Xavier Quidelleur

ON LOW-DEGREE SPHERICAL HARMONIC MODELS OF PALEOSECULAR VARIATION

Abstract Geomagnetic field directions over the past few millions of years have recently been compiled and these data bases can be used to build low-degree spherical harmonic models of the time-averaged field (TAF) and its paleosecular variation (PSV) variance structure. Such models can be constrained by a number of statistical distributions derived from the data, such as latitude variations of the angular standard deviation (ASD) of virtual geomagnetic poles (VGP), which is widely used in paleomagnetism. Our approach has been to test predictions of the giant Gaussian field model of Constable and Parker ( J. Geophys. Res. , 93: 11569–11581, 1988) with respect to six such distributions: ASD of VGPs, VGP distribution with latitude and longitude and in so-called ‘common-site’ longitude, and standard deviations of declination and inclination. Model predictions have been calculated at the actual sites and for the actual number of lava flows recorded in the database. We have next explored modifications of the TAF model when a single persistent term is added (up to degree and order four), or a combination of a few such terms. No model with persistent terms and the Constable and Parker (1988) PSV structure can fit the data satisfactorily, and a change in that variance structure is required. We explore several hypotheses, and find that assuming isotropic PSV in the non-dipole field components improves the situation significantly. This could be related to systematic underestimation of paleomagnetic uncertainties. However, the best fit is obtained if one assumes that the PSV structure of spherical harmonic terms is non-uniform. An excellent fit is obtained if the standard deviation of the (2, 1) quadrupole is assumed to be significantly larger than that of the (2, 0) and (2, 2) terms. This is in good agreement with recent theoretical findings. The significance of persistent harmonics of degree equal to or higher than three in the TAF is doubtful. There are also indications that the lack of symmetry between the normal and reverse states of the paleofield may have been overestimated. It remains to be seen how much of the TAF and PSV structure that is found is still biased by the poor geographical distribution of the sites, as is clearly the case for the so-called ‘right-handed effect’ in VGP distribution.

An attempt to constrain the age, duration, and eruptive history of the Karoo flood basalt: Naude's Nek section (South Africa)

We have carried out paleomagnetic sampling of a similar to 750 m sequence of the Karoo large igneous province (Naudes Nek Pass, South Africa). K-Ar dating (Cassignol-Gillot) has been performed on four samples from the 650 m upper unit (mean age 179.2 +/- 1.8 Ma) and a sample from the lower unit (184.8 +/- 2.6 Ma). A succession of two phases of volcanism is suggested. The lower 25 flows (115 m thick) have recorded a reversed polarity; the next 23 flows (135 m thick) are transitional and contribute a detailed record of the Van Zijl (1962) Jurassic reversal. The upper 38 flows (500 m thick) have normal polarity. Directional groups (DGs) of lava flows with quasi-identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events. Altogether, 19 DGs and 10 sheet lobes yield a sequence of 29 distinct directions. This could correspond to a total eruptive activity shorter than 3000 years, less than one per mil of the total duration over which the section was emplaced. We obtain a new paleomagnetic pole for South Africa at similar to 180 Ma (lambda = 75.2 degrees N, phi = 276.4 degrees E, A(95) = 5.8 degrees, N = 19), which is consistent with earlier reports.

The geological evolution of Merapi volcano, Central Java, Indonesia

Merapi is an almost persistently active basalt to basaltic andesite volcanic complex in Central Java (Indonesia) and often referred to as the type volcano for small-volume pyroclastic flows generated by gravitational lava dome failures (Merapi-type nuées ardentes). Stratigraphic field data, published and new radiocarbon ages in conjunction with a new set of 40K–40Ar and 40Ar–39Ar ages, and whole-rock geochemical data allow a reassessment of the geological and geochemical evolution of the volcanic complex. An adapted version of the published geological map of Merapi [(Wirakusumah et al. 1989), Peta Geologi Gunungapi Merapi, Jawa Tengah (Geologic map of Merapi volcano, Central Java), 1:50,000] is presented, in which eight main volcano stratigraphic units are distinguished, linked to three main evolutionary stages of the volcanic complex—Proto-Merapi, Old Merapi and New Merapi. Construction of the Merapi volcanic complex began after 170xa0ka. The two earliest (Proto-Merapi) volcanic edifices, Gunung Bibi (109u2009±u200960xa0ka), a small basaltic andesite volcanic structure on Merapi’s north-east flank, and Gunung Turgo and Gunung Plawangan (138u2009±u20093xa0ka; 135u2009±u20093xa0ka), two basaltic hills in the southern sector of the volcano, predate the Merapi cone sensu stricto. Old Merapi started to grow at ~30xa0ka, building a stratovolcano of basaltic andesite lavas and intercalated pyroclastic rocks. This older Merapi edifice was destroyed by one or, possibly, several flank failures, the latest of which occurred after 4.8u2009±u20091.5xa0ka and marks the end of the Old Merapi stage. The construction of the recent Merapi cone (New Merapi) began afterwards. Mostly basaltic andesite pyroclastic and epiclastic deposits of both Old and New Merapi (<11,792u2009±u200990 14C years BP) cover the lower flanks of the edifice. A shift from medium-K to high-K character of the eruptive products occurred at ~1,900 14C years BP, with all younger products having high-K affinity. The radiocarbon record points towards an almost continuous activity of Merapi since this time, with periods of high eruption frequency interrupted by shorter intervals of apparently lower eruption rates, which is reflected in the geochemical composition of the eruptive products. The Holocene stratigraphic record reveals that fountain collapse pyroclastic flows are a common phenomenon at Merapi. The distribution and run-out distances of these flows have frequently exceeded those of the classic Merapi-type nuées ardentes of the recent activity. Widespread pumiceous fallout deposits testify the occurrence of moderate to large (subplinian) eruptions (VEI 3–4) during the mid to late Holocene. VEI 4 eruptions, as identified in the stratigraphic record, are an order of magnitude larger than any recorded historical eruption of Merapi, except for the 1872xa0AD and, possibly, the October–November 2010 events. Both types of eruptive and volcanic phenomena require careful consideration in long-term hazard assessment at Merapi.

