Breccia-Cored Columnar Rosettes in a Rubbly Pahoehoe Lava Flow

Geoscience Frontiers 8 (2017) 1299e1309

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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 during 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. 3e). The boundary Brown et al., 2011). Simple flows dominate the upper parts of the between the FTB upper crust and the columnar tier below is thus sequence and many of them are pahoehoe and rubbly pahoehoe, highly irregular. An outcrop only meters from the outcrops in with some ‘a’a( Bondre et al., 2004; Duraiswami et al., 2008, 2014). Fig. 3cee shows fans of short and thick joint columns diverging and It is generally assumed that during the emplacement and cooling of widening from the FTB upper crust (Fig. 3f). Note that the outcrops thick rubbly pahoehoe flows, their crusts and interiors do not un- shown in Fig. 3 all contain a FTB upper crustal zone in place, and dergo large-scale remixing. However, Sheth et al. (2011) presented fans of the columns diverging from the locally depressed lower field, petrographic and geochemical evidence from some rubbly boundary of that zone, rather than rosettes of joint columns around pahoehoe flows of the Deccan Traps showing that their broken loose and suspended FTB blocks in the flow interior as seen in the flow-top breccia (FTB) crusts became incorporated into the flows’ Deccan examples of Fig. 2. molten interiors where, strongly affecting the internal temperature Observations inside the abandoned quarry also show an up- distribution, they led to radial cooling joint columns growing out- permost zone of FTB, followed downwards by a zone of very chaotic wards from them. To our knowledge, these Deccan “FTB-cored ro- columnar jointing patterns, followed downwards by a massive and settes” (Fig. 2) have not been described from any other flood basalt structureless zone (Fig. 4a). It is hazardous to try to climb to the province, though many examples are known worldwide of upper FTB crust but large blocks of the FTB upper crust, left by the columnar rosettes without obvious cores, such as the “war bonnet” quarrying, are found at the bottom of the quarry near its entrance H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309 1301

Figure 2. (aed) Outcrop photographs from the Koyna quarry at Koynanagar showing the various FTB-cored rosettes in a thick rubbly pahoehoe flow. Note the radial arrangement and gradual widening of columns with distance away from the FTB cores. The rosette in (c) lacks a scale as it is exposed high up on a vertical face, but it is many meters in size. (e) Large FTB-cored rosette in the same flow 32 km away (as identified from the geochemical-isotopic data of Sheth et al., 2011), exposed in a road cut near Sajjangad. Geologists providing a scale are Rudranarayan Chatterjee (a) and Cliff Ollier (b, d, e).

(Fig. 4b). The thick flow core underlying the FTB crust shows a large 3. Samples, petrography, and geochemistry radial jointing structure with no obvious FTB core (circled in Fig. 4c), and adjacent to this, random and widely spaced columns We sampled the FTB upper crust (sample IP1) and an underlying (Fig. 4d). The field appearance of this flow is somewhat reminiscent column (sample IP2) from the outcrop in Fig. 3f. We also sampled of that with the abundant FTB-cored rosettes in the Koyna quarry the FTB core (sample IP3) and one of the attached columns (sample (Fig. 2aed). IP4) from the large landslide boulder shown in Fig. 5c. Inside the At one place along the eastern fault, a large landslide has quarry, we collected a sample from the fresh, fine-grained, non- produced much bouldery rubble including boulders with slicken- vesicular and massive lower part of the flow (sample ELF1), and sides and boulders of FTB upper crust (Fig. 5a). The top of this another (sample ELF2) from a loose block of joint columns on the landslide face shows a FTB inclusion within a complete columnar path outside the quarry that came from a nearby outcrop. Sample rosette (Fig. 5b), whereas a large boulder several meters in size ELF1 thus represents the relatively fresh, massive lower part of the shows a core of FTB upper crust to which columnar lava is welded flow, whereas samples ELF2, IP2 and IP4 represent various on two opposite sides (Fig. 5c). columnar zones in the flow. The base of the flow is marked by a red 1302 H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309

