Earth and Planetary Science Letters 417 (2015) 142–150

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Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Neogene marine isotopic evolution and the erosion of Lesser

Himalayan strata: Implications for Cenozoic tectonic history

Paul M. Myrow a,∗, Nigel C. Hughes b, Louis A. Derry c, N. Ryan McKenzie d,e, Ganqing Jiang f, A. Alexander G. Webb g, Dhiraj M. Banerjee h, Timothy S. Paulsen i, Birendra P. Singh j

a Department of Geology, Colorado College, Colorado Springs, CO 80903, USAb Department of Earth Sciences, University of California, Riverside, CA 92521, USAc Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USAd Jackson School of Geosciences, University of Texas, Austin, TX 78713, USAe Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USAf Department of Geosciences, University of Nevada, Las Vegas, NV 89154, USAg Department of Geology & Geophysics, Louisiana State University, LA 70803, USAh Department of Geology, University of Delhi, Delhi, 110007, Indiai Department of Geology, University of Wisconsin, Oshkosh, WI 54901, USAj Department of Geology, Panjab University, Chandigarh, 160014, 54901, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 September 2014Received in revised form 29 January 2015Accepted 12 February 2015Available online xxxxEditor: A. Yin

Keywords:HimalayatectonicsgeochemistryisotopesNeogene

An extensive, northward deepening blanket of Neoproterozoic and Cambrian sedimentary rocks once extended from the Himalayan margin far onto the Indian craton. Cambrian deposits of this “upper Lesser Himalayan” succession, which include deposits of the “outer” Lesser Himalaya tectonic unit, are enriched in radiogenic 187Os. They make up part of a proximal marine facies belt that extends onto the craton and along strike from India to Pakistan. By contrast, age-equivalent facies in the Tethyan Himalaya are more distal in nature. Neoproterozoic to Cambrian strata of the upper Lesser Himalayan succession are now missing in much of the Lesser Himalaya, with their erosion exposing older Precambrian Lesser Himalayan strata. We suggest that exhumation and weathering of the upper Lesser Himalaya and related strata caused dramatic changes in the 187Os/188Os and 87Sr/86Sr Neogene record of seawater starting at ∼16 Ma. First-order estimates for the volume of upper Himalayan strata, as well as the volume of all LH rock eroded since this time, and geochemical box modeling, support this idea. Exhumation at 16 Ma is a fundamental event in the evolution of the Himalayan orogeny and the geochemical evolution of the oceans, and will be a critical part of the construction of future models of Himalayan thrust belt evolution.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The uplift and erosional history of the Himalayan orogen had fundamental influence on climate and secular changes in ocean chemistry (Derry and France-Lanord, 1996; France-Lanord and Derry, 1997; Galy et al., 2007). Of key interest are the links be-tween Neogene uplift and both the erosion of Himalayan bedrock and the record of the isotopic variations of Os and Sr in seawa-ter. Quantification of the erosional history of the Himalayan oro-gen requires restoration of the geology prior to major unroofing. This objective, however, has been hampered by uncertainties in

* Corresponding author.E-mail address: pmyrow@coloradocollege.edu (P.M. Myrow).

http://dx.doi.org/10.1016/j.epsl.2015.02.0160012-821X/© 2015 Elsevier B.V. All rights reserved.

the timing of exhumation of lithotectonic zones of the Himalaya (Fig. 1), and debates on the pre-deformational configuration of the north Indian margin (e.g., Yin, 2006). Recent studies of the Neoproterozoic–early Paleozoic successions of the ancient northern Indian margin, both along and across the strike of the Himalayan orogen, provide insights into the stratigraphic, depositional, and tectonic relationships between these zones; in other words, the pre-collisional nature of the margin (Myrow et al., 2003; Hughes et al., 2005; Myrow et al., 2006; McQuarrie et al., 2008; Myrow et al., 2009, 2010; Long et al., 2011; McKenzie et al., 2011; Webb et al., 2011b; McQuarrie et al., 2013).

