Geoscience Frontiers 8 (2017) 1299e1309 HOSTED BY Contents lists available at ScienceDirect China University of Geosciences (Beijing) Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf Research Paper Breccia-cored columnar rosettes in a rubbly pahoehoe lava flow, Elephanta Island, Deccan Traps, and a model for their origin Hetu Sheth a,*, Ishita Pal a,b, Vanit Patel a, Hrishikesh Samant c, Joseph D’Souza a a Department of Earth Sciences, Indian Institute of Technology Bombay (IITB), Powai, Mumbai 400076, India b Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0225, USA c Department of Geology, St. Xavier’s College, Mumbai 400001, India article info abstract Article history: Rubbly pahoehoe lava flows are abundant in many continental flood basalts including the Deccan Traps. Received 4 October 2016 However, structures with radial joint columns surrounding cores of flow-top breccia (FTB), reported from Received in revised form some Deccan rubbly pahoehoe flows, are yet unknown from other basaltic provinces. A previous study of 6 December 2016 these Deccan “breccia-cored columnar rosettes” ruled out explanations such as volcanic vents and lava Accepted 18 December 2016 tubes, and showed that the radial joint columns had grown outwards from cold FTB inclusions incor- Available online 12 January 2017 Handling Editor: S. Glorie porated into the hot molten interiors. How the highly vesicular (thus low-density) FTB blocks might have sunk into the flow interiors has remained a puzzle. Here we describe a new example of a Deccan rubbly fl Keywords: pahoehoe ow with FTB-cored rosettes, from Elephanta Island in the Mumbai harbor. Noting that (1) fl fl Rubbly pahoehoe thick rubbly pahoehoe ows probably form by rapid in ation (involving many lava injections into a Columnar jointing largely molten advancing flow), and (2) such flows are transitional to ‘a’a flows (which continuously shed Flow-top breccia their top clinker in front of them as they advance), we propose a model for the FTB-cored rosettes. We Volcanism suggest that the Deccan flows under study were shedding some of their FTB in front of them as they Flood basalt advanced and, with high-eruption rate lava injection and inflation, frontal breakouts would incorporate Deccan Traps this FTB rubble, with thickening of the flow carrying the rubble into the flow interior. This implies that, far from sinking into the molten interior, the FTB blocks may have been rising, until lava supply and inflation stopped, the flow began solidifying, and joint columns developed outward from each cold FTB inclusion as already inferred, forming the FTB-cored rosettes. Those rubbly pahoehoe flows which began recycling most of their FTB became the ‘a’a flows of the Deccan. Ó 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). 1. Introduction Hart, 2008; Reidel et al., 2013), the PlioceneePleistocene South Caucasus flood basalts (Sheth et al., 2015), <4500 yr old flood Many flood basalt provinces of the world contain abundant and basalt fields in Saudi Arabia (Murcia et al., 2014), and the voluminous lava flows of rubbly pahoehoe, i.e., flows with 1783e1784 Laki eruption in Iceland (Guilbaud et al., 2005). Rubbly extensively brecciated upper crusts (Keszthelyi and Thordarson, pahoehoe lavas are also recognized on the planet Mars (Keszthelyi 2000, 2001). Rubbly pahoehoe flows have been reported from et al., 2006). Given the significant environmental impact of the the mid-Cambrian Kalkarindji flood basalts (Marshall et al., 2016), historical Laki eruption (Guilbaud et al., 2005), and noting that the Triassic/Jurassic Boundary CAMP flood basalts (El Hachimi many prehistoric flood basalt events closely correlate with bio- et al., 2011), the Early Cretaceous Kerguelen oceanic plateau logical mass extinctions (e.g., Rampino and Stothers, 1988; (Keszthelyi, 2002), the Late CretaceousePalaeocene Deccan Traps Wignall, 2001), topics such as the physical emplacement of flood (Duraiswami et al., 2008), the Miocene Columbia River flood ba- basalt lava flows, their emplacement duration, and volatile release salts (e.g., Swanson and Wright, 1981; Self et al., 1997; Bondre and are of major interest (e.g., Self et al., 1997, 2014; Parisio et al., 2016). The Deccan Traps currently occupy w500,000 km2 in western and central India, and in the Western Ghats escarpment (Fig. 1a) * þ Corresponding author. Fax: 91 22 25723480. they attain a stratigraphic thickness of w3.4 km over a w500 km E-mail address: [email protected] (H. Sheth). Peer-review under responsibility of China University of Geosciences (Beijing). distance (e.g., Beane et al., 1986). Walker (1971) described many http://dx.doi.org/10.1016/j.gsf.2016.12.004 