ model (Goebel et al., 2008). Perhaps the most spectacular challenge to the ‘Clovis First’ model was the reported presence of human footprints within a basaltic ash (Xalnene Ash, Valsequillo Basin, Central Mexico), dated to 38.04 8.57 ka using optically stimulated luminescence (OSL; Gonzalez et al., 2006a). However, Renne et al. (2005) challenged the validity of the footprints by dating lapilli from the Xalnene Ash using 40Ar/39Ar and reported an age of 1.30 0.03 Ma (2s). Renne et al. (2005) also reported a reversed palaeomagnetic polarity for the ash, consistent with deposition during chron C1r.2r (Cande and Kent, 1995). Such antiquity casts considerable doubt on the interpretation of the impressions as human footprints. Gonzalez et al. (2006b) questioned the 40Ar/39Ar age and highlighted the heterogeneous nature of the lapilli as a potential problem for 40Ar/39Ar geochronology. The lapilli contain phenocrysts and include xenocrysts. Olivine phenocrysts can be contaminated with excess Ar (ArE; McDougall et al., 1969) and hence the dating of

Paleomagnetism of the Cretaceous Morelos and Mezcala Formations, southern Mexico

A paleomagnetic study of platform-facies carbonate rocks of the mid-Cretaceous Morelos Formation and deep-water carbonate rocks of the overlying Upper Cretaceous Mezcala Formation, sampled at Zopilote canyon, in Guerrero State, southern Mexico, indicates that their characteristic magnetization was acquired contemporaneously with folding of these rocks during the Late Cretaceous Laramide orogeny. The remanence carrier is interpreted to be magnetite, although other mineral phases of high coercivity carry recent secondary overprints. The overall mean is of Dec=323.1j and Inc=36.5j (k=162.7; a95=2.7j; N=18 sites; 64% unfolding). Comparison with the North America reference direction indicates that this area has experienced a small, yet statistically significant, counterclockwise direction of 19.2F4.0j. Similar rotations are documented in other localities from southern Mexico; rotations are linked to mid-Tertiary deformation associated with the left-lateral strike-slip fault system that accommodated motion of the Chortis and Xolapa blocks. D 2002 Elsevier Science B.V. All rights reserved.

Paleomagnetic study of Jurassic and Cretaceous rocks from the Mixteca terrane (Mexico)

Three sites from Cretaceous limestone and Jurassic sandstone in northern Oaxaca, Mexico, were studied paleomagnetically. Thermal demagnetization isolated site-mean remanence directions which diAer significantly from the recent geomagnetic field. The paleopole for the Albian‐Cenomanian Morelos formation is indistinguishable from the corresponding reference pole for stable North America, indicating tectonic stability of the Mixteca terrane since the Cretaceous. Rock magnetic properties and a positive reversal test for the Bajocian Tecomazuchil sandstone suggest that the remanence could be of primary origin, although no fold test could be applied. The Tecomazuchil paleopole is rotated 108258 clockwise and displaced 248258 towards the study area, with respect to the reference pole for stable North America. Similar values were found for the Toarcien‐Aalenian Rosario Formation, with 358268 clockwise rotation and 338268 latitudinal translation. These data support a post-Bajocian southward translation of the Mixteca terrane by around 258, which was completed in mid-Cretaceous time. # 1999 Published by Elsevier Science Ltd. All rights reserved.

Precision of the paleomagnetic method: An example from the Quaternary Eifel volcanics (Germany)

The total within-site dispersion of paleomagnetic data results from natural misalignment processes and experimentally produced dispersion. Although some of the sources of dispersion may be determined by experiments, this is not possible for others. The total accuracy of the paleomagnetic method so far has been determined only on historic lava flows. Thirty-seven Quaternary basaltic lava flows from the Eifel, Germany, have been re-sampled using the same outcrops as before. The new site-mean directions do not differ significantly from those of the original study. The angular distance between pairs of site-mean directions may be approximated by a Fisher distribution. Assuming that both studies are affected by the same natural dispersion processes, we obtain as the best value for the experimental dispersion sexp = 4.33°. The total within-site dispersion shows a log-normal distribution with a mean of stot = 5.39°. From these values we calculate that the dispersion due to natural processes in the Eifel volcanic field is snat = 3.21°.