Figure 3. Structures in the Elephanta Island rubbly pahoehoe flow, viewed outside the abandoned quarry and along the eastern fault (Samant et al., 2017). (a) Columns fanning from the contact with the FTB upper crust; Geologist for scale is Keegan Carmo Lobo. A part of the columnar tier is enlarged in (b); Panels (c) to (e), in that order, show outcrops adjacent or only a few meters apart along a south-north direction and at almost the same height; subhorizontal columns (c); subvertical columns (d); FTB upper crust (e), and the contact between FTB crust and the columnar zone (f). Note widening of the columns downwards. bole horizon partly exposed at the quarry entrance, possibly rep- the major and a few trace elements on a SPECTRO ARCOS induc- resenting alteration of the flow’s glassy lower chilled margin as tively coupled plasma atomic emission spectrometer at the So- shown by Duraiswami et al. (2008). The red bole is affected by the phisticated Analytical Instrumentation Facility (SAIF), IIT Bombay. eastern fault and shows well-developed slickensides as reported by Several U. S. Geological Survey rock standards covering a large Samant et al. (2017). compositional range were dissolved along with the samples. The In thin section, sample ELF1 shows a very fine-grained texture standards DNC-1, BIR-1, BCR-2 and BHVO-2 were used for cali- with microphenocrysts of olivine and clinopyroxene (Fig. 6a), brating the instrument, whereas the standard W-2a was analyzed whereas the joint column samples also all show a very fine-grained as an unknown to estimate the analytical accuracy. Loss on igni- groundmass with olivine, plagioclase and clinopyroxene micro- tion (LOI) values were determined by heating the rock powders at phenocrysts (Fig. 6b) which sometimes form clots comprising 1000 C in platinum crucibles, after overnight drying in an oven at many individuals (Fig. 6c). 110 C to drive away adsorbed moisture (H2O ). The geochemical Small, fresh chips of these samples were cleaned in an ultra- data are presented in Table 1, along with the CIPW norms and Mg# sonic bath and ground to powders of <75 mm grain size using a values obtained after adjusting the data on an LOI-free basis with Retsch PM-100 planetary ball mill and stainless steel grinding the program SINCLAS (Verma et al., 2002). The three samples have balls. Solutions of the sample powders were prepared following closely similar compositions. They are moderately evolved with the methods described in Vijayan et al. (2016), and analyzed for MgO contents of 6e7 wt.% and Mg# of 52.3 to 55.3, contain a little H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309 1303

Figure 4. Structures in the Elephanta Island rubbly pahoehoe flow, viewed inside the abandoned quarry. (a) Section through the flow with the FTB crust at the top, a thick zone of random and chaotic jointing below, and massive, largely joint-free lava at the base; (b) a large boulder of FTB upper crust at the entrance to the quarry; (c) a large rosette without any obvious FTB core on the quarry’s northern wall; and (d) large irregular joints on the same wall only a few meters to the east. Preserved FTB upper crust is seen overlying the flow core in (d). The FTB boulder shown in (b) fell from the eastern (right) end of the FTB upper crust seen in (d). or no normative olivine or quartz, and are classified as subalkalic (Fig. 1a) in the northern Deccan. For our samples ELF1 and ELF2, the basalts by SINCLAS. TiO2 vs. Zr/Y plot of Peng et al. (2014), involving three alteration- resistant elements, indicates a character transitional between the 4. Discussion Poladpur and Ambenali formations (both of which are in the upper part of the stratigraphically 3.5 km thick Western Ghats sequence), 4.1. Current understanding but more data including SreNd isotopic data would be needed to establish their exact geochemical correlations (if any) to that The FTB-cored rosettes of Sheth et al. (2011) shown in Fig. 2 are sequence (a topic beyond the scope of this study). in a lava flow of the Ambenali Formation, exposed at Koynanagar The field observations of Sheth et al. (2011) on the Deccan FTB- and near Sajjangad (Fig. 1a) which are 32 km from each other. cored rosettes can be summarized as follows: The flows hosting Another example described by them comes from near Burhanpur these structures are thick (>20 m) and typically chaotically 1304 H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309

