Joseph Meert

The huge Neoproterozoic glaciations require to find plausible mechanisms that could explain such a cooling as well as the associated carbon isotopic decrease recorded in the carbonates. Although the exact geometry of the continents at this time remains a hard task given the limitations of paleomagnetic data, it is commonly accepted that a supercontinent Rodinia, supposedly formed at 1100 Ma and broke apart at around 800--750 Ma. Apart from the probable increase of organic carbon deposition due to the new created continental margins, we can expect another effect of the dislocation of a supercontinent on the atmospheric CO_2. Indeed, at geological time scale, the atmospheric CO_2 content is regulated by the balance between the main sources, volcanism and metamorphism, and the main sinks, silicate weathering and organic carbon deposition. Moreover, it has been shown that among the factors that modulate the rates of these processes, the paleogeography and the climate are important by their impact on the silicate weathering as it is dependant on the temperature and the runoff. In order to quantify this effect, we have coupled the Climber 2.3 global climate model to the COMBINE global geochemical model. This new approach allows to take into account the spatial variation in longitude and in latitude of the climatic parameters used in the geochemical model, runoff and temperature which are used to calculate the rate of silicate weathering. Our simulations address the issue of the relation between the paleogeographic evolution and the existence of a major glaciation during the Neoproterozoic. Our results show that the effect on the atmospheric CO_2 can be considerable as the transition from a supercontinent to a time of continental dispersion results in a decrease of CO_2 reaching more than 1000 ppm. In order to confirm this preliminary results, we will also vary the outgassing rates of volcanic CO_2 to evaluate the sensitivity of the results to this poorly constrained parameter.

Volcanic eruptions are known to have short-term effects on global cimate through the release of aerosols in the stratosphere. Large volcanic eruptions are thus considered as potential candidates for initiating major climatic and biological crises in the Earth's history. On the other hand, changes in tectonic activity and average volcanism is gen- erally thought as one of the major driving forces of climate change at the geological timescale (> 1 my), through the release of CO2 in the atmosphere and the associ- ated greenhouse warming. At this timescale, the volcanic release of CO2 into the atmosphere-ocean system is balanced by its consumption during silicate weathering followed by carbonate deposition. This equilibrium is reached dynamically through the negative feedback of silicate weathering, as the system evolves towards an hypo- thetic steady state. Thus, in this simplified view of the long-term carbon cycle, vol- canic activity is thought to play a role on the source of CO2, but does not act directly on its sink. This assertion fails to be true in the case of subaerial basaltic volcan- ism, where the eruption not only releases CO2 to the atmosphere, but also produces balsaltic rocks which weather much more rapidly than the average continental crust, enhancing CO2 consumption. As shown recently by some of us (Dessert et al., Earth Planet. Sci. Lett., 188:459-474, 2001), the emplacement of the Deccan basaltic traps at the K-T boundary may have led to a transient increase of atmospheric CO2 over a few hundred thousand years, followed by a drop towards CO2 levels lower, and climate cooler, than prior the emplacement. This trend towards a lower CO2 level is still ef- fective today and will persist until the Deccan traps are completely weathered. Hence, basaltic emplacements appear to act both as short- and long-term climatic factors. The succession of basaltic emplacements which occurred during the Cenozoic may explain at least part of the climatic cooling recorded over the same period. Similarly, the em- placement of the Siberian traps at the Permo-Trias boundary had a profound impact on the Triassic climate. Model simulations are presented to investigate the possibility that sharp cooling or glacial periods may have been produced by the formation of large basaltic provinces, especially if these occur under warm and wet tropical climates.

The configuration of the Precambrian supercontinent Rodinia and the subsequent assembly of Gondwana are under considerable debate due to a paucity of high quality paleomagnetic data. The Indian continent is crucial to this topic and plays an essential role in the history of East Gondwana amalgamation. In proto-Gondwana reconstructions the location of the central Indian craton is important in testing the existence and age of the proposed Rodinia supercontinent. Improved paleomagnetic and geochronologic data collected from numerous dikes in central India will help to better constrain the details of the supercontinent. We have collected samples from 4 late stage mafic dikes that intrude the Jalore Granite in the Malani Igneous Suite (MIS) in Rajasthan, Central India. The MIS is primarily composed of felsic rocks that erupted in initial voluminous flows, which were shortly intruded by granitic plutons. The large (up to 5 m wide) mafic dikes mark the final phase of igneous activity and were the targets of our investigation. Previous age constraints from the Malani suite are either reported as personal communications or are unreliable Rb/Sr dates and do not provide a complete picture of the tectonic evolution of the continent during the Rodinia breakup. We obtained a paleomagnetic direction with declination=349.8° and inclination=64.1° (k=116.44 and α95=11.5°), that overlaps with previously reported results. In addition, fine-grained mafic dikelets show reversed directions with a declination=202° and inclination=-61° (k=58.43 and α95=16.3°) and also record an overprint of normal polarity from the larger dikes. We also report a U/Pb age of 771 ±5 Ma from zircons in the Malani rhyolitic tuff and we are currently in the process of calculating an Ar/Ar whole-rock age. These data combined with a baked contact test truly solidify the paleomagnetic pole, and thus give insight into the complicated Proterozoic and Early Cambrian history of India.