We comprehensively studied the spatial distribution of late Neoproterozoic–Cambrian successions across the northern Indian subcontinent in order to evaluate the uplift and erosion of var-ious potential source rocks during propagation of thrust faults

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Fig. 1. (a) Overview Himalayan geologic map. (b) Simplified geological map of the northern Indian Himalaya west of Nepal (modified after Valdiya, 1980; Yin, 2006; Célérier et al., 2009b; Webb et al., 2011b; Webb, 2013).

associated with Himalayan deformation. Such eroded rocks may include the late Neoproterozoic–Cambrian strata of the Lesser Himalaya, some of which are enriched in radiogenic 187Os, par-ticularly a shale unit in the Tal Group (Singh et al., 1999;Pierson-Wickmann et al., 2000). The spatial and temporal pattern of erosion and chemical weathering of these strata may have been an important driving factor for secular changes in Neogene seawa-ter 187Os/188Os and 87Sr/86Sr. If so, changes in the isotopic record of seawater may record significant changes in the thrust belt evo-lution of the Himalaya, including tectonic uplift and exhumation of changing source rocks. Therefore, we explore the feasibility, via geochemical modeling, that successive exhumation and weather-ing of two distinct Lesser Himalayan (LH) stratigraphic successions can quantitatively explain the observed trends in Neogene seawa-ter 187Os/188Os and 87Sr/86Sr. The proposed exhumation history of the LH proposed here is consistent with foreland basin sedimen-tation and detrital zircon records, as well as the marine Os and Sr isotopic evolution.

2. Geologic background

Current convention is to divide the Himalaya into lithotectonic zones (e.g., Yin, 2006) (Fig. 1). The northernmost of these units, the Tethyan Himalaya (TH), is situated in the hanging wall of the South Tibetan Fault System (STFS) and consists of late Neoproterozoic to Eocene sedimentary successions. A central belt of high-grade metamorphic rocks, the Greater Himalaya (GH), is situated in the hanging wall of the Main Central Thrust (MCT) (but see Webb et al., 2011b, 2011a for discussion of various MCT definitions). The Lesser Himalaya (LH) is situated in the footwall of the Main Cen-tral Thrust (MCT) and consists mostly of Proterozoic strata with packages of younger Phanerozoic rocks scattered across the orogen. A series of thrust faults that place Himalayan bedrock structurally

against Cenozoic basin deposits are generically referred to as the Main Boundary Thrust system (MBT) and uplifted foreland basin deposits reside in the hanging wall of the southernmost Frontal Thrust system (FT), which marks the boundary between the thrust belt and the foreland basin.

A prominent ∼500 million year unconformity that separates late Paleoproterozoic and older rocks (>1.6 Ga) from late Meso-proterozoic and younger rocks (<1.1 Ga) has been recognized across the Indian margin (McKenzie et al., 2011, 2013). In the Himalaya, this unconformity is generally recognized within the LH, and the terms “lower Lesser Himalaya” and “upper Lesser Hi-malaya” have been applied to the overlying and underlying units (e.g., Robinson et al., 2001, Richards et al., 2005; Robinson et al., 2006; McQuarrie et al., 2008; Gehrels et al., 2011 McKenzie et al., 2011). However, rocks with ages that are comparable to those above and below this unconformity have been recognized within the GH (cf. Yin et al., 2010; Webb et al., 2011b), demonstrating this is not a diagnostic feature of the LH, but occurs more widely. Therefore, we will use the broad terms “upper Lesser Himalayan succession” and “lower Lesser Himalayan succession” to refer to strata deposited above and below this unconformity, respectively.

Rocks of the upper and lower Lesser Himalayan successions are variably exposed along the orogen. Sedimentary rocks of both age groups are present in the LH of the eastern Himalaya in Bhutan (McQuarrie et al., 2008; Long et al., 2011; McQuarrie et al., 2013) and Arunachal Pradesh (Tewari, 2001), whereas rocks of the up-per Lesser Himalayan succession are reportedly absent (due to later erosion) throughout the LH of Nepal (Robinson et al., 2001;DeCelles et al., 2004; Gehrels et al., 2011; Martin et al., 2011). Neo-proterozoic and Cambrian rocks are also known along strike south of the Main Central Thrust in Pakistan, within the sub-Himalaya of the Salt Range of Pakistan, and on the Indian craton itself in Ra-jasthan, south of the Himalayan Frontal Thrust.

