Annick Chauvin

Style and history of Andean deformation, Puna plateau, northwestern Argentina

Topographically, the Puna plateau of northwestern Argentina is the southern continuation of the Bolivian Altiplano. Its thickening and consecutive uplift result from the Andean orogeny. To better constrain the structural style and its progressive development, we have studied field data, topographic and satellite imagery, balanced cross sections, seismic reflection data, kinematic analysis of fault slip data, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, and apatite fission track (AFT) data. Across the Puna plateau, Precambrian and Paleozoic basement ranges, bounded by high-angle reverse faults (dips ≥ 60°), alternate with Cenozoic intermontane basins. Major thrusts trend NNE-SSW and do not show a preferred vergence. Intermontane basins have various degrees of symmetry, depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets. There is a close correlation between the surface expression of a basin and the amount of internal deformation. A line-balanced cross section of the Puna at 25°S has yielded a Cenozoic shortening of 10–15%, in a direction subperpendicular to the orogen. By kinematic analysis of Cenozoic fault slip data we have obtained principal directions of strain rate across the Puna. Shortening axes are subhorizontal and trend on average WNW-ESE (∼N110°), stretching axes are subvertical, and intermediate axes are subhorizontal and trend on average NNE-SSW. Strain ellipsoids are dominantly of plane strain type, and they represent dip-slip thrusting. From paleomagnetic and AMS data, shortening axes form a radial pattern around the eastern edge of the central Andes. The pattern is attributed to an inhomogeneous stress field, reflecting the eastward convex shape of the central Andean thrust front. From the history of burial and uplift, Andean shortening reached the northeastern part of the Puna in the late Eocene and the adjacent Eastern Cordillera in the late Eocene or early Oligocene. This shortening was presumably due to the Incaic phase of the Andean orogeny. In the eastern part of the orogen the onset of shortening was probably guided by preexisting Paleozoic and Mesozoic structures, so that Andean deformation propagated unevenly eastward.

Dating transitionally magnetized lavas of the late Matuyama chron : Toward a new 40Ar/39Ar timescale of reversals and events

The K-Ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed, but rarely dated, the transitions between polarity intervals. Subsequent 40Ar/39Ar dating indicated that the ages of some polarity transitions had been underestimated by about 6%. Although the accepted ages of the polarity chron boundaries have increased, their precise temporal definition remained uncertain. We have taken a different approach and used incremental-heating techniques to obtain 18 new 40Ar/39Ar ages from basaltic lavas within flow sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Hawaii. These lavas record transitional paleomagnetic directions corresponding to four mid-Pleistocene polarity reversals or events. Three lavas from Punaruu Valley previously thought to record the Cobb Mountain Normal Polarity Subchron (CMNS) gave a mean age of 1.105 ± 0.005 Ma, indicating that they were erupted about 76 kyr after the CMNS; this period of transitional field behavior is designated the Punaruu event. In addition, seven new 40Ar/39Ar ages from the Punaruu Valley indicate that the Jaramillo Normal Polarity Subchron (JNS) lasted about 67 kyr, starting at 1.053 ± 0.006 Ma and ending 0.986 ± 0.005 Ma. This agrees with astronomical estimates but conflicts with JNS ages proposed by Spell and McDougall [1992] and Izett and Obradovich [1994] on the basis of 40Ar/39Ar dating of rhyolite domes in the Valles Caldera. Indistinguishable 40Ar/39Ar ages of seven lavas, including one from Punaruu Valley and six from Haleakala that record broadly similar intermediate paleodirections, suggest that the Kamikatsura event occurred at 0.886 ± 0.003 Ma. Moreover, these data indicate that the Kamikatsura event occurred 20–40 kyr after another geomagnetic event, most probably taking place at 0.92 Ma. We designate this earlier field behavior the Santa Rosa event, adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the JNS in the 1968 study of Doell et al. [1968]. The discovery of these new short-lived polarity events during the Matuyama reversed chron suggests that the 400 kyr period between 1.18 and 0.78 Ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse. This newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks.

Paleointensity of the Earth's magnetic field recorded by two Late Quaternary volcanic sequences at the Island of La Réunion (Indian Ocean)

A paleomagnetic study has been undertaken on two late Quaternary volcanic sequences from the Piton de la Fournaise volcano (island of La Reunion) in order to provide absolute paleointensities of the Earths magnetic field from a site in the southern hemisphere. The pattern of secular variation recorded by both sequences clearly indicates that the extrusion rate of the lava flows was not constant with time. A detailed investigation of the magnetic properties of the samples was carried out in parallel with paleointensity experiments which were performed with the Thellier method. Samples with minimum overprint, composed by single-domain or pseudo-single-domain magnetite showing good thermal stability and a narrow blocking temperature spectrum were the most successful during Thelliers experiments. Values of the paleofield strength from 19 to 55 mT have been obtained on 23 flows among the 30 which were sampled. Few variations (7.5–9.9×1022 A m2) are observed in the virtual dipole moments (VDMs) determined on the youngest sequence (5–12 ka). Those values are within the range of fluctuations of the worldwide variations of VDMs for this period. On the other hand, data from the oldest sequence (82–98 ka) are the first paleointensity values obtained in that time period, and the observed VDMs extend from 4.1 to 8.8×1022 A m2. These results suggest that for this time interval, the field did not depart from its average behavior.

