Reply to ‘evidence for simple volcanic rifting not complex subduction initiation in the laxmi basin’

REPLYING TO Clift et al. _Nature Communications_ https://doi.org/10.1038/s41467-020-16569-y (2020) Pandey et al.1 proposed relict subduction initiation in the Laxmi Basin (LB) as a viable

alternative to numerous competing tectonic models for the evolution of western Indian margin. We thank Clift et al.2 for their interest in our study. They contend that basalts from volcanic

rifted margins may also exhibit analogous geochemical signatures. However, we do not agree with their views for several reasons. Clift et al.2 argue that enrichments in certain elements such

as Th, U, Pb and Sr, typical of subduction magmatism, may also imply possible crustal assimilation. However, they2 overlooked the extent of such enrichments. In fact, the scale of Th

enrichments for both lava types differ almost by an order of magnitude2. Low average Th/Yb ratios for LB (~0.2) are linked to a shallower source, e.g., mid-ocean ridge basalt (MORB), similar

to subduction related ophiolites3. In contrast, rifted margin basalts exhibit much higher Th/Yb ratios (>1) pointing at much deeper sources e.g., ocean island basalts. Similarly, TiO2/Yb

vs. Nb/Yb systematics2 of the LB and IBM forearc both overlap significantly with notably low TiO2 contents—distinct from volcanic margin basalts. Clift et al.2 further state that Nd/Sr

isotopes may indicate continental input instead of subduction initiation (SI)1 during its petrogenesis. Their contention is, however, inconsistent when examined with existing isotopic data.

Average εNd values4 of lower and upper continental crust are −10 and −20 respectively while global subducting sediments (GLOSS) is restricted below −5. In contrast, LB lavas, having εNd

values between +6 to +8, are very close to Depleted Mantle. Moreover, average Ba concentrations of continental crust, LB lava and Muslim Bagh ophiolites are ~259, 140 and 110 ppm4

respectively4. Therefore, isotopic, total trace and rare earth elements (REE) characteristics of LB lava are akin to SI, instead of volcanic margins. Although Clift et al.2 did not include

isotopic data for comparison, we suspect that the 87Sr/86Sr values from rifted volcanic margins would be significantly higher than the modest LB lava values (7037–0.7044), which are similar

to Neotethyan ophiolites and forearc lavas1. Highly mobile elements (e.g., Rb, Sr, Ba, U) are concentrated into aqueous slab-derived fluids, whereas Th and other light rare earth elements

(LREE) are partitioned into sediment-derived melts5,6,7. Enrichment in Ba and to minor extent Th again points towards SI. Similarly, Ba/Th vs. Th variations1 show evidence of shallow,

fluid-derived enrichments8. Using unaltered MORB dataset of Jenner and O’Neill9, one can clearly see enrichment of Ba in LB lava relative to MORB. Often Sr and Ba additions can result from

seawater hydrothermal alteration processes, however LB lavas show no discernible correlation between Sr, Ba and Loss on ignition (_R_2 = 0.01 for both). In fact, samples with the lowest loss

on ignition values, exhibit the highest Ba. This implies that the high Ba relative to MORB of the LB lavas, similar to the Muslim Bagh ophiolite, is a primary feature of LB lavas and the

result of shallow slab-derived fluid input and not seawater alteration. The most important distinction between LB and volcanic margin basalts come from REE signatures, which Clift et al.

have ignored2. LB lavas exhibit highly depleted LREEs and low total REEs in contrast to volcanic margin lava (see Fig. 1). Crustal inputs at volcanic margins typically result in

significantly elevated concentrations of large ion lithophile elements and LREEs, which are absent in LB (Fig. 1). A plot of chondrite normalized [La/Yb]_N_ ratios vs. SiO2 (Fig. 2) also

highlights differences with significantly higher [La/Yb]_N_ values for North Atlantic Igneous Province lavas (~0.6 to as high as 30) in comparison to LB lavas (~0.6–1.4). The lack of LREE

enrichments in LB lava (similar to forearc basalts of the IBM with [La/Yb]_N_ less than 1.4) is therefore noteworthy. Furthermore, boninitic-like lava from LB exhibit characteristic U-shaped