The late Neoproterozoic, c. 550 Ma, marks the estimated final assembly of the supercontinent Gondwana. Paleogeography of major cratons is relatively well established within Gondwana, but individual continental paleolocations prior to final Gondwana suturing are often poorly constrained. It is generally accepted that components of western Gondwana were not a coherent group prior to the late Neoproterozoic, and that the Neoproterozoic assembly of these cratons was complex and polyphase. The amalgamation of eastern Gondwana cratons is more elusive, and is commonly described as one of two contrasting models. Eastern Gondwana cratons either existed as a longstanding stable group throughout the mid to late Proterozoic, or were assembled in a more complex manner in the late Proterozoic with the suturing of Gondwana. Key temporal and spatial constraints on continent locations have crucial implications for testing models of both eastern and western Gondwana amalgamation. New paleomagnetic and geochronologic data from the Malani Igneous Suite (MIS), Rajasthan, Central India, improve the paleogeographic reconstruction of the Indian subcontinent (eastern Gondwana) between dispersal of the supercontinent Rodinia and Neoproterozoic assembly of Gondwana. The MIS comprises a voluminous initial phase of felsic and mafic volcanism followed by granitic plutonism. A zircon U-Pb age on a rhyolitic tuff constrains initial volcanism in the MIS to 771 ±5 Ma. Large (up to 5 m wide) mafic dikes mark the final phase of igneous activity. A virtual geomagnetic pole from 4 mafic dikes has a declination=358.8° and inclination=63.5° (with k=91.2 and α95=9.7). This normal polarity direction includes a fine-grained mafic dikelet that showed a reversed direction with declination=195.3° and inclination=-59.7° (k=234.8 and α95=8.1) and also records an overprint of normal polarity from the larger dikes. The VGP obtained from this study on mafic dikes is combined with previous studies of the Malani suite to obtain a mean paleomagnetic pole of 67.8°N, 72.5°E (A95=8.8). Supported by a tentative baked contact test, we argue that this pole is primary, and permits improved reconstruction of the Indian subcontinent at around 770 Ma. Data on the MIS, and equivalent data on Seychelles at 750 ±3 Ma, are compared with paleomagnetic data on the 755 ±3 Ma Mundine Well dikes in Australia, to indicate a latitudinal separation of nearly 45° between the reconstructed Indian plate and its location in a traditional Gondwana fit. This suggests that East Gondwana was not amalgamated at c. 750 Ma and therefore that these two cratons were assembled later into the Gondwana supercontinent, during the Pan-African c. 550 Ma Kuunga Orogeny.

A paleomagnetic and geochronologic investigation was undertaken on the Dzabkhan volcanics in the Altay region of southwestern Mongolia. Our goal is to define the paleolocation of the Dzabkhan microcontinent during the Neoproterozoic for the higher purpose of addressing the puzzling nature of pre-Phanerozoic glaciations and the tectonic evolution of the Ural-Mongol belt. A total of 184 samples (24 sites) were analyzed from a bimodal suite of volcanic rocks. Alternating field treatments up to 130 mT removed 90% of the natural remanent magnetization (NRM). Thermal demagnetization was applied up to temperatures of 565°C for the mafic samples and up to 680° C for some of the more felsic samples. Based on a preliminary analysis of the data, we identify 3 components of remanent magnetization in the Dzabkhan complex. Present day overprints unblocked at a temperature range of 100-250°C. Other components unblocked at higher temperature ranges of 530-550°C. The first component yielded a direction (in-situ) of 246.1/ -69.5, (k=86, a95=5). This component had a negative fold test. The second component had a shallower NW direction with in-situ mean 282.9/ -42.9 (k=109, a95=5.8). The sites were from a monocline and the tilt-corrected direction is 319/-75.4. The third component (also monoclinally dipping) yielded and in-situ mean of 306.4/24.7, (k=184, a95=5.7) and tilt corrected values 289.9/-59. All three components are high-temperature/high coercivity but a mean of the three components (either in-situ or tilt- corrected) is poorly grouped. A fold test conducted for components 1 and 3 is negative whereas a fold test for components 2 and 3 is positive above the 95% confidence interval. The overall mean for this tilt-corrected direction is 301.7/-69.1 (k=44.6, a95=6.6). This component is similar to, but statistically distinct from components observed in the overlying Tsagaan Oloom and Bayan Gol formations, but still may represent a remagnetization prior to folding. Intraformational conglomerates of the Dzabkhan volcanic clasts yield a low- temperature overprint with a mean direction of 267/-60.8 (similar to the post-folding direction observed in component #1) and random high temperature directions. The prefolding magnetization yielded a paleolatitude of ~53°. The Dzabkhan Formation underlies the diamictites at the base of the Tsagaan Oloom formation. We are in the process of determining an age of the felsic volcanics. Previous ages ranged from 850-700Ma. This new age will also provide some age constraints for the associated overlying diamictite and carbonates. We will discuss these directions and their significance to the paleoposition of the Dzabkhan microcontinent and the overall history of its amalgamation into the Eurasian continent.

The Ural-Mongol fold belt stretches from the North Urals to Kazakhstan and Tien Shan to Altai and Mongolia to the Pacific. This belt comprises many microcontinents with Precambrian crust and numerous island-arc domains separated by ophiolitic sutures. Many scientists acknowledge a very important role of the UMB in formation of Eurasia, and numerous controversial models of its evolution have been proposed. The history of the belt is poorly understood, especially its Neoproterozoic and Early Paleozoic stages. This is largely due to nearly complete lack of reliable age determinations and paleomagnetic data from microcontinents. With this shortcoming in mind, we undertook a geochronological and paleomagnetic study of the Kurgan formation, which is the youngest member of the Neoproterozoic succession in the Lesser Karatau microcontinent in South Kazakhstan. This formation consists of weakly metamorphosed sedimentary rocks, mostly redbeds and silicic tuffs, which are deformed into simple folds without penetrative deformation; with erosional and some angular unconformity, these sediments are overlain by dolomites of Nemakit-Daldynian age and Cambrian to Ordovician limestones. Numerous zircon grains from upper and lower tuffaceous units yielded concordant U-Pb ages of 766.4 ± 3.6 Ma and 831 ± 7.5 Ma respectively. Out of 33 sites sampled, 20 are fully demagnetized up to date. Apart from a low-temperature component aligned along the present-day field, two more components are successfully isolated from most samples. An intermediate-temperature component of ubiquitously reverse polarity is of postfolding origin and was likely to have been acquired in the late Paleozoic. A high-temperature component shows two nearly antipodal groups of directions and statistically significant maximum in grouping upon full unfolding; hence this remanence is of pre-Paleozoic age and likely to be primary. In our presentation, we will discuss different implications of this data on Neoproterozoic paleogeography and evolution of Eurasia.

