Abstract A relative palaeointensity determination was obtained using the pseudo-Thellier technique on sediment from Ther, Tirna Basin, Latu-Osmanabad District, Maharashtra, India. The stability of the natural remanent magnetization was investigated by alternating field (AF) demagnetization. Rock magnetic studies suggest that the main carriers of magnetization are ferrimagnetic minerals, predominantly pseudo-single-domain magnetite. To varying degrees, the smoothed palaeoinclination and palaeodeclination patterns of the Tirna Basin are similar to other Asian palaeosecular variation records CALS3k.4, CALS10k.1 and SED3k.1, with an age offset. Measurements of intensity of the natural remanent magnetization left after AF demagnetization v. intensity of anhysteric remanent magnetization gained at the same peak were carried out on a set of samples. A jackknife re-sampling scheme provides error estimates for the palaeointensity. A good agreement pattern can be observed between the Tirna Basin relative palaeointensity proxy and other global curves with an age shift. Although some temporal offsets of palaeointensity features between different records have been recognized, their similar shape suggests that the palaeointensity can give a globally coherent signal and may be used as a relative dating technique. For the first time, relative palaeointensity data for the past 2 kyr from India is presented here, which complement the existing archaeological records but with an additional input.

ABSTRACT We conducted a detailed rock magnetic and mineralogical study of bole beds from the Deccan magmatic province, India. Magnetic susceptibility of 15 bole beds showed two contrasting patterns, with susceptibility values either increasing or decreasing up the profile. We then focused on two representatives red boles located in the Western Ghats, the RBB and RBAN profiles, to unravel the nature and origin of these contrasting magnetic susceptibility patterns. The presence of smectite argues against significant secondary thermal alterations. Major-elemental compositions obtained by X-ray fluorescence spectrometry of RBB and RBAN red boles are comparable to the parent basalt and show significant and typical depletion of mobile elements such as sodium and calcium compared to the parent basalt. The Ti/Al ratio of both the red boles and their overlying clay layers is close to the typical value of Deccan basalt (0.2), suggesting that the material of the red boles has been derived from weathering of the parent basalt. The chemical index of alteration varies from 40–50 in the parent basalt to 80–90 at the top of the bole beds, consistent with moderate to intense weathering of the bole beds. However, similar to other Deccan bole beds, indices of lateritization below 50 suggest that the state of lateritization has not been reached. Although the RBB and RBAN profiles share similar mineralogical signatures, their magnetic mineral assemblages are distinctly different. In the RBB profile, magnetic susceptibility decreases up-profile as a result of oxidation/dissolution of primary titanomagnetite inherited from the parent basalt, with subsequent formation of pedogenic hematite and superparamagnetic particles. In contrast, magnetic susceptibility in the RBAN profile, which contains magnetite, some hematite, and goethite, increases up-profile. The increase in the magnetic signal is mainly due to the increasing amounts of phyllosilicate and goethite, while the content of magnetite and hematite remains constant along the profile. We attribute the variation in the magnetic mineral assemblage to contrasting humid and dry environments during weathering, leading to the preferential formation of goethite or hematite, respectively. The combined mineralogical and rock magnetic data suggest the existence of a single weathering profile involving soil formation in the two studied red boles, with few or no contributions from an external source.

Abstract Understanding the Deccan Trap Large Igneous Province in western India is important for deciphering the India–Seychelles rifting mechanism. This book presents 13 studies that address the development of this province from diverse perspectives including field structural geology, geochemistry, analytical modelling, geomorphology and geophysics (e.g., palaeomagnetism, gravity and magnetic anomalies, and seismic imaging). Together, these papers indicate that the tectonics of Deccan is much more complicated than previously thought. Key findings include: the Deccan province can be divided into several blocks; the existence of a rift-induced palaeo-slope; constraints on the eruption period; rift–drift transition mechanisms determined for magma-rich systems; the tectonic role of the Deccan or Réunion plumes; sub-surface structures reported from boreholes; the delineation of the crust–mantle structure; the documentation of sub-surface tectonic boundaries; post-Deccan-Trap basin inversion; deformed dykes around Mumbai, and also from the eastern part of the Deccan Traps, documented in the field.

The seismic structure of the transition-zone discontinuities was studied beneath the forty-nine hotspot locations of the catalog of Courtillot et al. (2003) , using a global data set of SS precursors. Some of these hotspots are proposed to originate from plumes rising in the upper mantle or from the core-mantle boundary region. I found thin transition zones in approximately two-thirds of the twenty-six hotspot locations for which precursor observations could be made. This observation agrees with the expectation for the olivine phase transition of a systematically thin transition zone in high-temperature regions. Other hotspot locations showed a clear deepening of both the 410- and 660-km discontinuities, which is consistent with a phase transition from majorite garnet to perovskite at a depth of 660 km. Predictions from mineral physics suggest that this transition is more important than the olivine phase transition in regions with high mantle temperatures. So, a hotspot location with a deep 410-km discontinuity in combination with either a shallow or deep 660-km discontinuity might be consistent with hot upwellings rising from the lower into the upper mantle. Hotspot locations with a shallow 410-km discontinuity are not in agreement with a positive thermal anomaly from the surface down to the mantle transition zone. This new interpretation of seismic discontinuities in the transition zone has important implications for our understanding of geodynamics in potential mantle plume locations.