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Fig. 2. Stratigraphic sections of Neoproterozoic and Cambrian rocks from the Himalayan margin and Indian craton. Data sources in Supplementary Data 1.

West of Nepal, the Tons Thrust divides the Indian LH into “in-ner” (iLH) and “outer” (oLH) zones (Valdiya, 1980; Ahmad et al., 2000) with oLH rocks sitting in the hanging wall of the Tons Thrust and iLH rocks in the footwall (Célérier et al., 2009a, 2009b; Webb et al., 2011b; Webb, 2013). Presently, strata from only the upper LH succession have been confirmed in the oLH, most of which range from Cryogenian to Cambrian in age (Jiang et al., 2002; Hughes et al., 2005; Célérier et al., 2009a; McKenzie et al., 2011). iLH rocks include two main structural units, the Damtha–Deoban duplex and the Berinag Thrust sheet. In this region of India, the iLH contains strata of both the lower Lesser Himalaya succession and the low-ermost part of the upper Lesser Himalayan succession (McKenzie et al., 2011). Klippen of GH rock occurs structurally above both iLH and oLH rocks (Célérier et al. 2009a, 2009b; Webb et al., 2011b;Mandal et al., 2014).

3. Continuity of Cambrian strata and depositional systems

3.1. Regional lithofacies

Regional lithofacies relationships for Neoproterozoic and Cam-brian strata across the TH and LH, as well as cratonic successions, demonstrate northward deepening across the northern Indian mar-gin. Sedimentological data (references in Supplemental Data 1) from deposits south of the MCT (Fig. 2) suggest that this area com-prises a proximal marine facies realm relative to the GH and TH to the north. Up to 900 km south of the Himalayan Frontal Thrust in the Marwar basin of Rajasthan are successions of Neoprotero-zoic evaporite-bearing carbonate and Cambrian sandstone (Malone et al., 2008; McKenzie et al., 2011). South of the Main Boundary Thrust (MBT) in the Salt Range of Pakistan is an evaporite-bearing

Neoproterozoic carbonate, a Cambrian succession with evaporites, and early Cambrian trilobites and brachiopods (Jell and Hughes, 1997). The Krol–Tal belt of the oLH includes a Neoproterozoic glacial diamictite, a thick evaporite-bearing carbonate succession (Krol Formation), and a dominantly siliciclastic Cambrian unit (Tal Group) with distinctive phosphatic shale and various shelly fossils (Hughes et al., 2005). Lateral continuity of facies along strike is provided by strata exposed in Abbottabad, Pakistan, which have comparable thick carbonate deposits and Cambrian phosphatic shale (Pogue et al., 1992) with similar fauna (Hughes et al., 2005)(Fig. 2). In addition, in the eastern Himalaya a likely Neoprotero-zoic carbonate/siliciclastic suite in the LH of Bhutan and Arunachal Pradesh (Buxa Formation) (Long et al., 2011) represents a pos-sible equivalent to the Krol Formation. Collectively, rocks of this proximal facies realm were characterized by periodic episodes of condensed and hypersaline sedimentation, and all are capped un-conformably by Permian or younger deposits.

In contrast, Neoproterozoic and Cambrian deposits in the hang-ing wall of the STFS are thick successions that represent generally more distal marine facies associations, although both facies realms share a late Neoproterozoic diamictite (Draganits et al., 2008). In the TH, (1) shale units form the distal equivalents of the prox-imal Neoproterozoic Krol Formation carbonate of the LH (Jiang et al., 2002), (2) shale of the Phe Formation is age equivalent to lower/middle Tal Group shallow water strata (including phosphatic facies) (Myrow et al., 2003), and (3) Cambrian deltaic sandstone of the Parahio Formation are age equivalents of the upper Tal Group fluvial sandstone in the LH proximal facies realm (Myrow et al., 2003, 2006). Parts of the GH have been precisely corre-lated with protolith TH rocks of the Cambrian Parahio Formation (Myrow et al., 2009). Ordovician and younger Paleozoic strata un-

