Evidence for simple volcanic rifting not complex subduction initiation in the Laxmi Basin

Download PDF Matters Arising Open access Published: 01 June 2020 Evidence for simple volcanic rifting not complex subduction initiation in the Laxmi Basin Peter D. Clift  ORCID:

orcid.org/0000-0001-6660-63881, Gérôme Calvès2 & Tara N. Jonell  ORCID: orcid.org/0000-0002-6811-38163  Nature Communications volume 11, Article number: 2733 (2020) Cite this article

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Subjects GeochemistryGeophysicsTectonics Matters Arising to this article was published on 01 June 2020

The Original Article was published on 21 May 2019

Arising from Pandey et al. Nature Communications https://doi.org/10.1038/s41467-019-10227-8 (2019)

In their recent paper Pandey et al.1 use geochemical data from an International Ocean Discovery Program (IODP) site in the Laxmi Basin to propose a previously unrecognized proto-subduction

zone in the eastern Arabian Sea in the region of Laxmi Basin (LB), offshore western India, during the Late Cretaceous-Early Cenozoic. A short section of basaltic basement (~16 m penetration)

was recovered at Site U1457 on the eastern flank of Laxmi Ridge (LR) (Fig. 1). The geochemistry of the volcanic rocks exhibited subduction-related characters, such as relative Nb and Ta

depletions, while Nd and Sr isotopes indicate some involvement of continental material during petrogenesis, interpreted as the incorporation of subducted sediment within mantle melts. On the

basis of these data this study proposes a significant revision of the Cretaceous-Cenozoic plate motions involving complex shearing and rotation of blocks during the opening of the Indian

Ocean. However, we here argue that the primary lines of evidence used in this study are misinterpreted and that other data sets in the region, not accounted for by these authors, render this

new model untenable.

a Shaded bathymetric map of the Arabian Sea and surrounding regions, showing the location of IODP Site U1457, proposed continent-ocean boundaries (yellow dashed lines), the extent of known

seaward dipping reflectors from Calvès et al4. Magnetic anomalies (thin grey numbered lines) and transform fault (dashed white lines). SDR seaward-dipping reflector, SR Somnath Ridge, SH

Saurashtra High, GR Gop Rift, LR Laxmi Ridge, LB Laxmi Basin, RS Raman Seamount. b Seismic reflection profile across LR and LB showing presence of SDRs, position of ocean-continent boundary

(OCB) and the Deccan Large Igneous Province (DLIP). c) Close-up of LR showing east-dipping SDRs in α and β.

Exactly where a LB proto-trench would have been located is not clear, nor how the LR, presumably representing a proto-forearc massif, was involved. In contrast, most studies consider the LR

to be a rifted continental fragment derived from the Indian margin. In the model proposed by Pandey et al.1 the LR is required to be oceanic crust. Pandey et al.1 improperly cite the study

by Krishna et al.2 who interpret the LR to be stretched and thinned continental lithosphere. A continental origin for the LR, albeit with magmatic underplating related to excess melting

above a plume prior to opening of the LB, was also inferred from seismic refraction data3. The new model1 further ignores the fact that the LR is part of a volcanic margin sequence running

south from two well-defined volcanic ridges, the Somnath Ridge and the Saurashtra High, which together form a pre‐Deccan continental flood basalt province (∼75–65.5 Myr ago)4. Indeed, Calvès

et al.4 report the presence of seaward-dipping reflectors on either side of LB that represent subaerial volcanic flows erupted during the initial extension of the basin (Fig. 1b, c). The

presence of seaward-dipping reflectors is incompatible with the presence of a proto-subduction zone but is a classic signature of the more widely accepted volcanic rifted margin setting5.

The shallower than normal oceanic crust depths to basement in Laxmi Basin (2.5–2.7 km sediment unloaded modern depths in the region of the drilling sites6 compared with 5.2 km for 66 Myr old

normal crust7) does not imply presence of subduction-related crust but simply continental crust that has not been extended to the degree required for seafloor spreading to occur (i.e., the

crust is thicker than normal oceanic crust and composed of less dense material). Indeed, a rifted origin was proposed by the first two authors of the Pandey et al.1 study based on simple

flexural extension models6 and despite the fact that it is incompatible with the subduction model they concurrently advocate in this second study. This more conventional approach to the

origin of LB is moreover predicted by restoration of the flat-topped Raman Seamount at 66 Myr ago to close to sealevel. This prediction is consistent with the seamount morphology, implying a

wave-cut top. A typical thermal subsidence model does a good job of matching the vertical tectonic constraints. Similarly, subsidence analysis of the LR shows a thermal subsidence history

compatible with regular oceanic crust, albeit one affected by dynamic support from the Réunion plume in the early stages of basin rifting8. If this region is really an extinct forearc massif

then this match would be purely coincidental.