REE profiles which are primarily attributed to proto-forearc and forearc environments10. Further, TiO2 is among the least mobile elements and compared to other trace elements, solubility of

TiO2 is very low in common mantle or subduction zone fluids. TiO2 concentrations may depend on the rate of crystallization of Fe-Ti phases. With respect to high LREE, Th/Yb and enrichment

of Ti, we argue that the LREE (and HREE) enrichments relative to MREE seen in U-shaped REE profiles is typical of boninitic-like lavas and is not related to crustal contamination or

alteration. Prior studies from this region envisaged numerous intra-oceanic weak zones at ~70 Ma (see Pandey et al.1). Rapid northward drift of India coupled with frequent rotations since

the late Cretaceous period is well established. After its spectacular journey from the southern latitudes, India presently resides in the northern hemisphere. Accordingly, considerable

amount of missing/unaccounted for crust in the NW Indian Ocean inferred by kinematic modelling implies synchronous compressional tectonics in this region (see Pandey et al.1). Clift et

al.2’s comments are largely drawn from a particular school of thoughts based on indirect geophysical models proposed well before drilling in LB in 2015. Using same regional seismic

profiles11,12,13 different groups interpret Laxmi ridge and basin variably either as continental/oceanic crust. Equivocal crustal models remained inconclusive about whether this margin is a

magma rich or magma poor type. Likewise, sporadic intra-basement magmatic reflections on seismic profiles are interpreted as seaward dipping reflectors (SDR)/axial anomalies/extinct

spreading centres/volcanic flows/ intrusives etc1, 11,12,13. No precise knowledge is available regarding basin-wide extent of SDRs in the LB, in contrast to what is reported by Clift et al2.

Therefore, a sweeping portrayal2 of SDRs in LB and surrounding regions appears to be conjectural. Identification of a proto-trench would require detailed morphological and crustal imaging

of the region, in time and space, which was beyond the scope of Pandey et al1. Clift et al.2 further link formation of the Laxmi ridge to that of the Saurashtra Volcanic Province. Due to

inconclusive interpretations and lack of any samples/rocks/data from both regions, we prefer not to speculate about its precise affinity. However, geochemical signatures of LB lava are

categorically different from that of Deccan volcanics14. Two-dimensional flexural modeling primarily dealt with post-rift evolution of the LB (awaiting geochemical results from Site U1457)

and confirmed significant residual bathymetry in LB and surrounding regions (~2-3 km at ~61 Ma). This means that any prior magmatism must have occurred under considerably deep-water

settings, in consonance with shipboard observations about rapid quenching of the LB basalts after their emplacement. Regional crustal uplift however, would depend upon several factors

including the onset, extent and duration of the impingement of a potential thermal regime. Finally, we would like to conclude by pointing out that our study1 reports direct data from LB

through crustal sampling for the first time. Geochemical and isotopic imprints of LB lava distinctly imply relict subduction initiation. Therefore, new findings based on direct observations

cannot be undermined merely to support specific geophysical models. Indeed, additional regional basement sampling, radiometric dating and revised plate kinematic modelling would provide

important insights about the complexity of the western Indian margin. DATA AVAILABILITY No new data are reported in this manuscript. See Pandey et al.1 and Clift et al.2 for data sources

related to Fig. 1 and 2. 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 https://doi.org/10.1038/s41467-019-10227-8 (2019). Article  ADS  Google Scholar  * Clift, P. et al. Evidence for simple volcanic rifting not complex subduction initiation in the Laxmi

Basin. _Nat. Commun._ https://doi.org/10.1038/s41467-020-16569-y (2019). * Whattam, S. A. & Stern, R. The “subduction initiation rule”: a key for linking ophiolites, intra-oceanic arcs

and subduction initiation. _Contributions Mineral. Petrol._ 162, 1031–1045 (2011). Article  ADS  CAS  Google Scholar  * Rudnick, R. & Gao, S. in _Composition of the Continental Crust_.