Paleomagnetic constraints on the ages of the Holocene Malpaís de Zacapu lava flow eruptions, Michoacán (México): Implications for archeology and volcanic hazards

Four monogenetic Holocene lava flows located within the Michoacán-Guanajuato Volcanic Field, Mexico, were sampled for paleomagnetic dating. These flows (namely, El Infiernillo, Malpaís Las Víboras, El Capaxtiro, and Malpaís Prieto) are within the heartland of the pre-Hispanic Tarascan civilization and were inhabited repeatedly since at least 100 BC, but no relation with the volcanic evolution has been proposed so far. The stratigraphically oldest lava flow, El Infiernillo, has a radiocarbon age of 3200 ± 30 yr BP (cal. 1525–1420 BC), and it was used to validate the method. Using full-vector paleomagnetic data from three sites as input for paleomagnetic dating applying the global paleosecular variation model SHA.DIF.14k, an age range of 1500–1370 BC was obtained. Two age ranges of 1340–1230 and 1030–940 BC were obtained for Malpaís Las Víboras. A younger age range of 200–80 BC was obtained for the Capaxtiro lava flow and, finally, the Malpaís Prieto lava flow erupted within the range of AD 830–960. The human occupation history of these flows started around 100 BC during the late Pre-Classic, probably shortly after the Capaxtiro eruption. Archeological records indicate an abandonment of the entire area around AD 900 (late to terminal Classic), which coincides with the paleomagnetic age of the Malpaís Prieto eruption. Interestingly, this area was heavily repopulated again only few hundred years later around AD 1250 and belongs to the core region in which the Tarascan civilization has its roots. The eruption recurrence interval of roughly 1000 years indicates that a new monogenetic eruption should be expected to occur again in the future and that this area deserves to be studied in greater detail with particular emphasis on the impact of past eruptions. This could help to better evaluate volcanic hazards and design preparedness strategies to minimize the impact of a future eruption.

Paleointensities of the Hawaii 1955 and 1960 Lava Flows: Further Validation of the Multi-specimen Method

The Kilauea 1955 and 1960 lava flows (Big Island of Hawaii, USA), both emplaced in a field of ∼36 μT, were studied using the multi-specimen parallel differential partial thermoremanent magnetization (pTRM) paleointensity (PI) method. In nineteen specimens from an upper cooling unit of the 1955 flow, the pTRMs were acquired at a temperature of 450°C and a PI of 34.3 ± 1.5/1.6 μT was obtained, while 13 specimens of the lower cooling unit heated to 230°C resulted in a PI of 38.5 ± 3.3 μT. The 1960 flow was studied at various temperatures of 400, 440, 480, 500 and 550°C. At 400 and 550°C overestimates of the PI were obtained: 47.3 μT and 41.5 μT respectively. The other temperatures yielded PI values ranging from 32.2 to 34.2 μT, just 6–11% lower than the expected field and similar to the 1955 flow results. The 550°C PI result is biased because of mineral alteration as indicated by an 18% susceptibility decrease which reduces the pTRM capacity leading to an overestimate of the PI. The overestimate of the PI at 400°C may be due to the comparatively small pTRM acquired and thus to larger high-temperature pTRM tails. Results obtained with the multi-specimen method for both flows compare favorably with the best other PIs obtained by the Thellier–Coe, Thellier–Thellier and microwave methods. The success rate is high with more than 90% of specimens contributing to a PI. Only the Thellier–Coe method used on single plagioclase crystals and the microwave method have similar success rates.