New archeointensity data from Spain and the geomagnetic dipole moment in western Europe over the past 2000 years

Archeomagnetic studies on 14 kilns, a group of jar fragments, and a collection of baked bricks dated between 1000 and 1959 AD plus one Roman pottery kiln have been conducted in order to obtain high-quality archeointensity data to enhance the western European database. The Thellier method with corrections for anisotropy of thermoremanent magnetization (TRM) and for cooling rate dependence upon TRM acquisition was used. The effect of TRM anisotropy is only important for the bricks where the corrected and uncorrected mean intensities differ by more than 5%. Cooling rate correction factors determined per sample are up to 15% and per site up to 7.7%. Our 17 new data together with 62 previously published results were used to obtain, by Bayesian modeling, the geomagnetic field intensity over the past two millennia for western Europe. Our results indicate that geomagnetic intensity remained more or less constant between the 1st and 4th centuries and between the 14th and 16th centuries (mean values, at Paris, around 65 and 57 μT, respectively), whereas an important decrease occurs between 1600 and 1800 AD. The detailed evolution of geomagnetic field intensity during the High Middle Ages is not yet well established. Despite their differences, geomagnetic global models predict our results reasonably well. This work indicates the need to obtain a number of archeointensity data for each time interval in order to reliably record variations of the geomagnetic field and to test whether any relationship exists between field intensity and climate

Vertical axis rotations across the Puna plateau (northwestern Argentina) from paleomagnetic analysis of Cretaceous and Cenozoic rocks

Between 10°S and 30°S, the central Andes are marked by both a major topographic anomaly, the Altiplano-Puna plateau, and a westward concave geometry whose origin remains controversial. The arcuate shape is accompanied by a remarkable pattern of rotations about vertical axes. Indeed, in the central Andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc (throughout Peru, northernmost Chile, and northern Bolivia) and clockwise rotations on the southern limb (throughout southern Bolivia, northwestern Argentina, and northern Chile). To fill a gap in data from northern Argentina and to contribute to the ongoing debate on the origin of rotations in the central Andes, we have undertaken a paleomagnetic study of 373 cores, taken at 29 sites (grouped into seven localities). The samples are from sediments and lava flows of Cretaceous to Tertiary age located in intermontane basins of the Puna plateau in northwestern Argentina. Vertical axis rotations, calculated from paleomagnetic declinations, are clockwise for all localities and confirm the pattern of clockwise rotations associated with the southern central Andes. However, significant variations in the amount of rotation occur from one locality to another, suggesting that they are, at least in part, influenced by local tectonics. As most faults in the Puna plateau have reverse dip-slip components, we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults. Whether or not this mechanism has operated across the entire area of thickened crust in the central Andes remains to be demontrated. Even if such faulting has locally influenced rotations, Cenozoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the central Andes.

First paleomagnetic data from the sedimentary cover of the French Penninic Alps: evidence for Tertiary counterclockwise rotations in the Western Alps

Abstract We present a paleomagnetic study performed in the Brianconnais sedimentary cover of the western Alpine Arc (France). Sampling was focused on Upper Jurassic rocks of the Brianconnais cover in the Briancon–Guillestre area. More than 100 samples from 11 sites were collected essentially from Ammonitico rosso limestones and sedimentary dykes of the Upper Jurassic. Thermal and alternating field demagnetizations revealed three components of magnetization carried dominantly by magnetite: (1) a low-temperature component with present-day magnetic field direction of viscous origin; (2) a well defined reverse-polarity component with a maximum unblocking temperature of 440°C (A component); and (3) a weak high-temperature component (B component) that experienced a viscous overprint during heating and was difficult to isolate. The A component exhibits a negative fold test and shows a better clustering when corrected for tilting associated with late Alpine extension. This component is interpreted as a secondary remagnetization acquired during the cooling path related to Late Eocene–Early Oligocene metamorphism. The mean A component direction, corrected for late Alpine extensional tilting, is D=142°, I=−57°, k=44, α95=8°. This direction significantly differs by 47° ± 13° from the expected direction for Eurasia. We interpret this difference to be related to a post-Eocene counterclockwise rotation about a vertical axis of the Penninic zone relative to stable Europe.