The Neoproterozoic to early Paleozoic Ural-Mongol belt that runs through Central Asia is crucial for determining the enigmatic amalgamation of microcontinents that make up the Eurasian subcontinent. Two unique models have been proposed for the evolution of Ural-Mongol belt. One involves a complex assemblage of cratonic blocks that have collided and rifted apart during diachronous opening and closing of Neoproterozoic to Devonian aged ocean basins. The opposing model of Sengor and Natal"in proposes a long-standing volcanic arc system that connected Central Asian blocks with the Baltica continent. The Aktau-Mointy and Dzabkhan microcontinents in Kazakhstan and Central Mongolia make up the central section of the Ural-Mongol belt, and both contain glacial sequences characteristic of the hypothesized snowball earth event. These worldwide glaciations are currently under considerable debate, and paleomagnetic data from these microcontients are a useful contribution to the snowball controversy. We have sampled volcanic and sedimentary sequences in Central Mongolia, Kazakhstan and Kyrgyzstan for paleomagnetic and geochronologic study. U-Pb data, 13C curves and abundant fossil records place age constraints on sequences that contain glacial deposits of the hypothesized snowball earth events. Carbonates in the Zavkhan Basin in Mongolia are likely remagnetized, but fossil evidence within the sequence suggests a readjusted age control on two glacial events that were previously labeled as Sturtian and Marinoan. U-Pb ages from both Kazakhstan and Mongolian volcanic sequences imply a similar evolution history of the areas as part of the Ural-Mongol fold belt, and these ages paired with paleomagnetic and 13C records have important tectonic implications. We will present these data in order to place better constraints on the Precambrian to early Paleozoic tectonic evolution of Central Asia and the timing of glacial events recorded in the area.

The assembly of the eastern part of Gondwana (eastern Africa, Arabian-Nubian shield, Seychelles, India, Madagascar, Sri Lanka, East Antarctica and Australia) resulted from a complex series of orogenic events spanning the interval from ~750 Ma to ~530 Ma. Although the assembly of Gondwana is generally discussed in terms of the suturing of East and West Gondwana, such a view oversimplifies the true nature of this spectacular event. Previous models argue for a Mesoproterozoic assembly of East Gondwana coincident with the formation of the Rodinia supercontinent. Here we will present a detailed examination of the geochronologic database from key cratonic elements in eastern Gondwana that suggest a multiphase assembly. Our model precludes a united East Gondwana until early Cambrian time. It is possible to identify at least two main periods of orogenesis within eastern Gondwana. The older orogen resulted from the amalgamation of arc-terranes in the Arabian-Nubian shield region and oblique continent-continent collision between eastern Africa (Kenya-Tanzania and points northward) with an, as of yet, ill-defined collage of continental blocks including parts of Madagascar, Sri Lanka, Seychelles, India and East Antarctica during the interval from ~750 to 620 Ma. This is referred to as the East Africa Orogen in keeping with both the terminology and the focus of the paper by Stern (1994). The second major episode of orogenesis took place between 570 and 530 Ma and resulted from the oblique collision between Australia and an unknown portion of East Antarctica with the elements previously assembled during the East African Orogen. This epidsode is referred to as the Kuunga Orogeny following the suggestion of Meert et al. (1995). Paleomagnetic data are currently too few to provide a rigorous test of this proposal, but the extant data do not conflict with the notion of a polyphase assembly of eastern Gondwana.

The island of Madagascar experienced widespread magmatism at ca. 90 Ma due to its interaction with the Marion hotspot. Previous paleomagnetic data from igneous rocks in the southwestern and northwestern regions of the island indicated that the Marion hotspot has remained fixed for the past 90 Ma. We report paleomagnetic data from northeastern Madagascar (d'Analava Complex). Samples were collected from basalts, rhyolites, gabbros and a dolerite dyke. Sixty samples from 5 sites yield a paleomagnetic pole at 66.7°S, 43.5°E (A95 = 10.7°) and a grand mean pole (GMP) calculated from 10 different studies covering the entire island of Madagascar falls at 68.9°S, 49.0°E (A95 = 4.4°). This pole translates to a paleolatitude for the Volcan de l'Androy (focal point of the hotspot) at 45.2° + 6°/− 5°S compared to the current location of the Marion hotspot at ∼ 46°S. Our results confirm, and expand upon, previous studies that argue for the fixity of the Marion hotspot for the past 90 Ma.