The most important line of evidence supporting the Pandey et al.1 model is the subduction signature of the volcanic basement. Although lavas erupted in the earliest stages of subduction show

very little subduction imprint9, the same is true of rocks from mid-ocean ridge or rifted margin settings. Many Tethyan passive margin volcanic rocks contain hints of a subduction-linked

petrogenesis but are not generated above a subduction zone10. The same can be said of the late Oligocene to Recent rift volcanic rocks of the western United States11 where subduction

signatures are proposed to reflect earlier, pre-rift enrichment and modification of the lithospheric mantle. These melts are then remobilized during rifting.

We use multi-element spider diagrams (Fig. 2) normalized to normal mid ocean basalt (N-MORB) values to highlight the fact that water-mobile elements, such as U, Pb and Ba are strongly

enriched compared to water-immobile elements, although Sr is slightly depleted and K and Rb are close to N-MORB values. Enrichment in water mobile elements is a feature of subduction

magmatism but may also indicate alteration. Lack of enrichment in some water-mobile elements is not typical of subduction-related magmatism. If Sr concentrations are affected by alteration

then Sr isotopic values cannot be considered original and magmatic. Alteration would make the Laxmi volcanic rocks shift towards an appearance of subduction influence in the Pandey et al.’s

εNd vs 87Sr/86Sr isotope plot and away from the original MORB-like composition. More detailed discussion of the isotopes was not possible because the data were not made available on request.

Furthermore, subduction magmatism is usually associated with relative depletions of Nb and Ta relative to other water-immobile elements, such as La and Ce, yet these depletions are not

observed (Fig. 2a). Indeed, Ta is relatively enriched. We note that the positive relative Zr and Hf enrichments in the Laxmi Basin volcanic rocks are commonly recorded in Mariana forearc

boninites9, but are not recorded in forearc basalts of the type favored by Pandey et al.

a N-MORB normalized multi-element spider diagrams for the LB lavas. Water-mobile elements are plotted left of Ba, with immobile elements on the right side of the plot starting at Nb.

Compatibility in mantle phases increases to the right within the immobile elements. N-MORB values are from Gale et al.15. b Nb/Yb–Th/Yb discrimination diagram16 highlights variable input

from subduction, AFC processes, inheritance, and mantle source enrichment that can displace rock compositions from and along the MORB-OIB array. Data were filtered prior to plotting to

remove fractionated samples with >55 wt% SiO2, significant crystallized oxides (Ti/Ti* < 0.85) and strong crustal contamination (Th/Nb > 0.2) following Pearce et al.16. Open symbols identify

LB lavas and IBM boninites that do not meet these requirements. Analytical uncertainties are within the size of the plotted symbols. Negative Nb anomalies, common in rocks formed in

subduction zones, result in compositions to be displaced sub-vertically from MORB. Many basalts unrelated to subduction can carry apparent subduction signatures as a result of crustal

assimilation and inheritance and may similarly plot in the volcanic arc array. c The Nb/Yb–TiO2/Yb diagram16 can further help discriminate intra-plate from plate margin basalts as it is less

sensitive to crustal input. Volcanic rifted margins, LB lavas and the Réunion plume plot along a steep diagonal from the OIB to MORB array. Increased extension and attenuation of

continental lithosphere drives compositions from the OIB to MORB field as depth of melting shallows. Subduction-influenced rocks, such as basalts and boninites from the IBM, that involve

higher degrees of fluid-flux melting under MORB-like temperatures, remain and evolve along the MORB array. Field labels from Pearce et al.16. Data compiled from GEOROC, IBM9, and Nauret et

al.17. Alk alkaline; BABB back-arc basin, B-FAB boninitic forearc basalt, Bon boninite, BTP British Tertiary Province, E enriched type, E-MORB Enriched Mid-Ocean Ridge Basalt, FAB forearc,

IBM Izu-Bonin-Mariana, LB Laxmi basin, MORB mid-ocean ridge basalt, N normal type, N. Atlantic North Atlantic margin, OIB Ocean Island Basalt. Data plotted in these figures are provided in

Supplementary Data 1 and from the Mendeley databank at https://doi.org/10.17632/723c5d795r.1.