Figure 5. (a) Large landslide face in the Elephanta Island rubbly pahoehoe flow along the eastern fault trace. A FTB-cored rosette is seen near the top of the cliff, and is enlarged in (b) where it is seen completely surrounded by lava columns. A large boulder in the talus fallen from the cliff shows a FTB core meters in size, to which columnar lava is welded on opposite sides, as seen in the enlarged view in (c); The features in panels (b) and (c) are both FTB-cored rosettes, and provide unambiguous evidence that loose blocks of the FTB upper crust became incorporated into the molten flow interior, as shown by Sheth et al. (2011) for the other Deccan examples. jointed, and the many FTB-cored rosettes they contain are gener- propagated outwards from them. The “war bonnet” structures of ally unconnected to the upper FTB crust and occur suspended at the Columbia River flood basalts (so named because of their various heights in the flow. The FTB crusts and cores are highly resemblance to the radial feathers on the headdress of a native vesicular (with the vesicles now filled by quartz and zeolites). The American) are large radial jointing structures tens of meters across, basalt lava forming the columns is quite non-vesicular and dense, have no obvious cores, and were interpreted as filled lava tubes by and it was chilled against the FTB core as inferred from its glassy Waters (1960) and Spry (1962), with the radial arrangement of margin. The joint columns, narrow close to the FTB core because of the columns due to the isotherms being concentric around their faster cooling (Grossenbacher and McDuffie, 1995), become pro- centers. Greeley et al. (1998) noted the absence of concentric gressively wider away from the core before finally merging with arrangement of vesicle zones in the war bonnet structures, the the massive flow interior (Sheth et al., 2011). Because joint col- textural uniformity of the columnar basalt with the rest of the flow, umns should develop perpendicular to isotherms (parallel to the and their continuous transition from one to the other with the highest thermal gradient, e.g., Budkewitsch and Robin, 1994; Lyle, columns widening radially outwards (Waters, 1960). Greeley et al. 2000), Sheth et al. (2011) inferred that each FTB core in the Deccan (1998) concluded that the war bonnet structures could not be FTB-cored rosettes had acted as a cold inclusion that strongly lava tubes, and Sheth et al. (2011) concluded the same for the FTB- warped the isotherms around itself, and the columns grew out- cored rosettes. wards from this inclusion. Simple ingress of meteoric water into the flow interior along Sheth et al. (2011) showed that past interpretations of these cooling joints (e.g., Long and Wood, 1986; DeGraff and Aydin, 1987; features as breccia-filled volcanic vents (Agashe and Gupte, 1971) Lyle, 2000) could well explain a highly irregular and uneven and progressively inward-closing lava tubes (Waters, 1960; Spry, boundary between FTB upper crust and the lava interior, and the 1962; Misra, 2002) were untenable. These features are too small resultant distorted isotherms would explain the highly variable to be vents, they are present within individual thick flows, being joint column orientations from near-horizontal to vertical just exposed in vertical sections they would imply horizontal feeder below this boundary as governed everywhere by the local shape of conduits, and the eruptive material they should have produced is the isotherms (Fig. 3). However, this mechanism would not explain missing. Sheth et al. (2011) also pointed out that in a progressively the sizeable (several meters) FTB cores which were undoubtedly cooling lava tube the central core should be the last to solidify, derived from the highly vesicular FTB upper crust of the same flow whereas in the FTB-cored rosettes the cores (the FTB inclusions) as shown by trace element matches and, in particular, Nd isotopic had been cold to begin with, and the columnar jointing had data (Figs. 2 and 5b,c). H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309 1305

Table 1 Major oxide and LOI (wt.%) and trace element (ppm) data and CIPW norms (italics) for the Elephanta Island rubbly pahoehoe lava ﬂow.

Comp. B, sa B, sa B, sa Ref. Meas.