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Fig. 3. Comparison of detrital zircon U–Pb age distributions of siliciclastic rocks from major structural units and foreland basin deposits of the Kumaon–Garhwal region of the Indian Himalaya and Neoproterozoic–Cambrian rocks of the Marwar Supergroup of the Indian craton. Neoproterozoic–Cambrian strata in all zones yield similar age distributions, whereas older Paleoproterozoic rocks of the inner Lesser Himalaya distinctly lack grains younger than ∼1.6 Ga. No discernable changes in detrital zircon age populations are observed in foreland basin deposits, implying the iLH was not a major contributor of detritus prior to at least 11 Ma. Age data are from Myrow et al. (2003, 2010), Malone et al. (2008), McKenzie et al. (2011), Ravikant et al. (2011), and Webb et al. (2011b).

conformably cap successions in the distal facies realm. These can be used to identify klippe of TH strata exposed atop the LH, rela-tive to Neoproterozoic–Cambrian successions indigenous to the LH, which are capped by Permian strata.

The stratigraphic data indicate that a contiguous Neoproterozo-ic–Cambrian sedimentary blanket may have extended from the craton across the northern Indian margin, and along strike from western to eastern syntaxes of the Himalaya. Sedimentological data and stratigraphic correlation to less deformed successions in Pak-istan, west of the western syntax where Himalayan deformation is minimal, show that the Krol–Tal Belt upper Lesser Himalayan suc-cession was part of the proximal marine facies realm, and thus an erosional remnant of the extensive Neoproterozoic–Cambrian blanket. Recent structural analysis suggests that a large swath of rocks in Nepal of the oLH structural unit was removed within the last 5 m.y. (Yu et al., in press), further supporting the idea of widespread erosion of upper Lesser Himalayan strata. Distinctive phosphatic and evaporitic strata can be used to correlate proximal-facies rocks far along strike within the LH. Strata of the proximal-facies realm are not known from the TH, which primarily contains more distal marine facies rocks.

3.2. Detrital zircon age distributions of Cambrian strata across the northern Indian margin

Continuity of sediment transport across the Indian margin is confirmed by similarities in detrital zircon age distributions of Neoproterozoic through Ordovician sandstone units from cratonic, LH, GH, and TH samples (Myrow et al., 2010; McKenzie et al., 2011; Webb et al., 2011b) (Fig. 3). Samples taken from across the ancient northern Indian Himalayan margin show remarkably uniform signatures that include ages from Archean to Ordovician, with peaks at ∼2.5 Ga and ∼1.9–1.7 Ga, various peaks from ∼1.3to 0.8 Ga, and a large peak at ∼0.6–0.5 Ga (Myrow et al., 2010;McKenzie et al., 2011; Mandal et al., 2014). The ∼0.5 Ga peak

represents erosion of granitic plutons emplaced during the final assembly of Gondwanaland, including parts of northern India (e.g., LeFort et al., 1986). A large fraction of bedrock currently exposed in the LH has depositional ages that are older than 1.6 Ga and the stratigraphic influx of abundant zircon grains older 1.6 Ga into syn-tectonic foreland basin deposits shed from the Himalaya had been used to date the initial timing of uplift of the LH (e.g., DeCelles et al., 1998a). Younger detrital zircon ages (0.5–1.6 Ga) and less negative εNd values, typical of GH and TH rocks, have also been documented from the LH of Bhutan (McQuarrie et al., 2008; Long et al., 2011; McQuarrie et al., 2013), and the upper LH succes-sion, lower LH succession, and Indian craton (Myrow et al., 2003;McKenzie et al., 2011) (Fig. 3). This indicates, in combination with lithofacies analysis, that rocks of such age extended from craton to the TH, thus spanning the entire region, including the area where rocks of the iLH are presently exposed.