The most relevant comparison with the LR are the rifted volcanic margins of the North Atlantic. Ocean Drilling Program (ODP) Site 917 offshore SE Greenland penetrated a > 900-m-thick

sequence of volcanic rocks erupted during initial break-up, prior to the onset of seafloor spreading. The lower and middle lava sequences at Site 917 have clear subduction signatures that

reflect remobilization of older melts, rather than nascent subduction initiating in the North Atlantic at that time12 (Fig. 2b). Likewise, volcanic rocks of the British Tertiary Province on

the conjugate margin also show a subduction-type trend on discrimination diagrams (Fig. 2b, c). In this context the continental signature inferred from Sr isotope geochemistry of the LB

samples would not represent sediment subduction but simply contamination of mantle melts rising through modified, extended continental lithosphere prior to eruption.

To generate melt within a subduction zone requires significant downflexing of hydrated material into the mantle. A conceptual model following that of Stern and Bloomer13 is indeed shown, yet

there is no geophysical or bathymetric evidence to support a proto-subduction feature within the LB. Furthermore, the nature of subduction initiation along an oceanic transform14 elicits

questions on the kinematics required to permit subduction in the LB. Considering the limited extension within the basin that correspondingly produced similarly aged and buoyant oceanic

material across the proposed transform boundary, it is questionable if enough gravitational instability would have existed across any transform in this region to initiate spontaneous SW

polarity subduction, depicted by Pandey et al.1. This lack of buoyancy contrast, lack of structural evidence supporting significant NE-SW compression that could otherwise induce subduction,

and no supporting morphological evidence make an incipient subduction model along the western Indian passive margin physically improbable. In any case, seismic reflection profiles across the

LR moreover show no indication of a transform margin along the western side of the ridge, as predicted4.

Instead, most evidence supports the Laxmi Basin forming an extensional basin with seaward-dipping reflectors and a normal thermal subsidence history6. The age of the basin, which has still

not been demonstrated from the drilled volcanic rocks, places it firmly within a wider pre-Deccan volcanic province4. Some contamination by older subduction-related rocks is a common feature

in magmas from rifted continental margins and does not require nascent subduction at the time of eruption, even assuming that the supposed subduction signature is not simply the product of

post-eruption alteration. All geophysical and geochemical evidence put forth by Pandey et al.1 to argue incipient subduction along the western Indian margin can be more simply explained by a

series of rifts following pure shear strain accommodation, accompanied by varying degrees of involvement from plume-related asthenosphere. Occam’s razor does not favour a complex subduction

model of the type advocated when a simple volcanic rifting model also matches the data. The break-up of Gondwana and the opening of the Indian Ocean is unlikely to be as complex as

suggested.

The geochemical data plotted in Fig. 2 was compiled from literature and is available in Supplementary Data 1 and from the Mendeley databank at https://doi.org/10.17632/723c5d795r.1.

References Pandey, D. K., Pandey, A. & Whattam, S. A. Relict subduction initiation along a passive margin in the northwest Indian Ocean. Nat. Commun. 10, 2248 (2019).

Article  ADS  Google Scholar 

Krishna, K. S., Rao, D. G. & Sar, D. Nature of the crust in the Laxmi Basin (14°-20°N), western continental margin of India. Tectonics 25, https://doi.org/10.1029/2004TC001747 (2006).

Article  Google Scholar 

Minshull, T. A., Lane, C. I., Collier, J. S. & Whitmarsh, R. B. The relationship between rifting and magmatism in the northeastern Arabian Sea. Nat. Geosci. 1, 463–467 (2008).

Article  ADS  CAS  Google Scholar 

Calvès, G. et al. Seismic volcanostratigraphy of the western Indian rifted margin: The pre-Deccan igneous province. J. Geophys. Res. 116, B01101 (2011).

Article  ADS  Google Scholar 

Mutter, J. C., Talwani, M. & Stoffa, P. L. Origin of seaward-dipping refectors in oceanic crust of the Norwegian margin by ‘subaerial sea-foor spreading’. Geology 10, 353–357 (1982).