Alteration of undisturbed igneous material used for argon dating work often results in inaccurate estimates of the crystallization age. A new quantitative technique to detect alteration has been developed (see Baksi, this volume , Chapter 15), utilizing the 36 Ar levels observed in rocks and minerals. The method is applied to data in the literature for rocks linked to hotspot activity. For subaerial rocks, argon dating results are critically examined for the Deccan Traps, India. The duration of volcanic activity and its coincidence in time with the K-T boundary are shown to be uncertain. The bulk of dated seafloor material (recovered from the Atlantic, Indian, and Pacific oceans) proves to be altered. Ages determined using large (hundreds of milligram) samples are generally unreliable, due to inclusion of altered phases. Such analyses include studies suggesting an age of ca. 43 Ma for the bend in the Hawaiian-Emperor chain. More recent attempts, using much smaller sub-samples (∼10 mg) that have been acid leached to remove alteration products, are generally of higher reliability. Plagioclase separates sometimes yield reliable results. However, many whole-rock basalts from the ocean floor yield ages that are, at best, minimum estimates of the time of crystallization. Most “rates of motions,” calculated from hotspot track ages, are shown to be invalid. Seafloor rocks are recovered at considerable expense but often are not suitable for dating by the argon methods. Most are severely altered by prolonged contact with sea-water. A method is recommended for testing silicate phases prior to attempts at argon dating. This involves a quantitative determination of the 36 Ar content of the material at hand; dating phases without pretreatment—leaching with HNO 3 for material containing ferromagnesian phases, and HF for feldspars—is strongly discouraged.

Evolution of sedimentary basins took place in the Barmer, Jaisalmer, and Bikaner regions during K-T (Cretaceous–Tertiary) time in western Rajasthan, India. These intracratonic rift basins developed under an extensional tectonic regime from Early Jurassic to Tertiary time. Rift evolution resulted in alkaline magmatism at the rift margins. This magmatism is dated at 68.5 Ma and has been considered an early phase of Deccan volcanism. Deccan volcanism, sedimentary basin development, and the alkaline magmatism of western Rajasthan have thus been considered the products of Réunion plume activity. However, sedimentary basin evolution began in western Ra-jasthan prior to Deccan volcanism and K-T alkaline magmatism. Gondwanaland fragmentation during the Mesozoic caused the development of the rift basins of Gujarat and western Rajasthan. This resulted in the opening of the Jurassic rift system and mildly alkaline magmatism at ca. 120 Ma in western India. This event was pre-K-T, and plume activity has not hypothesized for it. Continental fragmentation under an extensional tectonic regime during K-T time resulted in the magmatism and basin tectonism in western Rajasthan. Crustal development during the K-T period in western Rajasthan resulted from an extensional tectonic regime and is not the manifestation of Réunion plume activity.

From the mantle plume model it would be expected that one to a few kilometers of regional, domal lithospheric uplift occurred 5–20 m.y. before the onset of flood basalt volcanism. This uplift resulted from heat conduction out of and dynamic support by the hot, buoyant, rising plume head. Field evidence for such uplift would comprise sedimentary sequences that reflect progressive basin shallowing before volcanism or (in the case of differential uplift along faults) widespread conglomerates derived from the basement rocks and underlying the first lavas. Local uplifts and subsidences cannot be used to invoke or rule out plume-caused uplift. Over large areas of the Late Cretaceous Deccan flood basalt province, the base of the lava pile is in the subsurface. Basalt-basement contacts are observed along the periphery of the province and in central India (the Satpura and Vindhya ranges), where substantial post-Deccan uplift is evident. Here, extensive horizontal Deccan basalt flows directly overlie extensive low-relief planation surfaces cut on various older rocks (Archean through Mesozoic) with different internal structures. Locally, thin, patchy Late Cretaceous clays and limestones (the Lameta Formation) separate the basalts and basement, but some Lameta sediments are known to have been derived from already erupted Deccan basalt flows in nearby areas. Thus, the eruption and flowage of the earliest Deccan basalt lava flows onto extensive flat planation surfaces developed on varied bedrock, and the nearly total absence of basement-derived conglomerates at the base of the lava pile throughout the province, are evidence against prevolcanic lithospheric uplift (both regional and local), and thereby the plume head model. There has been major (∼1 km) post-Deccan, Neogene uplift of the Indian peninsula and the Sahyadri (Western Ghats) Range, which runs along the entire western Indian rifted margin, well beyond the Deccan basalt cover. This uplift has raised the regional Late Cretaceous lateritized surface developed on the Deccan lava pile to a high elevation. This uplift cannot reflect Deccan-related magmatic underplating, but is partly denudational, is aided by a compressive stress regime throughout India since the India-Asia collision, and is possibly also related to active eastward flow of the sublithospheric mantle. The easterly drainage of the Indian peninsula, speculated to be dome-flank drainage caused by the plume head, predates the uplift. Field evidence from the Deccan and India is in conflict with a model of plume-caused regional uplift a few million years before the onset of volcanism.