In previous literature, the “Kibara belt” has often been portrayed as a Mesoproterozoic belt trending NE over 1300 km across the Central African Congo craton, from the Angola-Zambia-D.R.Congo border triple-junction in the SW, through Katanga and Kivu-Maniema (DRC), Rwanda and Burundi, up to SW Uganda and NW Tanzania in the NE. However, north of Katanga in the DRC, there is a clear break in continuity of the thus-defined “Kibara belt”, cross-cut by Palaeoproterozoic (Rusizian) terranes, in structural continuity with the NW-SE trending Ubende shear belt further south in Tanzania.In this paper, we redefine the use of the term “Kibara belt” (“KIB”), restricting it henceforward to the belt occurring SW of the Ubende-Rusizian terranes, i.e. in the Kibara Mountains type area of Katanga (DRC). The other belt situated NE of the Ubende-Rusizian terranes and east of the Western Rift, previously referred to as the “Northeastern Kibaran Belt” (NKB), is henceforward and for clarity reasons re-named “Karagwe-Ankole belt” (“KAB”). In our re-definitions, we do not take into account the Kivu-Maniema (DRC), because of its geological complexity apparent from satellite imagery and the lack of recent field data, although “some continuity” of the KAB in Kivu-Maniema is obvious.For the KAB, we document 10 new SHRIMP U–Pb zircon ages, in addition to new 40Ar/39Ar and laser-ablation zircon Hf data, all of them obtained from previously already isotopically “dated” rock specimens. Contrary to previous belief, magmatism in the KAB (and the KIB) is punctuated by the profuse emplacement of bimodal intrusions between 1380 and 1370 Ma. Moreover, the occurrence of Palaeoproterozoic basement within the KAB is confirmed. The prominent c. 1375 Ma bimodal magmatism in the KAB consists of (1) the 350 km long Kabanga-Musongati (KM) alignment of mafic and ultramafic, Bushveld-type, layered complexes, originating from an enriched lithospheric mantle source and (2) voluminous S-type granitoid rocks with accompanying subordinate mafic intrusive rocks. Both coeval magmatic suites are interpreted to have been emplaced under extensional regime in a regional-scale intra-cratonic setting. During ascent the mantle-derived magmas have taken advantage of the regionally occurring crustal-scale zone of weakness in the KAB, i.e. the rheological boundary between the Archaean craton of Tanzania, to the east, and the adjacent Palaeoproterozoic basement (2.1 Ga mobile belt), to the west, both overlain by Mesoproterozoic (meta)sedimentary rocks. The mantle-derived magmas initiated concomittantly and under extension, large-scale crustal melting preferentially of the Palaeoproterozoic basement, and characterised by the absence of a thick lithospheric profile in contrast to the nearby Archaean craton. Such petrogenetic processes have intra-plate characteristics and are thus not associated with normal plate boundary processes nor with their typical magmatism. On the contrary, they may include rift-related packages, characteristically associated with successful or attempted, though unsuccessful, continental break-up as was the case here.In the KAB, later magmatic events occurred respectively at c. 1205 (A-type granitoids) and c. 986 Ma (“tin-granites”). They represent minor additions to the crust.For decades the term “Kibaran” has been used to name the orogenic cycle and/or orogeny occurring in (Central) Africa in “late” Mesoproterozoic times (1.4–1.0 Ga), which was considered to have a protracted character. Here, we propose to restrict henceforward the term “Kibaran” only to the prominent tectono-magmatic “event”, giving rise to the coeval c. 1375 Ma bimodal magmatism emplaced under extensional regime. This “Kibaran event” pre-dates compressional deformation, reflecting far-field effects of global orogenic events, external to the craton and possibly related to Rodinia amalgamation.

Three boreholes, PLTG-1, PLTG-2 and PLTG-3, were drilled in the Platanares, Honduras geothermal system to evaluate the geothermal energy potential of the site. The maximum reservoir temperature was previously estimated at 225–240°C using various types of chemical and isotopic geothermometry. Geothermal gradients of 139–239°C/km, calculated from two segments of the temperature-depth profile for borehole PLTG-2, were used to project a minimum depth to the geothermal reservoir of 1.2–1.7 km. Borehole PLTG-1 exhibited an erratic temperature distribution attributed to fluid movement through a series of isolated horizontal and subhorizontal fractures. The maximum measured temperature in borehole PLTG-1 was 150.4°C, and in PLTG-2 the maximum measured temperature was 104.3°C. PLTG-3 was drilled after this study and the maximum recorded temperature of 165°C is similar to the temperature encountered in PLTG-1.Heat flow values of 392 mWm−2 and 266 mWm−2 represent the first directly-measured heat flow values for Honduras and northen Central America. Radioactive heat generation, based on gamma-ray analyses of uranium, thorium and potassium in five core samples, is less than 2.0 μWm−3 and does not appear to be a major source of the high heat flow. Several authors have proposed a variety of extensional tectonic environments for western Honduras and these heat flow values, along with published estimates of heat flow, are supportive of this type of tectonic regime.

Heat flow values were calculated from direct measurements of temperature and thermal conductivity at thirteen sites in the Arkansas-Missouri Ozark Plateau region. These thirteen values are augmented by 101 estimates of heat flow, based on thermal conductivity measurements and temperature gradients extrapolated from bottom-hole temperatures. The regional heat flow profile ranges from 9 mW m−2 to over 80 mW m−2, but at least two distinct thermal regimes have been identified. Seven new heat flow determinations are combined with three previously published values for the St. Francois Mountains (SFM), a Precambrian exposure of granitic and rhyolitic basement rocks, average 47 mW m−2. Radioactive heat production of 76 samples of the exposed rocks in the SFM averages 2.4 μW m−2 and a typical continental basement contribution of 14 mW m−2 is implied. Conversely, the sedimentary rock sequence of the plateau is characterized by an anomalously low heat flow, averaging approximately 27 mW m−2. Groundwater transmissivity values that are based on data from 153 wells in deep regional aquifers demonstrate an inverse relationship to the observed heat flow patterns. The areas of high transmissivity that correspond to areas of low total heat flux suggest that the non-conservative vertical heat flow within the Ozark sedimentary sequence can be attributed to the effects of groundwater flow.