4. Evidence for ∼16 Ma exhumation of the upper LH succession and associated proximal-facies strata

Recent interpretations suggest the oLH, including the upper LH strata of the Krol–Tal Belt, is a far transported thrust sheet that roots along the TH (Célérier et al. 2009a, 2009b). In this case, the oLH would have been juxtaposed against the lower LH strata of the iLH by the Oligocene with the oLH subsequently overthrust by the MCT. An alternative model suggests that the oLH is a short-traveled thrust sheet that was emplaced through footwall accretion via break-forward in-sequence thrusting of the sole décollemont of the MCT (Webb et al., 2011b; Webb, 2013). Our stratigraphic data demonstrates that the Krol–Tal Belt of the oLH was part of the upper LH proximal marine facies realm, distinct from the age equivalent, generally more distal, marine facies now exposed in the TH zone. All of these strata were part of a Neoproterozoic–Cambrian succession that overlaid the older rocks exposed in the LH, and extended south onto the Indian craton and north across

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the Tethyan realm. This interpretation is consistent with a prox-imal pre-deformational setting of the oLH (Webb et al., 2011b;Webb, 2013). The similarity in detrital zircon spectra between the upper LH strata of the oLH and TH strata also supports a proxi-mal to distal continuous depositional system (Myrow et al., 2003, 2010).

These data are important for reconstructing the timing of uplift of LH-associated rocks. The ages of detrital zircon grains from Pa-leocene to late Miocene foreland basin deposits have been linked to a south-directed, break-forward sequence of thrusting within the Himalaya, and thus to the sequential uplift of specific litho-tectonic zones. The detrital zircon signature of samples from lower Eocene to middle Miocene foreland basin deposits were postulated to have been derived from the GH and TH, whereas later breach-ing and erosion of the LH is recorded by upward coarsening, an influx of abundant >1.6 Ga detrital zircon grains (DeCelles et al., 1998a), and decreases in εNd values in the upper part of the lower Siwalik Group and middle Siwalik Group (Najman et al., 2000;Robinson et al., 2001). Based on our estimates of stratigraphic thickness and estimates of areas of exposed strata, we calcu-late that ∼9300 m of Neoproterozoic and younger oLH-age strata (Supplemental Data 2) must have been eroded to expose Pale-oproterozoic rocks, the source of abundant >1.6 Ga grains. The geochronological and geochemical signature of these strata would have included abundant 0.5 to 1.6 Ga grains and much less neg-ative εNd values (Myrow et al., 2003; Richards et al., 2005;McKenzie et al., 2011; Webb et al., 2011b), similar to GH and TH rocks to the north. Detrital zircon U–Pb age data from fore-land basin deposits in northern India showed no change in prove-nance from the Eocene Subathu Formation through to lower Siwa-lik Group, which was deposited between 13 and 11 Ma, as based on Ravikant et al.’s (2011) data. All of these foreland basin U–Pb age distributions are representative of upper Lesser Himalayan sources, which imply that the lower Lesser Himalaya was not sig-nificantly exposed prior to 11 Ma.

It appears that some amount of upper Lesser Himalayan strata was removed prior to the Neogene, based on stratigraphic and structural relationships of small, isolated outcrops of Cretaceous and Paleocene–Eocene rocks that rest on Precambrian strata in both the iLH and oLH zones (e.g., Webb et al., 2011b). These are, however, difficult to assess due to poor exposure, and the possibil-ity exists that in some cases the contacts are in fact faulted. There are no reported orogenic events from the late Paleozoic through Cretaceous in the Himalayan region that may have caused sub-stantial uplift and erosion of upper Lesser Himalayan rocks prior to Himalayan uplift, although some erosion may have taken place without uplift. Thus, the degree of erosion prior to Himalayan up-lift is difficult to quantify, but our goal is to provide first-order approximations of volumes of eroded strata and the effects of this erosion.

During Himalayan deformation, erosion of lower Lesser Hi-malaya strata of the iLH would have began after erosion of the thick Neoproterozoic to Cambrian blanket of upper Lesser Hi-malaya strata, so the stratigraphic appearance of abundant zir-con grains older than 1.6 Ga in the Siwalik Group records the erosional breaching of enough lower Lesser Himalayan Paleopro-terozoic (2.5–1.6 Ga) rocks to provide abundant grains of this age. Using geologic map and stratigraphic data, we estimate that ∼930 000 km3 (±300 000 km3) of upper Lesser Himalayan strata was removed from the LH (Supplementary Data 2).