ELF1 ELF2 IP2 W-2a W-2a

SiO2 48.68 47.15 48.12 52.68 53.17 TiO2 2.15 2.14 1.95 1.06 1.11 Al2O3 13.92 13.90 14.74 15.45 15.25 T Fe2O3 14.81 14.55 12.81 10.83 11.36 MnO 0.20 0.19 0.18 0.17 0.17 MgO 6.96 7.11 6.77 6.37 6.80 CaO 11.42 11.51 11.46 10.86 10.94

Na2O 2.31 2.16 2.26 2.20 2.23 K2O 0.21 0.17 0.36 0.63 0.63 P2O5 0.21 0.19 0.18 0.14 0.13 LOI 0.95 1.85 1.34 Total 101.82 100.92 100.17 100.39 101.79 Mg# 52.3 53.3 55.3

Q eee Or 1.25 1.03 2.17 Ab 19.62 18.68 19.56 An 27.10 28.34 29.68 Di 23.42 23.68 22.40 Hy 18.37 15.54 16.53 Ol 2.37 4.83 2.53 Mt 3.29 3.29 2.90 Il 4.10 4.15 3.79 Ap 0.49 0.45 0.43

Sc 36.8 38.5 e 36 36.6 Cr 637 375 e 92 87.7 Ni 114 109 e 70 69.1 Cu 253 215 e 110 107 Sr 210 220 e 190 196 Y 31.0 30.1 e 23 23.4 Zr 136 127 e 100 99.3 Ba 60.8 44.4 e 170 174

Notes: Sample ELF1 represents the massive, non-vesicular lower part of the rubbly pahoehoe ﬂow, and samples ELF2 and IP2 come from columnar zones above, sampled at different locations mentioned in the text. Composition “B, sa” implies þ þ þ subalkalic basalt. Mg# ¼ 100 Mg2 /(Mg2 þ Fe2 ), atomic, assuming 85% of the þ total Fe to be in the Fe2 form. For U.S.G.S. reference material W-2a, the reference (ref.) and measured (meas.) values provide an idea of analytical accuracy. The former values are from Stephen A. Wilson of the U.S. Geological Survey (http://crustal.usgs. gov/geochemical_reference_standards/diabase.html).

into a ﬂow interior as apparently required by the Deccan FTB-cored rosettes.

4.2. A model Figure 6. Photomicrographs of samples from the massive base (a) and columnar zones (b, c) of the Elephanta Island rubbly pahoehoe flow, showing the very fine-grained groundmass and microphenocrysts of clinopyroxene (cpx), olivine (ol) and plagio- We propose a model for the origin of these FTB-cored rosettes in clase (pl), and very rarely Fe-Ti oxide (ox). All photomicrographs were taken under which the incorporation of FTB upper crust into the molten flow crossed polarized light. interior does not require sinking of the FTB crust into the molten interior. We note that pahoehoe flows of all sizes typically grow by Sheth et al. (2011) noted that though liquid basalt is about 10% inflation (e.g., Hon et al., 1994; Self et al., 1997; Reidel, 1998). A key lower in density than solid basalt (Philpotts and Ague, 2009), the aspect of inflation is that initial flow thickness during emplace- highly vesicular and therefore lower-density FTB crust of the rubbly ment is much less than the flow thickness during final solidifica- pahoehoe flows should have floated on the molten interior, rather tion (Fig. 7). A lava lobe (single cooling unit of size ranging from than having sunk into it. Nevertheless, they thought that temporary centimeters to meters) would swell (inflate) as lava that continued gravitational instabilities or even limited lava convection and to be supplied lifted the upper crust (Fig. 7a). A viscoelastic upper crustal overturn (perhaps due to new injection of hotter and crustal layer under the top brittle crustal layer underwent ductile therefore lower-density lava into the flow), may have caused the stretching and accommodated the incoming lava. The lobe, broken FTB crust to sink into the molten interior. Whereas bodily inflating like a water-filled balloon, eventually burst and a new sinking of gravitationally stable crystal mush through the molten lobe emerged and itself grew by inflation, this process repeating as interior of the flow was suggested by Philpotts and Dickson (2002) long as lava continued to be supplied from the vent. If fresh lava for the Holyoke flood basalt flow (representing the 200 Ma Central inputs into the flow lobes were very rapid, the various lobes Atlantic Magmatic Province, CAMP, in the northeastern USA), no would coalesce and a lava flow that was a single thick cooling unit physical explanation has been available for the sinking of FTB crust would form. 1306 H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309