The Nyanzian System lavas of western Kenya are believed to be the oldest rocks of the Tanzanian Craton. Intrusive age relationships suggest an age ⩾2850 Ma although direct attempts at dating the Nyanzian have produced disparate results. Our study involves a suite of samples collected from the Nyanzian basalts, pillow basalts, andesites and rhyolites from sixteen sites in western Kenya. These rocks yield a tilt-corrected paleomagnetic pole at 14°N, 150°E (K=59, dp=5°, dm=7°). This pole is constrained to be older than the first (D1) deformation (>2472±30 Ma) by positive fold, conglomerate and reversal tests. Analysis of the paleomagnetic data base for three African cratonic nuclei (Tanzanian, Kaapvaal/Zimbabwe and West Africa) for the time period from 2.0 Ga to 3.0 Ga demonstrates a paucity of well-dated poles, although there are several poles from the Kaapvaal/Zimbabwe and Tanzanian Cratons which allow “spot-readings” of their relative positions. We demonstrate, based on these data, that the Kaapvaal/Zimbabwe and Tanzanian Cratons were drifting independently at ∼ 2875 Ma, ∼ 2700 Ma and ∼ 2450 Ma. This independent motion of the Tanzanian and Kaapvaal/Zimbabwe Cratons indicates that previously proposed models involving African cratonic coherence can no longer be considered valid for the time period from 2850 to 2500 Ma.

The utility of paleomagnetic data gleaned from the Bhander and Rewa Groups of the “Purana-aged” Vindhyanchal Basin has been hampered by the poor age control associated with these units. Ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources, including 87Sr/86Sr and δ13C correlations with the global curves and Ediacara-like fossil finds in the Lakheri–Bhander limestone. New analyses of the available paleomagnetic data collected from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone and sandstones from the Marwar SuperGroup suggest that the Upper Vindhyan sequence may be up to 500 Ma older than is commonly thought. Paleomagnetic analysis generated from the Bhander and Rewa Groups yields a paleomagnetic pole at 44°N, 214.0°E (A95 = 4.3°). This paleomagnetic pole closely resembles the VGP from the well-dated Majhgawan intrusion (36.8°N, 212.5°E, α95 = 15.3°).Detrital zircon analysis of the Upper Bhander sandstone identifies a youngest age population at ∼1020 Ma. A comparison between the previously correlated Upper Bhander sandstone and the Marwar sandstone detrital suites shows virtually no similarities in the youngest detrital suite sampled. The main 840–920 Ma peak is absent in the Upper Bhander. This supports our assertion that the Upper Bhander is older than the 750–771 Ma Malani sequence, and is likely close to the age of the 1073 Ma Majhgawan kimberlite on the basis of the paleomagnetic similarities. By setting the age of the Upper Vindhyan at 1000–1070 Ma, several intriguing possibilities arise. The Bhander–Rewa paleomagnetic pole allows for a reconstruction of India at 1000–1070 Ma that overlaps with the 1073 ± 13.7 Majhgawan kimberlite VGP. Comparisons between the composite Upper Vindhyan pole (43.9°N, 210.2°E, α95 = 12.2°) and the Australian 1071 ± 8 Ma Bangamall Basin sills and the ∼1070 Ma Alcurra dykes suggest that Australia and India were not adjacent at this time period.

Palaeomagnetic data are used to study the configurations of continents during the Proterozoic. Applying stringent reliability criteria, the positions of the continents at 12 times in the 2.45- to 1.00-Ga period have been constructed. The continents lie predominantly in low to intermediate latitudes. The sedimentological indicators of palaeoclimate are generally consistent with the palaeomagnetic latitudes, with the exception of the Early Proterozoic, when low latitude glaciations took place on several continents.The Proterozoic continental configurations are generally in agreement with current geological models of the evolution of the continents. The data suggest that three large continental landmasses existed during the Proterozoic. The oldest one is the Neoarchaean Kenorland, which comprised at least Laurentia, Baltica, Australia and the Kalahari craton. The protracted breakup of Kenorland during the 2.45- to 2.10-Ga interval is manifested by mafic dykes and sedimentary rift-basins on many continents. The second ‘supercontinental’ landmass is Hudsonland (also known as Columbia). On the basis of purely palaeomagnetic data, this supercontinent consisted of Laurentia, Baltica, Ukraine, Amazonia and Australia and perhaps also Siberia, North China and Kalahari. Hudsonland existed from 1.83 to ca. 1.50–1.25 Ga. The youngest assembly is the Neoproterozoic supercontinent of Rodinia, which was formed by continent–continent collisions during ∼1.10–1.00 Ga and which involved most of the continents. A new model for its assembly and configuration is presented, which suggests that multiple Grenvillian age collisions took place during 1.10–1.00 Ga. The configurations of Kenorland, Hudsonland and Rodinia depart from each other and also from the Pangaea assembly. The tectonic styles of their amalgamation are also different reflecting probable changes in sizes and thicknesses of the cratonic blocks as well as changes in the thermal conditions of the mantle through time.

Palaeomagnetists' basic assumption that Earth's magnetic field is a GAD, that is, a geocentric axial dipole, has been challenged by anomalous magnetic data from ancient Canadian basalts. At a closer look, fast continental drift could explain this anomaly.