Modes of rifting in magma-rich settings: Tectono-magmatic evolution of Central Afar†

Recent research in Afar (northern Ethiopia) has largely focused on the formation of the present-day ocean-continent transition at active segments (e.g. Manda Hararo). However, the Oligo-Miocene history of extension, from the onset of rifting at ~25 Ma to the eruption of the massive Stratoid flood basalts at ~ 4 Ma, remains poorly constrained. Here, we present new structural data and radiometric dating from Central Afar, obtained along a zone stretching from the undeformed Oligocene Ethiopian plateau to the Manda Hararo and TatAle active volcanic segments. Basaltic and rhyolitic formations were mapped in two key areas corresponding to the proximal and distal parts of a half-rift. We present a balanced composite cross-section of Central Afar, reconstructed using our new data and previously-published geophysical data on the crustal structure. Our main findings are: 1) Extension during the Mio-Pliocene corresponds to a ‘wide rift’ style of rifting. 2) The lower crust has been underplated/intruded and re-thickened during rifting by magmatic injection. 3) Our restoration points to the existence of mid-crustal shear zones that have helped to distribute extension in the upper crust and to localize extension at depth in a necking zone. Moreover, we suggest that there is a close relationship between the location of a shear zone and the underplated/intruded material. In magma-rich environments such as Central Afar, break-up should be achieved once the initial continental crust has been completely replaced by the newly, magmatically accreted crust. Consequently, and particularly in Afar, crustal thickness is not necessarily indicative of break-up but instead reflects differences in tectono-magmatic regimes.

Active tectonics in Quito, Ecuador, assessed by geomorphological studies, GPS data, and crustal seismicity

The Quito Fault System (QFS) extends over 60u2009km along the Interandean Depression in northern Ecuador. Multidisciplinary studies support an interpretation in which two major contemporaneous fault systems affect Quaternary volcanoclastic deposits. Hanging paleovalleys and disruption of drainage networks attest to ongoing crustal deformation and uplift in this region, further confirmed by 15 years of GPS measurements and seismicity. The resulting new kinematic model emphasizes the role of the N-S segmented, en echelon eastward migrating Quito Fault System (QFS). Northeast of this major tectonic feature, the strike-slip Guayllabamba Fault System (GFS) aids the eastward transfer of the regional strain toward Colombia. These two tectonic fault systems are active, and the local focal mechanisms are consistent with the direction of relative GPS velocities and the regional stress tensor. Among active features, inherited N-S direction sutures appear to play a role in confining the active deformation in the Interandean Depression. The most frontal of the Quito faults formed at the tip of a blind thrust, dipping 40°W, is most probably connected at depth to inactive suture to the west. A new GPS data set indicates active shortening rates for Quito blind thrust of up to 4u2009mm/yr, which decreases northward along the fold system as it connects to the strike-slip Guayllabamba Fault System. The proximity of these structures to the densely populated Quito region highlights the need for additional tectonic studies in these regions of Ecuador to generate further hazard assessments.