(a) Lobe emplacement

pahoehoe upper crust direction of lava flow advance lobe

horizontal substrate surface 1 m

(b) Slabby pahoehoe formation thickening and fracturing of upper crust flow advance

rapid

inflation 5 m substrate

FTB falling over into front 10 m

rapid inflation continuing and brecciation of pahoehoe slabs substrate

(d) Breakout formation and incorporation of fallen FTB blocks into flow interior flow advance

new crust of breakout, fracturing to slabs (and later brecciating) as breakout inflates; the whole process (a-d) continues many times large surge in lava supply causing breakout 20 m

possible invasive flow behaviour, lifting up FTB blocks substrate

(e) Fully inflated stagnant flow, syn-solidification situation

FTB upper crust

isotherm subvertical subhorizontal thick isotherms 30 m fanning columns columns to form columns columns to form to form to form isotherm

poorly jointed massive base of flow FTB inclusions

(f) Final situation, post-solidification

FTB upper crust

subhorizontal columnar

subvertical 30 m fanning column tier column column set set set

FTB-cored rosettes poorly jointed massive base of flow H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309 1307

Shearing of the viscoelastic crust could occur if the flux of the and the FTB rubble was dispersed into the molten flow interior lava continuing to flow under the crust rose significantly leading to where, controlled by its density and motions in the lava, it rose to rapid flow inflation, or during large surges in the lava supply, as various heights. when lava temporarily ponded in a part of the lava flow was The whole process shown in Fig. 7aed would repeat many suddenly released in the downflow direction (e.g., Peterson and times as a flow continued to advance, supplied by lava from the Tilling, 1980; Hon et al., 1994). Such surges would lead to source area. When the lava supply finally stopped, the flow began stretching and extensional fracturing of the crust (Duraiswami to solidify, and the shape of the isotherms at the crust-core et al., 2003; Kilburn, 2004), forming slabby pahoehoe in which boundary (modified by meteoric water ingress, Long and Wood, the crustal slabs rotated, collided and fractured further as the flow 1986; Philpotts and Dickson, 2002) governed the shapes of the advanced (Fig. 7b). Further rapid inflation would lead to breccia- columnar joint sets as subvertical, subhorizontal or chevron (fan- tion of the pahoehoe slabs, producing flow-top breccia (FTB) and a shaped). The joint columns grew everywhere perpendicular to the continuously thickening lobe of rubbly pahoehoe (Fig. 7c). Slabby local shape of the isotherm (Fig. 7e). For example, where the FTB and rubbly pahoehoe are transitional lava types between pahoe- crust was locally unusually thick and protruded into the flow hoe and ‘a’a, the end member forms of basaltic lava (e.g., interior, the isotherms were locally vertical, leading to sub- Macdonald, 1953; Peterson and Tilling, 1980). Whereas pahoehoe horizontal column sets. A high irregularity in the isotherm shape flows have preserved, unbroken upper crusts, slabby pahoehoe at the crust-core contact would explain the subvertical, sub- and rubbly pahoehoe flows have extensively fractured and brec- horizontal and chevron column sets in immediate lateral juxta- ciated upper crusts (Fig. 7b and c) but with preserved bases position, as seen in the Elephanta Island rubbly pahoehoe flow (Duraiswami et al., 2003, 2008). Transitions from pahoehoe (Fig. 3). Within the flow interior, the isotherms were roughly to slabby pahoehoe and further to rubbly pahoehoe have spherical around the cold FTB blocks, which may also have expe- been documented in the Deccan, Saudi Arabia and Armenia rienced some initial rounding due to limited resorption into the (Duraiswami et al., 2003, 2014; Murcia et al., 2014; Sheth et al., enclosing molten lava (Fig. 7e). However, the roughly spherical 2015). The rubbly pahoehoe flows are in turn transitional to ‘a’a isotherms resulted in columnar joints growing radially away from flows, which contain both flow-top and flow-bottom breccia or the FTB blocks, and the FTB-cored rosettes formed in this way clinker and, being channelized, erode their base (e.g., Macdonald, (Fig. 7f). 