The Central Asian Orogenic Belt (CAOB) is widely recognized as a locus of Asia's main growth during the Neoproterozoic–Paleozoic, but its evolution remains controversial. The views on the most enigmatic, late Neoproterozoic to Cambrian, stages are critically dependent on the origin and subsequent kinematics of numerous microcontinents that comprise the structure of Kazakhstan, Tien Shan, Altai and Mongolia.We report new paleomagnetic data and U–Pb zircon ages from Neoproterozoic volcano-sedimentary rocks from the Lesser Karatau block in central Kazakhstan. The laser ablation U–Pb age of felsic tuff of the Kurgan Fm. is 766 ± 7 Ma. Thermal demagnetization revealed that most studied samples retained a dual-polarity pre-tilting component whose primary origin is supported by a conglomerate test. According these paleomagnetic data, the Lesser Karatau microcontinent was located at a paleolatitude of 34.2 ± 5.3°, N or S, at about 770 Ma. There is only one additional CAOB microcontinent, the Baydaric microcontinent in central Mongolia, for which reliable paleomagnetic data indicate a paleolatitude of 47 ± 14°, N or S, at about 770–805 Ma (Levashova et al., 2010).Several lines of evidence favor the view that the above CAOB microcontinents were originally parts of two larger domains, thus allowing extrapolation of the above paleomagnetic data to much larger territories, the Kazakhstan and Mongol domains that, in turn, might have belonged to major cratonic areas. A comparison of our paleomagnetic data with those from the larger cratonic nuclei provides first-order constraints on the origins of the CAOB microcontinents. We compare the existing tectonostratigraphic correlations between the Neoproterozoic to early Paleozoic sections of the microcontinents with coeval sections on the margins of Tarim, Australia, South China, Siberia, and North China. This combined analysis excludes a southern hemispheric location for the CAOB microcontinents at 750–800 Ma. Of the several cratons that were located in the northern hemisphere at that time we favor a hypothesis that the Kazakhstan and Mongol domains had originally belonged either to Tarim or South China.

The Precambrian geologic history of Peninsular India covers nearly 3.0 billion years of time. India is presently attached to the Eurasian continent although it remains (for now) a separate plate. It comprises several cratonic nuclei namely, Aravalli–Bundelkhand, Eastern Dharwar, Western Dharwar, Bastar and Singhbhum Cratons along with the Southern Granulite Province. Cratonization of India was polyphase, but a stable configuration between the major elements was largely complete by 2.5 Ga. Each of the major cratons was intruded by various age granitoids, mafic dykes and ultramafic bodies throughout the Proterozoic. The Vindhyan, Chhattisgarh, Cuddapah, Pranhita–Godavari, Indravati, Bhima–Kaladgi, Kurnool and Marwar basins are the major Meso to Neoproterozoic sedimentary repositories. In this paper we review the major tectonic and igneous events that led to the formation of Peninsular India and provide an up to date geochronologic summary of the Precambrian. India is thought to have played a role in a number of supercontinental cycles including (from oldest to youngest) Ur, Columbia, Rodinia, Gondwana and Pangea. This paper gives an overview of the deep history of Peninsular India as an introduction to this special TOIS volume.

The Cambrian explosion, c. 530–515 Ma heralded the arrival of a diverse assembly of multicellular life including the first hard-shelled organisms. Fossils found in Cambrian strata represent the ancestors of most modern animal phyla. In contrast to the apparent explosiveness seen in the Cambrian fossil record, studies of molecular biology hint that the diversification observed in Cambrian strata was rooted in ancestry extending back into the Ediacaran (635–542 Ma). Fossil evidence for this mostly cryptic phase of evolution is derived from the soft-bodied fossils of the Ediacaran biota found throughout the world and bilaterian embryos found in the Doushantuo lagerstätte in South China. The first appearance of Ediacara fauna is thought to have followed the last of the ~ 750–635 Ma Neoproterozoic glacial episodes by 20–30 million years. In this paper, we present evidence for the oldest discovery of the ‘Ediacara’ discoidal fossils Nimbia occlusa and Aspidella terranovica (?) that predate the early Cryogenian glaciations by more than fifty million years. There is considerable disagreement over the significance of discoidal Ediacaran fossils, but our findings may support earlier suggestions that metazoan life has roots extending deeper into the Proterozoic Eon. We also confirm the presence of a Late Cryogenian (e.g. “Marinoan”) glaciation on the Lesser Karatau microcontinent including dropstones and striated clasts within the glacial strata.►Late Cryogenian glaciation documented at Lesser Karatau, Kazakhstan. ►Ediacara-like discoidal fossils found in a ~ 770 Ma siliclastic bed below the glacial. ►Ages of volcanic sequences on microcontinents in Central Asia supports correlation.

We report the results of a preliminary paleomagnetic study on well-dated 1.9 Ga dykes from the Bastar craton, India. This suite of NW–SE trending dykes was linked to similarly-aged magmatic activity in the Dharwar craton and Cuddapah basin in India as part of a large igneous province. This igneous activity may have extended across many cratons in the “Columbia” supercontinent including the North China craton, Laurentia, Baltica, Australia, Siberia and the Kaapvaal and Zimbabwe cratons (southern Africa). The Bastar dykes along with the Cuddapah traps and dyke yield a dual-polarity magnetization with D = 126°, I = + 15.2° (k = 27, α95 = 11.9°) and a corresponding paleomagnetic pole at 31° N, 330° E (dp = 6.3°, dm = 12.2°). The relatively robust paleomagnetic and geochronologic database at 1.85–1.90 Ga allow us to test one of the configurations of the supercontinent “Columbia”. There are some critical differences between our paleomagnetically-based reconstruction and the archetypal and geologically-based “Columbia” configuration of Zhao et al. (2004). Most notably, Siberia, India and Australia cannot be linked to Laurentia in the archetypal Columbia fit. Proposed links between western Australia and the Kaapvaal and Zimbabwe cratons are possible as is the fit between Baltica and Laurentia. The robust paleomagnetic data reported in this paper require that either the Columbia supercontinent did not exist at 1.9 Ga or requires major modification. Given that our data provide only a snapshot on the Paleoproterozoic, we conclude that the Columbia supercontinent remains a viable possibility although relationships between individual elements should be re-evaluated as more data become available.► Present new paleomagnetic data from 1.9 Ga dykes from the Bastar craton. ► Test proposed Columbia supercontinental configuration. ► Position India at low latitudes in the Paleoproterozoic.