1674-9871/Ó 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1300 H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309 o structures of the Columbia River flood basalt province (Waters, 73 00' E a 1960; Spry, 1962; Scheidegger, 1978; De, 1996). A persisting problem, however, is how blocks of the FTB forming KACHCHH N the upper crust were incorporated into the molten flow interiors. SAURASHTRA Sheth et al. (2011) thought that pieces and blocks of the FTB upper Toranmal crust (highly vesiculated and therefore low-density) might have Burhanpur sunk into the molten interior owing to temporary gravity in- Dhule stabilities, or perhaps limited lava convection and crustal overturn, Igatpuri but noted that these explanations were unsatisfactory. In this pa- o per, we describe a new occurrence of FTB-cored rosettes in a rubbly 19 Mumbai & Ahmednagar 00' Elephanta pahoehoe flow on the island of Elephanta in the Mumbai harbor Pune fi N Mahabaleshwar INDIA (Fig. 1a and b), in the western Deccan Traps. We provide eld, ARABIAN Sajjangad petrographic and geochemical data on this flow, and present a Koynanagar Rajahmundry model for the FTB-cored rosettes that is based on well-understood SEA Traps mechanisms of emplacement of such flows and does not require WGE Belgaum 200 km sinking of blocks of the FTB upper crust into the flow interior. 2. Geology of Elephanta Island and its rubbly pahoehoe flow E72 o 56’ 06” E72o 57’ 18” b Elephanta Island in the Mumbai harbor rises 168 m above sea o jetty (Mumbai boats) N18 k Nhava level and is covered in large part with dense jungle (Fig. 1b). It is e e 58’12” r however well known for the ca. mid-6th century A.D. Hindu rock- C cut caves, a World Heritage Site of the UNESCO (The United Na- a 168 m v fi a tions Educational, Scienti c and Cultural Organization) since 1987. h N N port The caves are carved into small-scale (Hawaiian-size) compound pahoehoe flows (in the terminology of Walker, 1971). A detailed Elephanta description of these compound flows can be found in Sheth et al. (in 1 km press). The lava flows dip westenorthwest by w12 due to the abandoned Panvel flexure, a late-stage tectonic megastructure along the N18 o quarry Sheva western Indian rifted margin (Sheth, 1998; Samant et al., 2017). The 57’ 18” southeastern part of the island (Fig. 1b) exposes a 40 m thick lava area of exposures of flow of rubbly pahoehoe which underlies the compound flows of rubbly pahoehoe flow the Elephanta Caves. The rubbly pahoehoe flow was quarried dur- ing the early to mid-70th to provide construction material for the Figure 1. (a) Sketch-map of the Deccan Traps (shaded) showing the Western Ghats e escarpment (WGE, heavy dashed line) and the region with abundant rubbly pahoehoe then upcoming major port of Nhava Sheva 1 km east of the island flows documented (enclosed within the thin line, Duraiswami et al., 2008). Some (Fig. 1b), but the quarrying was stopped in a few years as it was important localities exposing these flows (Duraiswami et al., 2008; Sheth et al., 2011) found detrimental to the historical monument. The rubbly pahoe- fl are marked. Rubbly pahoehoe ows are also found in Saurashtra in the northwestern hoe flow is traversed by two subparallel, oblique-slip normal faults Deccan Traps (R. Duraiswami and H. Sheth, unpubl. data). (b) Google Earth image of Elephanta Island in the Mumbai harbor, with parts of NhavaeSheva port on the Indian with well-developed slickensides and easterly downthrows mainland immediately to the east. Box with black boundary shows the area of present (Samant et al., 2017). study. Observations of the rubbly pahoehoe flow made on the eastern fault surface, outside the abandoned quarry, show an upper tier of joint columns that dip in various orientations, and are overlain by individual lava flows of the Deccan Traps as “compound”, made up FTB upper crust, whereas the lower part of the flow is massive and of numerous constituent flow units or lobes. He described other structureless (Fig. 3). Fans of well-developed, long and slender, Deccan flows, which are single, thick and areally extensive, typi- subvertical columns are seen in outcrops along the trace of the cally columnar-jointed flow units, as “simple” flows. Compound eastern fault (Fig. 3a and b). When followed southwards, almost flows (dominantly pahoehoe, with minor ‘a’a) are abundant in the horizontal columns are seen (Fig. 3c) in immediate lateral contact lower part of the Western Ghats stratigraphic sequence (Walker, with subvertical columns (Fig. 3d), which are immediately juxta- 1971; Keszthelyi et al., 1999; Bondre et al., 2004; Sheth, 2006; posed against a meters-thick FTB crust (Fig.