1953; Lockwood and Hazlett, 2010). This proposed mechanism of FTB-cored rosette formation In our model, as a rubbly pahoehoe lobe inflated, some of the removes the major problem of how to make low-density FTB upper FTB rubble inevitably fell in front of the lobe and was overridden by crust sink into a molten flow interior. That physically implausible the same lobe when a large surge in lava supply produced a process did not have to occur. The breccia pockets, evidently breakout at the front (Fig. 7d). In this, the rubbly pahoehoe flow derived from the FTB upper crust, were incorporated into the flow showed a behavior akin to that of an ‘a’a flow: the continuous interior in quite a different manner, akin to processes in ‘a’a flows to recycling of flow-top breccia or clinker in an ‘a’a flow, by falling of which rubbly pahoehoe flows are transitional. Indeed, far from upper clinker in the front of a slowly advancing ‘a’a flow, and the sinking into the molten interior from the top, the FTB blocks came overriding of the clinker by the lava flow producing a basal clinker from below and may well have been rising, as expected from their horizon (e.g., Lockwood and Hazlett, 2010), is too well known to be low densities, into the molten interior, until the whole flow began elaborated here. solidifying and the chilling of molten lava at their margins froze We emphasize the significance, for our model, of the fact that them in place. rubbly pahoehoe is transitional to ‘a’a. We imply that some thick What may have limited the degree of shedding of the FTB rubble rubbly pahoehoe flows of the Deccan, during their advance, may in front of the advancing rubbly pahoehoe flows? We believe that it have been periodically dropping some of their upper crustal brec- did become continuous and extensive in some of the flows, which cias in front of them (rather than continuously dropping most or all completed the transition into ‘a’a flows sensu stricto, displaying of those breccias as ‘a’a flows would do). A rubbly pahoehoe flow both upper and bottom FTB zones (Brown et al., 2011; Duraiswami could then override and incorporate its own fallen breccia (Fig. 7c et al., 2014). True ‘a’a flows are uncommon in the Deccan, one of the and d). Note that lava breakouts at the front of an advancing rubbly important reasons being that the very low ground slopes (<1)on pahoehoe flow, as the name suggests, would have to break through which the flows were emplaced would have prevented channeli- the dipping FTB crust at the flow front, and the lava breakout would zation and ‘a’a flow formation (Brown et al., 2011; Duraiswami incorporate some of the FTB rubble through which it emerged, and et al., 2014). Very high eruption rates may have produced ‘a’a additional FTB rubble from the ground. It would in turn rapidly flows directly, as Brown et al. (2011) envisaged. Alternatively, inflate to the same thickness as the main part of the flow behind, greatly increased eruption rates during individual eruptions and developing a crust which underwent fracturing, slab formation and flow emplacement may have pushed some flows over the rubbly then brecciation to produce new FTB. The rapid lava supply means pahoehoe e ‘a’a transition. that the breakout had swollen and merged with the lobe behind it,