(ed Rudnick, R. L.) 1–64 (Treat, Geochm, 2005). * Elliott, T. et al. Element transport from slab to volcanic front at the Mariana arc _Jour_. _Geophys. Res._ 102, 14991–15019 (1997). Article

  ADS  CAS  Google Scholar  * Hawkesworth, C. et al. Elemental U and Th variations in island arc rocks: implications for U-series isotopes. _Chem. Geol._ 139, 207–221 (1997). Article  ADS 

CAS  Google Scholar  * Singer, B. S. et al. Along-strike trace element and isotopic variation in Aleutian Island Arc basalt: subduction melts sediments and dehydrates serpentine. _Jour.

Geophys. Res_ 112, B06206 (2007). Article  ADS  Google Scholar  * Pearce, J. A. et al. Geochemical mapping of the Mariana arc-basin system: implications for the nature and distribution of

subduction components. _Geochem., Geophysics, Geosystems_ 6, Q07006 (2005). ADS  Google Scholar  * Jenner, F. E. & O’Neill, H. S. C. Analysis of 60 elements in 616 ocean floor basaltic

glasses. _Geochem. Geophys. Geosyst._ 13, Q02005 (2012). ADS  Google Scholar  * Reagan, M. K. et al. Fore-arc basalts and subduction initiation in the Izu- Bonin-Mariana system. _Geochem.

Geophys. Geosyst._ 11, Q03X12 (2010). Article  Google Scholar  * Corfield, R. I. et al. Variability in the crustal structure of the West Indian Continental Margin in the Northern Arabian

Sea. _Pet. Geosci._ 16, 257–265 (2010). Article  Google Scholar  * Calvès G. et al. Seismic volcanostratigraphy of the western Indian rifted mar- gin: the pre-Deccan igneous province. _J.

Geophys. Res. Solid Earth_ 116, https://doi.org/10.1029/2010JB000862. (2011). * Misra, A. A., Sinha, N. & Mukherjee, S. Repeat ridge jumps and microcontinent separation: insights from NE

Arabian Sea. _Mar. Pet. Geol._ 59, 406–428 (2015). Article  Google Scholar  * Melluso et al. Constraints on the mantle sources of the Deccan Traps from the petrology and geochemistry of the

basalts of the Gujarat state, western India. _J. Petrol._ 36, 1393–1432 (1995). Article  ADS  CAS  Google Scholar  Download references ACKNOWLEDGEMENTS DP acknowledges financial support

from the Ministry of Earth Sciences (MoES), India bearing Grant number: MoES/PO(Seismo)/3(45)2012. This is NCPOR contribution number #J-03/2020-21. AUTHOR INFORMATION AUTHORS AND

AFFILIATIONS * National Centre for Polar & Ocean Research, Ministry of Earth Sciences, Vasco da Gama, Goa, 403804, India Dhananjai K. Pandey * H-V-3, NCPOR Campus, Vasco da Gama, Goa,

403804, India Anju Pandey * Department of Geosciences, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Saudi Arabia Scott A. Whattam Authors * Dhananjai K. Pandey View

author publications You can also search for this author inPubMed Google Scholar * Anju Pandey View author publications You can also search for this author inPubMed Google Scholar * Scott A.

Whattam View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS All authors (D.P., A.P. and S.W.) contributed to the discussion of the content of

this manuscript. CORRESPONDING AUTHOR Correspondence to Dhananjai K. Pandey. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing interests. ADDITIONAL INFORMATION PEER

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rifting not complex subduction initiation in the Laxmi Basin’. _Nat Commun_ 11, 2734 (2020). https://doi.org/10.1038/s41467-020-16570-5 Download citation * Received: 28 August 2019 *

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