The Precambrian history of the Earth is punctuated by a number of supercontinental assemblies and their disintegration. New paleomagnetic and geochronologic results from the Dharwar, Bundelkhand and Marwar cratons of the Indian subcontinent are presented here in an attempt to constrain the paleogeographic position of India within various proposed Precambrian supercontinents.Our paleomagnetic results from the Paleoproterozoic Gwalior traps of the Bundelkhand craton, all of a single polarity, yielded a combined tilt-corrected mean declination = 73.9° and an inclination of +4.4° (k = 22, α95 = 11.2°). The paleomagnetic pole was calculated using a site location of 26°N, 78°E and is located at 15.4°N, 173.2°E.The U–Pb isotopic studies on the zircons obtained from the alkaline mafic dyke sample from Anantapur dyke swarm of the Dharwar craton, southern India, yielded a concordant age of 1027.2 ± 13 Ma (2σ; MSWD = 5.0). An overall mean of our paleomagnetic studies combined with previously published results yielded a VGP at 10°N and 211°E with a mean declination = 65° and inclination = −57° (k = 31, α95 = 10).In an effort to constrain the lower age limit of the Malani Igneous Suite (MIS) we report new U–Pb isotopic ages for the Harsani granodiorite. The granodiorite forms the basement for the Malani igneous province in NW India. The zircon U–Pb analyses from Harsani granodiorite yielded and age of 827.0 ± 8.8 Ma that we interpret as the age of intrusion and the 786.4 ± 5.6 Ma may relate to a disturbance marking onset of Malani volcanism.Along with these new data, we also review the paleomagnetic results from our previous studies on the Harohalli alkaline dykes, Upper Vindhyan sequence, Majhgawan kimberlite, and a widespread paleomagnetic overprint that we interpret to be of ∼580 Ma in an attempt to constrain the paleogeography of the Indian subcontinent from 1.8 Ga to 580 Ma.

Since 1959 when Harland and Bidgood [l] first suggested that Neoproterozoic glaciation occurred in low palaeolatitudes, palaeomagnetic research into Neoproterozoic glaciation has led to a major palaeoclimatic paradox. Early palaeomagnetic studies of Neoproterozoic glaciogenic rocks in Norway, Greenland, Scotland and Canada in general suggested low palaeolatitudes of glaciation, although some of the data were equivocal or contentious. In 1980, when assessing the first two decades of palaeomagnetic work on Neoproterozoic ...

The Nama Group of southern Namibia is a candidate for the Terminal Proterozoic Global Stratotype Section and Point (GSSP). Desirable characteristics of a GSSP include a well-preserved index-fossil assemblage, little deformation or metamorphism. well-constrained isotopic ages, stable-isotope records and magnetostratigraphic control. The age of the Nama Group sediments is now constrained to between 570 and 510 Ma. Assuming the Gondwana assembly was nearly complete at this same time, there is a discrepancy between the previously published Nama poles, a revised 550-510 Ma apparent polar wander path for Gondwana and the preceding supercontinental assemblages of Rodinia and Panottia. For these reasons, the Nama Group sediments were resampled in an effort to evaluate the potential of detailing the magnetostratigraphy of the Nama Group and resolving the discrepancy between the Nama poles and the APWP of Gondwana. Collectively, both the previous studies of the Nama Group and this one show a complex series of overprints and no easily discernible primary direction of magnetization. We therefore urge caution in using the Nama Group poles in any tectonic models of the Neoproterozoic-Early Palaeozoic. Specifically, the N1 component of magnetization, previously identified as a primary magnetization, was discovered in a younger suite of samples. Therefore, previous tectonic models that used the N1 magnetization direction as representative of the time of Nama deposition should be revised in light of these recent findings.

New paleomagnetic and geochronologic data from the Malani Igneous Suite (MIS) in Rajasthan, northwest India, improve the paleogeographic reconstruction of the Indian subcontinent between dispersal of the Mesoproterozoic supercontinent Rodinia and late Neoproterozoic assembly of Gondwana. The MIS comprises voluminous phases of felsic and volumetrically insignificant mafic volcanism followed by granitic plutonism. Large (up to 5 m wide) felsic and mafic dikes represent the terminal phase of magmatism. A zircon U–Pb age on a rhyolitic tuff constrains the initial volcanism in the MIS to 771 ± 5 Ma. A paleomagnetic direction obtained from four mafic dikes has a declination = 358.8° and inclination = 63.5° (with κ = 91.2 and α95 = 9.7). It overlaps with previously reported results from felsic MIS rocks. This direction includes a fine-grained mafic dikelet that showed a reversed direction with declination = 195.3° and inclination = −59.7° (κ = 234.8 and α95 = 8.1°) and also records an overprint of normal polarity from the larger dikes. The VGP obtained from this study on mafic dikes is combined with previous studies of the Malani suite to obtain a mean paleomagnetic pole of 67.8°N, 72.5°E (A95 = 8.8°). Supported by a tentative baked contact test, we argue that this pole is primary, and permits improved reconstruction of the Indian subcontinent for 771–750 Ma. Data from the MIS and equivalent data from the Seychelles at 750 ± 3 Ma are compared with paleomagnetic data from the 755 ± 3 Ma Mundine Well dikes in Australia to indicate a latitudinal separation of nearly 25° between the Indian and Australian plates. These suggest that East Gondwana was not amalgamated at ca. 750 Ma and therefore these two cratonic blocks were assembled later into the Gondwana supercontinent, during the ca. 550 Ma Kuunga Orogeny.