Figure 7. (aef) A series of cartoons illustrating the new proposed model for formation of FTB-cored rosettes in rubbly pahoehoe flows. Bright yellow indicates the highest temperature (1200 C), orange somewhat lower temperatures and cooler (but still largely molten) lava, and pinkish red still lower temperatures, whereas shades of gray indicate solidified lava. (a) A small lobe is emplaced and develops an upper crust. This lobe may have been a breakout from an earlier lobe which is not seen itself. (b) The lobe rapidly inflates and its upper crust undergoes stretching and fracturing, producing the slabby pahoehoe lava type. (c) Further rapid inflation leads to brecciation of the slabs, producing flow-top breccia (FTB) and the rubbly pahoehoe lava type. As this lobe inflates, some of the FTB rubble inevitably falls in front of the lobe and is overridden by the same lobe during a breakout (d). The breakout itself rapidly inflates, developing a crust which undergoes fracturing, slab formation and then brecciation to produce new FTB. The entire process (aed) repeats as the flow advances from left to right. (e) The final inflated and largely molten lava flow, with the shape of the isotherms (heavy dashed lines) at the crust- core boundary and within the flow interior deciding the local shapes of the columnar joint sets. The dark gray outlines of the inclusions indicate the glassy rinds of the columnar lava coating them. (f) The completely solidified lava flow with various columnar joint sets and the FTB-cored rosettes in the flow interior. Note continuous thickening of the lava flow throughout (a) to (d); thin-pointed arrows in these panels indicate lava movement. Typical thickness of the lobe or flow at every stage is also shown, though there is a range of thicknesses at which the behavior displayed can be reached. A human figure (1.8 m tall) is added to the lower right corners of panels (e) and (f) so that the reader can easily visualize the flow thickness and the scale of the FTB-cored rosettes. 1308 H. Sheth et al. / Geoscience Frontiers 8 (2017) 1299e1309

