Polarity inversion in the Rajmahal lavas, north-east India: trap emplacement near commencement of the Cretaceous Normal Superchron

Abstract

S U M M A R YTo evaluate suggestions from earlier work that a reversal of magnetization is present in the Rajmahal Traps of mid-Cretaceous (probable Aptian) age in north-eastern India, we have sampled a 140m thick section comprising a minimum of nine flow units at 25 sites in the north-western part of the outcrop. A coherent N+R+N magnetostratigraphy is identified, with evidence for transitional behaviour a t the top and bottom of the succession.The mean characteristic remanence direction of the reversed sites (D/I = 145.7"/63.5", ay5 = 6.0") is approximately antiparallel to that of the normal polarity sites (D/I = 300.2"/-55.7", ay5 = 12.4"), and the group mean confirms a palaeomagnetic pole position near 125"E, 9"s (Ay5= 10.5") for the time of eruption of the traps. From 4oAr/39Ar evidence that the Rajmahal-Bengal Traps were emplaced over a short interval a t ca. 117 Ma, we interpret the brief reversed episode identified near the base of the succession as a record of the ISEA event. The polarity evidence therefore supports the view that the Rajmahal Traps were fed by the Crozet Hotspot a t the beginning of the Cretaceous Normal Superchron (-119-83 Ma).

Frequent Polarity Reversals in the Cretaceous Normal Superchron

“…Meanwhile, 40 Ar/ 39 Ar dating on the basalt at the base of DSDP site 879 below the ISEA subchron in the Takyo‐Daisan Guyot of the NW Pacific Ocean indicated an age younger than ∼118 Ma for the ISEA event (Pringle & Duncan, 1995; Figure 3q); 40 Ar/ 39 Ar dating on the basalt flows in the westernmost Tarim Basin yielded a reversed‐polarity interval at 113.3 ± 1.6 Ma (Gilder et al., 2003; Figure 3r), which is likely correlated with the upper one in DSDP site 402 (Figure 3f). Paleomagnetic data from the Sanbaoying andesitic basalt (Shi et al., 2004) and the andesite lava sequence in the Dalinghe Formation in NE China (Zhu et al., 2004) both revealed a reversal event occurring at an 40 Ar/ 39 Ar age of ∼116.0 Ma (Figure 3s and 3t); Whereas the Rajmahal Traps basalt lavas in NE India that emplaced during a short period of ∼2 Ma at ∼117 Ma (Baksi, 1995), its reversed event was correlated to the ISEA (Das et al., 1996; Rao & Rao, 1996; Figure 3u and 3v). Hence, apart from the ISEA event at ∼116.0 Ma, there is another much younger reversal occurred at ∼96 Ma and a potential one at ∼113.3 Ma.…”

“…Numerous numerical dynamo simulations have predicted that the nonreversing magnetic field during the superchron tends to have smaller PSVs and higher dipole moments than those in intervals with frequent reversals (Aubert et al., 2010; Cox, 1968; Glatzmaier et al., 1999; McFadden et al., 1991; Tarduno et al., 2001), which partly conflicts with the above mentioned diverging views. One possible interpretation is that, due to the limitation of model assumptions, the geomagnetic field has remained exceptionally stable throughout the CNS (Glatzmaier et al., 1999), which has been challenged by some reports that have indicated the existence of short episodes of polarity reversals, such as in the early (e.g., Das et al., 1996; Gilder et al., 2003; Lowrie et al., 1980; Tarduno, 1990; VandenBerg et al., 1978; Zhu et al., 2004), middle (Benammi et al., 2006), and late (Archibald & Irving, 1990; Fang et al., 1989; He et al., 2012; Kollofrath, 2012; Shcherbakova et al., 2012) CNS. In addition, despite the disputes on whether these observed reversals were polarity events, their occurrence times and frequencies are also a subject of considerable debate due to uncertain paleomagnetic records and age models (Cronin et al., 2001).…”

Detrital zircon U-Pb ages of the Cretaceous Dadaepo Formation in Busan, Korea and their geological implication

“…Paleomagnetically, most of the Cretaceous Period is known as the Cretaceous Normal Superchron (CNS: ~126-84 Ma; Gradstein et al, 2012;Helsley and Steiner, 1969). However, even during the CNS, the presence of short reverse polarities at various times in the early (e.g., VandenBerg et al, 1978;Lowrie et al, 1980;Tarduno, 1990;Das et al, 1996;Gilder et al, 2003;Zhu et al, 2004), middle (Benammi et al, 2006), and late (Fang et al, 1989;Archibald and Irving, 1990;He et al, 2012;Kollofrath, 2012;Shcherbakova et al, 2012) periods has been consistently confirmed. In addition, only two samples among many taken vertically at a narrow interval from the DF recorded reverse polarities (Kim, 1988).…”

Plate Tectonics

“…The late Tertiary portion of this model was not well constrained by radiometric ages, based on the lack of published age dates for the post-30 Ma portion of most hot spot tracks involved at that time. Paleopoles for India from the Rajmahal Traps (Das et al, 1996;Rao and Rao, 1996) result in a paleolatitude of the traps at their time of formation ($117 Ma, Baksi, 1995) at 47 S (AE400 km), whereas the Mü ller et al (1993) model places them at about 40 S (AE400 km). Therefore, the absolute motion of the Indian, Australian, and Antarctic Plates relative to the mantle has to be computed by plate circuit closure for these times.…”