Since the late 1980s, it has been hypothesized that the wide range of apparent argon ages seen within single K-feldspar samples might be due to a distribution of diffusion domain sizes within the mineral. To test and apply this idea, an analytical technique that combines conventional laboratory degassing experiments (resistance heating) with numerical inversion procedures has been developed to extract cooling history information from feldspars. A key part of the method involves careful control of temperature in the laboratory to constrain the diffusion parameters of the feldspar samples. In our study, we have K-feldspar data from single crystals that mimic the types of data seen in classic resistance heater fusion experiments. Our step-heating data are based on using a continuous argon-ion laser with no direct control on temperature. However, with only a single added free parameter in the model, we show that it is possible to analyze this data in the multi-domain style, and make some simple inferences on the nature of the cooling history of the Carion pluton in central Madagascar. The Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP dating of the pluton yields an emplacement age of 532.1 ± 5.2 Ma followed by relatively slow cooling as constrained by 40Ar/39Ar ages on hornblende, biotite and K-feldspar. Four hornblende samples yielded a mean 40Ar/39Ar age of 512.7 ± 2.6 Ma. A biotite sample yielded an age of 478.9 ± 1.0 Ma and modeled K-feldspar ages show cooling from 350° C at 466 Ma to 100° C by 410 Ma. Collectively, the data suggest that the pluton cooled from 850° C at 532.1 ± 5.2 (U-Pb zircon) Ma to 500° C at 512.7 ± 2.6 Ma (40Ar/39Ar hornblende), or approximately 18 °C/Ma slowing to ∼4 °C/Ma between 512 Ma and 478 Ma and finally to about 3°C/Ma between 478 and 410 Ma.

The paleomagnetic assumption that the Earth’s magnetic field is reduced to a geocentric axial dipole (GAD) when sufficiently sampled has been called into question for Mesozoic and earlier times. It has been suggested, for example, that modest contributions from axial quadrupolar (10%) and octupolar (25%) fields are resolvable using inclination only data from paleomagnetic studies. The underlying assumption in inclination only studies is that considerable continental drift has occurred over a sufficiently long period of time to render paleomagnetic sampling random in a paleogeographic sense. This assumption was stated in all previous studies dating back to 1976, but was never tested. We have developed a random walk model designed to test this assumption. Our model uses three different configurations for the continents in the random walk and allows the user to vary parameters such as maximum velocity, sampling distribution, sampling frequency and frequency of directional change. The model generates large sample sizes that cannot be adequately evaluated using the standard χ2 statistical test and therefore we introduce two statistical parameters used in structural equation models. Our models indicate that the ‘random paleogeographic sampling’ assumption used in the previous studies is not valid due primarily to the lack of an adequate sample size and temporal distribution. We show, for example, that even the most robust dataset compiled in 1998 is severely undersampled. A series of model runs on a GAD earth with sampling over a 600 Myr period demonstrates that detailed sampling will, on average, produce a GAD-like distribution only 30% of the time. Other model runs demonstrate that inadequate sampling can produce false quadrupolar and octupolar effects. It is our conclusion that time-averaged inclination only studies using the extant paleomagnetic database should be viewed with extreme caution.

A paleomagnetic investigation of the Alnø carbonatite complex dikes was undertaken in an attempt to refine the apparent polar wander path for Baltica. The currently available paleomagnetic database for the Ediacaran–Cambrian segments of the Baltica APWP is marked by disparate (and often supposedly) coeval poles. This has led to remarkable conclusions about rapid rates of drift or true polar wander. Our study shows that the Alnø Complex (584 ± 7 Ma) is coeval with the Fen Carbonatite Complex (583 ± 15 Ma) via 40Ar/39Ar dating of biotite and K-feldspar. We identify three components of remanent magnetization in the Alnø Complex. The first is a low unblocking/coercivity component with a mean declination of 51.2° and inclination of +70.2° (k = 22, a95 = 8.3°). A paleopole calculated from this direction falls at 62.7°N, 101°E. The high coercivity and unblocking components show a large spread in both declination and inclination. A somewhat artificial grouping of shallow–intermediate vectors (inclinations less than 60°) yields a mean direction with a declination of 108.1° and an inclination of 10.5° (k = 5.3, a95 = 32.1°). This direction is statistically indistinguishable from that obtained by Piper [Piper, J.D.A., 1981. Magnetic properties of the Alnøn complex. Geol. Foren. Stock. Forhandlingar, 103, 9–15 (Part 1)] and yields a paleomagnetic pole at 3.5°N, 269°E. Conversely, the remaining high unblocking/coercivity components (inclinations > 60°) yields a second grouping with a mean declination of 28° and inclination of 76.8° (k = 24; a95 = 16.1°; Fig. 5E). This direction is indistinguishable from the present Earth's field at the site. We conclude that the Alnø Complex poles (and indeed many of the Ediacaran poles for Baltica) should be viewed with scepticism when used for paleogeographic/geodynamic models.