5. Conclusions De, A., 1996. Entablature structure in Deccan Trap flows: its nature and probable mode of origin. Gondwana Geological Magazine 2, 439e447. DeGraff, J.M., Aydin, A., 1987. Surface morphology of columnar joints and its sig- Large rosettes (radial columnar jointing structures) with meter- nificance to mechanics and direction of joint growth. Geological Society of size cores of flow-top breccia (FTB) are found in some of the rubbly America Bulletin 99, 605e617. pahoehoe flows of the Deccan Traps. They are thus unlike the Duraiswami, R.A., Dole, G., Bondre, N., 2003. Slabby pahoehoe from the western Deccan volcanic province: evidence for incipient pahoehoee‘a’a transitions. columnar rosettes without breccia cores, described from other Journal of Volcanology and Geothermal Research 121, 195e217. basaltic provinces (e.g., Waters, 1960; Spry, 1962; Scheidegger, Duraiswami, R.A., Bondre, N.R., Managave, S., 2008. Morphology of rubbly 1978) and from the Deccan and Rajahmundry Traps (Fig. 1a; De, pahoehoe (simple) flows from the Deccan volcanic province: implications for style of emplacement. Journal of Volcanology and Geothermal Research 177, 1996; Sen and Sabale, 2011). The FTB cores of the Deccan exam- 822e836. ples, suspended into the flow interior, were undoubtedly derived Duraiswami, R.A., Gadpallu, P., Shaikh, T.N., Cardin, N., 2014. Pahoehoee‘a’a tran- from the FTB upper crust and, being cold, strongly modified the sitions in the lava flow fields of the western Deccan Traps, India e implications for emplacement dynamics, flood basalt architecture and volcanic stratigraphy. isotherms in the interior to roughly concentric shapes around In: Sheth, H.C., Vanderkluysen, L. (Eds.), Flood Basalts of Asia, Journal of Asian them, leading to radial joint columns growing away from them. Earth Sciences 84, pp. 146e166. However, how these FTB blocks from the upper crust may have El Hachimi, H., Youbi, N., Madeira, J., Bensalah, M.K., Martins, L., Mata, J., Medina, P., entered the molten interior below has remained an enigma. They Bertrand, H., Marzoli, A., Munhá, J., Bellieni, G., Mahmoudi, A., Ben Abbou, M., Assafar, H., 2011. Morphology, internal architecture and emplacement mecha- would be buoyant and would not have sunk into the interior owing nisms of lava flows from the Central Atlantic Magmatic Province (CAMP) of to their highly vesiculated nature and low densities. We show that Argana Basin (Morocco). In: Van Hinsbergen, D.J.J., Buiter, S.J.H., Torsvik, T.H., the key to understanding these FTB-cored rosette structures is in Gaina, C., Webb, C.J. (Eds.), The Formation and Evolution of Africa: A Synopsis of 3.8 Ga of Earth History, Geological Society London Special Publications, vol. 357, the inflation process that all pahoehoe (and slabby and rubbly pp. 167e193. pahoehoe) flows undergo, as well as in the transitional nature of Greeley, R., Fagents, S.A., Harris, R.S., Kadel, S.D., Williams, D.A., 1998. Erosion by ‘ ’ fl flowing lava: field evidence. Journal of Geophysical Research 103, rubbly pahoehoe to a a ows which continuously recycle their e fl 27325 27345. upper crusts. Thus, the Deccan rubbly pahoehoe ows in question Grossenbacher, K.A., McDuffie, S.M., 1995. Conductive cooling of lava: columnar may have already begun shedding their FTB in their fronts and, as joint diameter and stria width as functions of cooling rate and thermal gradient. these flows advanced, the FTB rubble was incorporated into the lava Journal of Volcanology and Geothermal Research 69, 95e103. fl Guilbaud, M.-N., Self, S., Thordarson, T., Blake, S., 2005. Morphology, Surface and may actually have been rising into it before in ation and the Structures, and Emplacement of Lavas Produced by Laki, A.D. 1783-84, vol. 396. flow advance stopped, the flow began to solidify, and the cold in- Geological Society of America, Special Papers, pp. 81e102. clusions led to the radial joint column formation around them, as Hon, K., Kauahikaua, J., Denlinger, R., Mackay, K., 1994. Emplacement and inflation of pahoehoe sheet flows e observations and measurements of active lava flows already inferred. Indeed, there seems to be no other way of on Kilauea volcano, Hawaii. Geological Society of America Bulletin 106, explaining the basic field observation that blocks of FTB upper crust 351e370. did get into the molten flow interiors. Keszthelyi, L., 2002. Classification of maficlavaflows from ODP Leg 183. Pro- fi e We would like to emphasize that the FTB-cored rosettes of the ceedings of the Ocean Drilling Program Scienti c Results 183, 1 28. Keszthelyi, L., Thordarson, Th, 2000. Rubbly Pahoehoe: A Previously Undescribed Deccan Traps, illustrated by Sheth et al. (2011) and in the present but Widespread Lava Type Transitional between ‘a’a and Pahoehoe. Meeting of study, remain unique in the world at our current state of knowl- Geological Society of America Abstracts with Programs, vol. 32, p. 7. edge. They have not been described from any other flood basalt Keszthelyi, L., Thordarson, Th, 2001. Rubbly Pahoehoe: Implication for Flood Basalt Eruptions and Their Atmospheric Effects. American Geophysical Union Fall province, due at least partly to exposure situations. However, given Meeting abstract #V52Ae1050. the abundance of rubbly pahoehoe in flood basalts on the terrestrial Keszthelyi, L., Self, S., Thordarson, Th, 1999. Application of recent studies on the fl planets (e.g., Keszthelyi et al., 2006; Duraiswami et al., 2008; emplacement of basaltic lava ows to the Deccan Traps. In: Subbarao, K.V. (Ed.), Deccan Volcanic Province, Geological Society of India Memoris 43, Marshall et al., 2016), we expect that many additional examples pp. 485e520. of FTB-cored rosettes will be discovered, and will match our field Keszthelyi, L., Self, S., Thordarson, Th, 2006. Flood lavas on Earth, Io and Mars. observations and conform to our genetic model. Journal of the Geological Society of London 163, 253e264. Kilburn, C.R.J., 2004. Fracturing as a quantitative indicator of lava flow dynamics. Journal of Volcanology and Geothermal Research 139, 209e224. 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