Recent studies in the Himachal Himalaya (White et al., 2002;Najman et al., 2009) also posit unroofing of a younger succes-sion starting around 17 Ma, prior to erosional exposure of the older rocks of the lower Lesser Himalayan succession at 11 Ma. In particular, they demonstrate that a fundamental shift in petro-logic and thermochronological character of foreland basin deposits

takes place at ∼17 Ma, where strata record a shift from sediment sourced from higher-grade GH rocks to low-grade sedimentary sources (mostly carbonate and shale). The shift at ∼17 Ma, coin-ciding with the transition from the lower to the upper Dharamsala Formation of the Himalayan foreland basin deposits, also shows a significant decrease in detrital white mica content, and a change in the mica 40Ar/39Ar thermochronological ages from <60 Ma(most 20–30 Ma) to >60 Ma and ranging into the Proterozoic. The younger micas in the lower Dharamsala Formation reflect rapid up-lift and erosion of the GH, which likely occurred during thrusting of the MCT, while the upper Dharamsala Formation micas reflect erosion of rocks in which the mica cooling ages were not reset by Himalayan metamorphism. The latter micas were attributed to erosion of Haimanta Group rocks that are of TH affinity (White et al., 2002). However, it is more likely that the shift in mica sources reflects erosion through the GH and into oLH rocks of the MCT footwall, as their 40Ar/39Ar ages would not likely have been reset by Himalayan metamorphism.

Deeken et al. (2011) used zircon (U–Th/He) thermochronomet-ric data to argue for uplift ∼100 km west of the westernmost exposure of the oLH via active thrusting on faults to the south of the MCT by at least ∼15 Ma, which is supported by data presented herein. Similarly, Bernet et al. (2006) propose LH exhumation in the Nepali Himalaya around ∼16 Ma. They postulate that uplift and erosion at this time led to widespread cooling, as recorded by zircon fission track ages, due to movement along the STFS, MCT, and the Ramgarh Fault, the latter of which includes iLH-equivalent rocks in its hanging wall. Bernet et al. (2006) do not mention up-per Lesser Himalayan rocks in Nepal, whose pervasive absence we attribute to extensive erosion of the youngest strata. However, such strata would have existed above the older lower Lesser Himalayan rocks of the Ramgarh thrust sheet (Yu et al., in press), and they would have been the first rocks to erode during uplift at 16 Ma, assuming that the younger strata were in the leading edge of the thrust. This may explain why the detrital zircon distributions in the Nepalese foreland basin do not show a pronounced shift to older grains until ∼11 Ma (cf. DeCelles et al., 1998a, 2004). Although spatial and temporal patterns of uplift along the length of the Hi-malaya at this time were potentially nonuniform, evidence points to a general shift in the locus of thrusting, which had direct effects on the provenance record of the foreland basin.

5. Implication for geochemical evolution of Neogene paleoseawater

Here, we present a combined tectonic–geochemical model that links earlier uplift of the LH to the Neogene paleoseawater record of 187Os/188Os and 87Sr/86Sr (Ravizza, 1993; Oslick et al., 1994;Peucker-Ehrenbrink et al., 1995; and Reusch et al., 1998). The 187Os/188Os record shows a period of stability from about 29 Ma to 16 Ma (Fig. 4) followed by a monotonic rise starting at ap-proximately 16 Ma. The initiation of this dramatic rise in os-mium isotopic ratios closely matches a sharp decrease in the rate of rise of the marine 87Sr/86Sr isotopic signal (Oslick et al., 1994). This record requires a change in the sources of Os and Sr to the oceans. Multiple workers have suggested that ero-sion of organic-rich sedimentary rock in the Himalaya led to the dramatic rise in Os isotopic values (e.g., Pegram et al., 1992;Ravizza, 1993). Links were specifically made to black shale in the early Cambrian Tal Group of the upper Lesser Himalayan succession, which is greatly enriched in radiogenic Os (Singh et al., 1999; Pierson-Wickmann et al., 2000), but the puta-tive discrepancy between the timing of exposure of Tal Group strata and the increase in 187Os/188Os of seawater led some workers to question the relationship between radiogenic sources and changes in marine Os isotope ratios. Based on discussions