Paleomagnetic study along the southeastern edge of the Altiplano - Puna Plateau: Neogene tectonic rotations

In northwestern Argentina the transition zone between the Puna and the Sierras Pampeanas was deformed in Late Miocene and Pliocene time. Structural observations suggest that local clockwise block rotations might be observed associated with dextral transpression. This study contributes new paleomagnetic results from four sites in Cretaceous rocks and 19 sites in Neogene sedimentary sequences. When compared with reference paleomagnetic data for stable continental South America, the results show an average inclination shallowing error of 10.5° and a pattern of clockwise rotations up to 29°. Study of the anisotropy of magnetic susceptibility (AMS) on 278 specimens provided, for most sites, AMS tensors with an oblate shape controlled by sedimentation and compaction, explaining the observed inclination shallowing. A slight AMS lineation was measured and we argue that this magnetic lineation reflects Late Miocene-Pliocene compression. A correlation observed between magnetic lineations, deduced from the AMS analysis, and rotation estimates from remanent magnetizations, demonstrates that the inferred shortening directions have also been rotated clockwise.

Archaeomagnetism, methodology and applications: implementation and practice of the archaeomagnetic method in France and Bulgaria

Abstract Recent improvements in archaeomagnetism applied to archaeological baked clay, in France and Bulgaria, are presented in this paper. After reviewing the historical development of the method in France and Bulgaria, and the principles of the method, we present sampling techniques for in situ structures (kilns and hearths) and sets of displaced materials (bricks or tiles). In the analysis protocol, we stress the importance of correcting the magnetic anisotropic effects especially for bricks. We also show how the problem of brittle specimens can be solved by induration. After a review of the published archaeomagnetic data currently available for France and Bulgaria, we present different smoothing techniques applied to data obtained in these countries. Finally, we present the usage of the variation curves of the geomagnetic elements in the past to calculate the archaeomagnetic dates. One of these techniques is based on a Bayesian approach, similar to the case of the dendro-chronological calibration of r...

Remanence anisotropy effect on the palaeointensity results obtained from various archaeological materials, excluding pottery

The effect of magnetic anisotropy on the palaeointensity results has been evaluated in different materials, including samples from archaeological structures of various ages, such as baked clay from prehistoric domestic ovens or pottery kilns, burnt soil from ancient fires, and bricks and bricks or tiles used in the kiln’s construction. The remanence anisotropy was estimated by the thermoremanent (TRM) anisotropy tensor and isothermal remanence (IRM) tensor methods. The small anisotropy effect (less than 5%) observed in the palaeointensity results of baked clay from the relatively thin prehistoric oven’s floors estimated previously through IRM anisotropy was confirmed by TRM anisotropy of this material. The new results demonstrate the possibility of using IRM anisotropy evaluation to correct baked clay palaeointensity data instead of the more difficult to determine TRM anisotropy ellipsoid. This is not always the case for the palaeointensity results from bricks and tiles. The anisotropy correction to palaeointensity results seems negligible for materials other than pottery. It would therefore appear that the palaeointensity determination is more sensitive to the degree of remanence anisotropy P and the angle between the natural remanent magnetization (NRM) vector and the laboratory field direction, than to the angle between the NRM and the maximum axis of the remanence anisotropy ellipsoid (Kmax).

Intensity of the geomagnetic field in Europe for the last 3 ka: Influence of data quality on geomagnetic field modeling

One of the main challenges of paleomagnetic research is to obtain high-resolution geomagnetic field intensity reconstructions. For the last millennia, these reconstructions are mostly based on archeomagnetic data. However, the quality of the intensity data available in the databases is very variable, and the high scatter observed in the records clearly suggests that some of them might not be reliable. In this work we investigate how the geomagnetic field intensity reconstructions and, hence, our present knowledge of the geomagnetic field in the past, are affected by the quality of the data selected for modeling the Earths magnetic field. For this purpose we rank the European archeointensity data in four quality categories following widely accepted paleomagnetic criteria based on the methodology used during the laboratory treatment of the samples and on the number of specimens retained to calculate the mean intensities. Four geomagnetic field regional models have been implemented by applying the revised spherical cap harmonic analysis to these four groups of input data. Geomagnetic field models strongly depend on the used data set. The model built using all the available data (without any preselection) appears to be the less accurate, indicating some internal inconsistencies of the data set. In addition, some features of this model are clearly dominated by the less reliable archeointensity data, suggesting that such features might not reflect real variations of the past geomagnetic field. On the contrary, the regional model built on selected high-quality intensity data shows a very consistent intensity pattern at the European scale, confirming that the main intensity changes observed in Europe in the recent history of the geomagnetic field occurred at the continental scale.