Eruptive history of the Karoo lava flows and their impact on early Jurassic environmental change

This paper reports new paleomagnetic and geochronologic data from a ~1500u2009m-thick composite section belonging to the Drakensberg group, the thickest remnant of the Karoo lavas in Northern Lesotho. Flow-by-flow analysis of paleomagnetic directions reveals twenty-one magnetic directional groups (DG), corresponding to single eruptive events (SEE), and sixteen individual lava flows. The new age determinations of lava flows range from 180.1u2009±u20091.4 to 182.8u2009±u20092.6u2009Ma. These data, combined with previous results allow us to propose that the main part of the Drakensberg group and the Karoo intrusive complex dated around 181-183u2009Ma may have been erupted over a period as short as 250 kyr, and may have coincided with the two main phases of extinction in the Early Toarcian. This scenario agrees well with the discontinuous rhythm of environmental and biotic perturbations in the late Pliensbachian-Toarcian interval.

Volcanic evolution of central Basse-Terre Island revisited on the basis of new geochronology and geomorphology data

Twenty-six new and seven previous K–Ar ages obtained on groundmass separates for samples from the Axial Chain massif (Guadeloupe, F.W.I.), associated with geomorphological investigations, allow us to propose a new model of the volcanic evolution of the central part of Basse-Terre Island. The Axial Chain is composed of four edifices, Moustique, Matéliane, Capesterre, and Icaque mounts, showing coeval activity from 681u2009±u200912 to 509u2009±u200910xa0ka, which contradicts a previous hypothesis that flank collapse affected them successively. Our geomorphological reconstruction shows that the Axial Chain can be considered as a single large volcano, named the Southern Axial Chain volcano (SCA), rather than a succession of several smaller volcanoes. It raises questions regarding the formation of a large depression within the SCA volcano, prior to the construction of the Sans-Toucher volcano between 451u2009±u200913 and 412u2009±u20098xa0ka. Given presently available evidence, a slump affecting the western part of the SCA volcano is the most probable scenario to reconcile the complete age dataset and the present-day morphology of central Basse-Terre. Finally, our study shows that the SCA volcano had a post-activity volume of 90xa0km3, implying a construction rate of 0.5xa0km3/kyr. This value strongly constrains interpretations of magma generation processes throughout the Lesser Antilles arc.

Construction and destruction of Mont Pelée volcano: Volumes and rates constrained from a geomorphological model of evolution

This study presents long-term volumes and construction rates for the Mont Conil-Mont Pelee volcano and rate estimates at which volcanic activity creates relief. An algorithm, ShapeVolc, is used to numerically model topographic surfaces. Volcano morphology is analyzed using current digital elevation model in combination with mapped geology to produce 10 paleotopographies at the end of four constructional stages and three destructional events. Volumes of each constructional stage were estimated at about 35.2u2009km3, 26.2u2009km3, 8.3u2009km3, and 2.5u2009km3 for a total cumulative erupted volume of 72.2u2009km3. We estimate that Mont Pelee accounted for about 10% of the Lesser Antilles arc production in the last 100u2009kyr. The volcano has been built at an average rate of 0.13u2009km3/kyr during the last 550u2009kyr. During that time, construction rates varied by a factor of 15, from 0.04u2009km3/kyr in early stages up to 0.52u2009km3/kyr after the second flank collapse. Volumes displaced by each flank collapse were estimated at 14.7u2009km3, 8.8u2009km3, and 3.5u2009km3, thus about 37% of the total constructed volume. Integrated over the volcanos lifetime, the rate at which flank collapses removed material off the island is 0.15u2009km3/kyr. In contrast, long-term erosion rates outside collapsed areas are estimated at about 0.05u2009±u20090.7u2009km3/kyr, or ~11u2009km3 of material removed. This latter rate is not negligible, which strengthens the importance of taking into account recurrent small erosional events on the geomorphological evolution of a volcanic island in a tropical context.

Towards reconstruction of the lost Late Bronze Age intra-caldera island of Santorini, Greece

During the Late Bronze Age, the island of Santorini had a semi-closed caldera harbour inherited from the 22 ka Cape Riva Plinian eruption, and a central island referred to as ‘Pre-Kameni’ after the present-day Kameni Islands. Here, the size and age of the intracaldera island prior to the Late Bronze Age (Minoan) eruption are constrained using a photo-statistical method, complemented by granulometry and high-precision K-Ar dating. Furthermore, the topography of Late Bronze Age Santorini is reconstructed by creating a new digital elevation model (DEM). Pre-Kameni and other parts of Santorini were destroyed during the 3.6 ka Minoan eruption, and their fragments were incorporated as lithic clasts in the Minoan pyroclastic deposits. Photo-statistical analysis and granulometry of these lithics, differentiated by lithology, constrain the volume of Pre-Kameni to 2.2–2.5 km3. Applying the Cassignol-Gillot K-Ar dating technique to the most characteristic black glassy andesite lithics, we propose that the island started to grow at 20.2u2009±u20091.0 ka soon after the Cape Riva eruption. This implies a minimum long-term lava extrusion rate of ~0.13–0.14 km3/ky during the growth of Pre-Kameni.