Article  ADS  Google Scholar 

Pandey, D. K. et al. Flexural subsidence analysis of the Laxmi Basin, Arabian Sea and its tectonic implications. Geol. Mag. 1–14. https://doi.org/10.1017/S0016756818000833 (2018).

Stein, C. A. & Stein, S. A model for the global variation in oceanic depth and heat flow with lithospheric age. Nature 359, 123–129 (1992).

Article  ADS  Google Scholar 

Misra, A. A., Banerjee, S., Kundu, N. & Mukherjee, B. in Tectonics of the Deccan Large Igneous Province 445 Special Publications (eds Mukherjee, S., Misra, A. A., Calvès, G., & Nemčok, M.)

119–149 (Geological Society, 2017).

Reagan, M. K. et al. Fore-arc basalts and subduction initiation in the Izu–Bonin–Mariana system. Geochem. Geophys. Geosyst. 11, https://doi.org/10.1029/2009GC00281 (2010).

Robertson, A. H. F. Late Palaeozoic–Cenozoic tectonic development of Greece and Albania in the context of alternative reconstructions of Tethys in the Eastern Mediterranean region. Int.

Geol. Rev. 54, 373–454 (2012).

Article  Google Scholar 

Fitton, J. G., James, D. & Leeman, W. P. Basic magmatism associated with Late Cenozoic extension in the western United States: Compositional variations in space and time. J. Geophys. Res.:

Solid Earth 96, 13693–13711 (1991).

Article  CAS  Google Scholar 

Fitton, J. G. et al. Magma sources and plumbing systems during break-up of the SE Greenland margin: preliminary results from ODP Leg 152. J. Geol. Soc. 152, 985–990 (1995).

Article  ADS  CAS  Google Scholar 

Stern, R. J. & Bloomer, S. H. Subduction zone infancy: examples from the Eocene Izu-Bonin-Mariana and Jurassic California arcs. Geol. Soc. Am. Bull. 104, 1621–1636 (1992).

Article  ADS  Google Scholar 

Stern, R. J. & Gerya, T. Subduction initiation in nature and models: a review. Tectonophysics 746, 173–198 (2018).

Article  ADS  Google Scholar 

Gale, A., Dalton, C. A., Langmuir, C. H., Su, Y. & Schilling, J.-G. The mean composition of ocean ridge basalts. Geochem. Geophys. Geosyst. 14, 489–518 (2013).

Article  ADS  CAS  Google Scholar 

Pearce, J. A. Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust. Lithos 100, 14–48 (2008).

Article  ADS  CAS  Google Scholar 

Nauret, F., Famin, V., Vlastélic, I. & Gannoun, A. A trace of recycled continental crust in the Réunion hotspot. Chem. Geol. 524, 67–76 (2019).

Article  ADS  CAS  Google Scholar 

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Author informationAuthors and Affiliations Department of Geology and Geophysics, E235 Howe-Russell, Louisiana State University, Baton Rouge, LA, 70803, USA

Peter D. Clift

Université Toulouse III, GET-OMP, 14 Avenue Edouard Belin, 31400, Toulouse, France

Gérôme Calvès

School of Earth and Environmental Sciences, The University of Queensland, St. Lucia, QLD, 4072, Australia

Tara N. Jonell

AuthorsPeter D. CliftView author publications You can also search for this author inPubMed Google Scholar

Gérôme CalvèsView author publications You can also search for this author inPubMed Google Scholar

Tara N. JonellView author publications You can also search for this author inPubMed Google Scholar

All authors contributed to this manuscript. T.N.J. lead the geochemical aspects, G.C. lead the geophysical section, while C.D.C. provided regional tectonic expertise.

Corresponding author Correspondence to Peter D. Clift.

The authors declare no competing interests.

Peer review information Nature Communications thanks Mark Reagan for their contribution to the peer review of this work.

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About this articleCite this article Clift, P.D., Calvès, G. & Jonell, T.N. Evidence for simple volcanic rifting not complex subduction initiation in the Laxmi Basin. Nat Commun 11, 2733

(2020). https://doi.org/10.1038/s41467-020-16569-y

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Received: 30 June 2019

Accepted: 07 May 2020

Published: 01 June 2020

DOI: https://doi.org/10.1038/s41467-020-16569-y

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