P.M. Myrow et al. / Earth and Planetary Science Letters 417 (2015) 142–150 147

Fig. 4. Osmium and strontium isotope evolution of the Neogene ocean. Both show a significant break in slope near 16 Ma. From 28 to 19 Ma the 187Os/188Os data can be fit with a zero-slope line with 187Os/188Os = 0.7337. From 16 to 1 Ma the Os data can be fit with a constant slope = 0.01329 Ma−1. The strontium isotope data can similarly be fit with two linear segments, from 22.2 to 15.8 (dR/dt = 6.86 × 10−5 Ma−1) and from 15.5 to 9.2 Ma (dR/dt = 2.67 × 10−5 Ma−1). Data sources: Ravizza (1993), Oslick et al. (1994), Peucker-Ehrenbrink et al. (1995), and Reusch et al. (1998).

above, we postulate that initial uplift and exhumation of up-per Lesser Himalayan strata (possibly the oLH) took place at ∼16 Ma, and that it is recorded in marked inflections in the marine Os and Sr records at that time. Decreased erosion of the GH and increased erosion of younger oLH strata could have plausibly driven the observed changes in the Os and Sr pale-oseawater curves beginning near 16 Ma, because radiogenic Os is concentrated in organic rich shale that accumulated in proxi-mal parts of the Neoproterozoic and Cambrian margin (e.g., Singh et al., 1999), and radiogenic Sr is concentrated in the high-grade rocks of the GH (France-Lanord et al., 1993; Ahmad et al., 2000).

Exhumation rates in the Himalaya varied both spatially and temporally over the Cenozoic, but most estimates are less than 2 mm/yr (White et al., 2002; Najman et al., 2009; Deeken et al., 2011). An average rate of 1.9 mm/yr would be required to unroof the estimated 9300 m of younger upper Lesser Himalayan strata over the interval of ∼5 million years (from 16 to 11 Ma) required to expose enough of the older lower Lesser Himalayan succession to provide its detrital signal to the foreland basin. In addition, analysis of Fe/Mn nodules (Chesley et al., 2000) from palaeosols in the foreland of the Ganges and Indus rivers, in an attempt to reconstruct the isotopic composition of past river fluxes, reveals increased radiogenic Os in the Ganges foreland by ∼15 Ma, which is consistent with our proposed estimate for initial oLH exhuma-tion near 16 Ma.

Exhumation of the oLH at ∼16 Ma would have resulted from southward fault propagation from the MCT to thrust faults to the south, e.g., the Tons Thrust, Berinag Thrust, and/or MBT. Structural data from Nepal support southward propagation at ∼15 Ma (Searle and Godin, 2003), including initiation of the emplacement of the Ramgarh–Munsari Thrust sheet (DeCelles et al., 1998b). In addition, ductile shearing and high temperature metamorphism along the MCT is thought to have ceased shortly after ∼18 Ma (Searle and Godin, 2003). Cooling of leucogranite bodies at ∼16 Ma (Horton and Leech, 2013) along the MCT was linked to cessation of move-ment and southward propagation of the thrust belt (Jessup et al., 2006). Finally, foreland basin deposits show a dramatic decrease in Himalayan exhumation rates at ∼17 Ma, as shown by a shift in lag times between the depositional ages of samples and the Ar–

Ar ages of the youngest detrital white micas (White et al., 2002;Najman et al., 2009).

