Correlation of the Deccan and Rajahmundry Trap lavas: Are these the longest and largest lava flows on Earth?

Abstract

We propose that the Rajahmundry Trap lavas, found near the east coast of peninsular India, are remnants of the longest lava flows yet recognized on Earth (∼ 1000 km long). These outlying Deccan-like lavas are shown to belong to the main Deccan Traps. Several previous studies have already suggested this correlation, but have not demonstrated it categorically. The exposed Rajahmundry lavas are interpreted to be the distal parts of two very-large-volume pāhoehoe flow fields, one each from the Ambenali and Mahabaleshwar Formations of the Wai Sub-group in the Deccan Basalt Group. Eruptive conditions required to emplace such long flows are met by plausible values for cooling and eruption rates, and this is shown by applying a model for the formation of inflated pāhoehoe sheet flow lobes. The model predicts flow lobe thicknesses similar to those observed in the Rajahmundry lavas. For the last 400 km of flow, the lava flows were confined to the pre-existing Krishna valley drainage system that existed in the basement beyond the edge of the gradually expanding Deccan lava field, allowing the flows to extend across the subcontinent to the eastern margin where they were emplaced into a littoral and/or shallow marine environment. These lavas and other individual flow fields in the Wai Sub-group may exceed eruptive volumes of 5000 km³, which would place them amongst the largest magnitude effusive eruptive units yet known.

We suggest that the length of flood basalt lava flows on Earth is restricted mainly by the size of land masses and topography. In the case of the Rajahmundry lavas, the flows reached estuaries and the sea, where their advance was perhaps effectively terminated by cooling and/or disruption. However, it is only during large igneous province basaltic volcanism that such huge volumes of lava are erupted in single events, and when the magma supply rate is sufficiently high and maintained to allow the formation of very long lava flows. The Rajahmundry lava fields were emplaced around 65 Ma during the later times of Deccan volcanism, probably just after the K/T environmental crisis. However, many lava-forming eruptions of similar magnitude and style straddled the K/T boundary.

Introduction

An important question about subaerial large igneous provinces (LIPs) is whether such widespread lava accumulations contain individual eruptive units (lava flow fields) that are also very extensive and, by implication, of very large volume. The relationship has been established for the relatively small Columbia River LIP (Tolan et al., 1989), where flood lava flows of the Columbia River Basalt Group (CRBG) ∼ 600 km in length (Hooper 1997) and up to 5000 km³ in volume (Reidel et al., 1989) have been described, but there is little information by which to examine the size of lava flows in older, major LIPS. We address this question by re-examining the long-suggested possibility that some outlying Deccan-like lavas near the east coast of peninsular India are the distal ends of lava flow fields belonging to the main Deccan lava series, and that these flow fields must therefore exceed 1000 km in length. A consequence of the great length of these lava flows would be that the parent pāhoehoe flow fields are of very large volume, and that they constitute the longest and, possibly, the largest lava flows yet identified on Earth. Lava flows up to 1000 km long have been reported from other planets (e.g., Mars, Venus) and conditions for forming such flows have been modelled (e.g., Zimbelman, 1998, Keszthelyi et al., 2000, and references therein).

The lavas highlighted here are the Rajahmundry Traps on the central east coast of India and their possible correlatives in the Western Ghats and southeastern region of the main Deccan province (Fig. 1). We bring new evidence to bear on the similarity of the main province lavas with the Rajahmundry Trap (RT) lavas. The idea that the RT lavas belong to the main Deccan series is not new, and was based upon the similar composition and apparent age of the RT lavas to the Deccan lavas. First Blandford (1867) and later Venkayya (1949) promulgated the idea, as discussed by Baksi et al. (1994) and references therein (see also Courtillot and Renne, 2003), but the suggestion has so far been without convincing proof, although Baksi, 2001, Baksi, 2005) draws this conclusion. The discussion developed in this paper is based upon published information on the RT lavas, with the addition of new samples of the Rajahmundry and main lavas of the SE and western Deccan collected during fieldwork by J. Cripps (with MW; Cripps, 2002) and A.E. Jay (with SS and MW). Geochemical data on the RT samples (J. Cripps and A.E. Jay, unpublished data 2003) and geochemical and paleomagnetic data on the main Deccan lavas (Jay, 2005) have been obtained. We believe that a sufficient body of evidence can now be amassed to demonstrate that the two lava groups are correlative. Moreover, we can indicate the specific parts of the main Deccan stratigraphy that correlate to these outlying lava flows, and these are in broad agreement with Baksi, 2001, Baksi, 2005) suggestions.

This paper introduces the problem, giving pertinent details on main Deccan and RT lavas, and then discusses the basis for the correlation of the main series and RT lavas. We also briefly describe the area of the main Deccan province where these lava flows-fields might have fed into valley-confined pathways. Application of a model for the transport of lava over very long distances (after Keszthelyi and Self, 1998), appropriate for RT lava conditions, completes the picture and supports the hypothesis that the RT lavas are the distal parts of very long pāhoehoe flows derived from the main Deccan province. We conclude with a discussion of plausible lengths and volumes of basaltic flood lavas on Earth.

A few terms are used to describe the constituent parts of basaltic pāhoehoe lava flows such as those that dominate the Deccan (Bondre et al., 2004): lava flow-field, lava flows, and lava lobes. These are adopted from Self et al. (1997) and have been recently defined by Guilbaud et al. (2005). Lava is used generically to describe the basaltic rock. The term lava flow-field designates the entire lava products of one prolonged eruption. We believe that eruptions in active flood basalt provinces were of long duration (perhaps, c. 10–50 years) and formed very complex and widespread lava layers (Thordarson and Self, 1998, Jerram and Widdowson, 2005). The dimensions of typical Deccan lava flow fields are not currently known. A lava flow is a lava body formed by an individual outpouring of lava from the vent and corresponds, ideally, to a single episode of magma effusion during an eruption. Flow fields are composed of many lava flows, which rarely form a single stretch of lava but consist of a succession of lobes that represent the sequential emplacement of the lava flow (see Hon et al., 1994). These lava lobes can occur on several scales. For instance, kilometre-scale lobes can be found to include cm to m-scale lava lobes in the field. One common type in flood basalt provinces is sheet lobes, which are typically several kilometres in lateral dimensions and tens of metres thick.