5.2. Geochemical model for oLH weathering and Neogene 187Os/188Os and 87Sr/86Sr

Assuming initiation of exhumation of the younger Lesser Hi-malayan succession at ∼16 Ma, and erosion of the highest strati-graphic deposits we constructed paired geochemical box models to test the effects of erosion and weathering of oLH and asso-ciated strata on the Os and Sr isotopic evolution of the Miocene oceans. The approach is a forward model (Supplementary Data 3) that can test the consequences of assumptions about the tempo-ral evolution fluxes of Os and Sr. We seek to evaluate whether the observed isotopic evolution curves for Os and Sr in paleoseawater are quantitatively consistent with our inferences about the expo-sure history of LH rocks, but do not imply that we can know those fluxes with certainty. For Os (Fig. 5), we made a first-order esti-mate of the volume of eroded lower Tal Group black shale using an area calculation of eroded Tal strata (Supplementary Data 2), and by assuming a uniform thickness of 150 m (supported by the notable lateral continuity of Cambrian lithofacies and unit thick-nesses along the Indian margin, Myrow et al., 2006) (Fig. 2). We provide two minimum (2900 km3, 3600 km3) and one maximum estimates (17 990 km3) respectively (Supplementary Data 2), and these can be significant, given that the release of Os from a unit volume of black shale is 1000 times greater than that of typical granitoids (Peucker-Ehrenbrink and Blum, 1998). The major sources of uncertainty in calculating the potential impact of the erosion of the Tal are 1) the volume eroded, and 2) the dissolved yield of Os associated with this denudation. If the eroded volume is close to our upper estimate, then a yield of 43% is sufficient to drive all of the observed increase in (187Os/188Os)sw during the interval from 16 to 11 Ma. If the eroded volume is close to our lower limit (2900 km3), the Tal can account for at most about half of the ob-served rise in (187Os/188Os)sw during that interval, which in itself is a significant statement.

Our Os model focuses on the lower Cambrian shale due to ex-isting constraints on the stratigraphic thickness and lateral extent of a particular Tal Group unit. However, additional potential 187Os

148 P.M. Myrow et al. / Earth and Planetary Science Letters 417 (2015) 142–150

Fig. 5. Osmium isotope evolution of the Neogene ocean from 28 to 11 Ma. Model curve presents steady state solution from 28 to 16 Ma. From 16 to 11 Ma, an additional radiogenic flux with Tal-like characteristics (187Os/188Os)16 = 2.95, 187Re/188Os = 185 that grows at a rate of 1.95 × 10−6 yr−1 is added. The time-integrated flux of this source from 16 to 11 Ma is 2.4 × 107 kg, compared to an estimated initial Os content of the Tal = 5.7 × 107 kg. The modeled isotopic growth rate = 0.0125 Ma−1, consistent with a least squares fit to the data from 16 to 11 Ma. Paleoseawater 187Os/188Os continues to grow at a near constant rate from 11 to 1 Ma, but the unroofing of the lower Lesser Himalaya implies that Himalayan sources became notably more radiogenic after that time, necessitating a correspond-ing decrease in the rate of flux growth to satisfy the observations. Data sources as in Fig. 4.

source rocks are known from the LH, which could have contributed radiogenic Os to the ocean at any time from 16 Ma to the present. Prominent Neoproterozoic age black shale enriched in 187Os un-derlay the Tal Group in the oLH (Singh et al., 1999). Older lower Lesser Himalayan organic-rich shale units and carbonate rocks with abundant and even more radiogenic Os (187Os/188Os = 7 to 21) are present in the Nepal LH (Chesley et al., 2000), and repre-sent additional sources of radiogenic Os from the LH that con-tributed to the rise of (187Os/188Os)sw after they were unroofed near 11 Ma. The detailed erosional history of LH strata is not known, but paleoriver archives from the Himalayan foreland record (187Os/188Os)river ≈ 2.0 by 16 Ma (Chesley et al., 2000), clearly in-dicating the presence of an anomalously radiogenic source. The model illustrates that a volumetrically minor oLH source driven by a plausible tectonic history can generate a quantitatively sig-nificant flux of radiogenic Os beginning ∼16 Ma, and is consistent with unroofing of the iLH near 11 Ma. The estimates above are a first-order attempt at such an analysis, and set the stage for future studies based on more refined stratigraphic, tectonic, and geochemical data.

The impact of enhanced erosion and weathering of oLH strata on the Sr isotopic flux from the Himalaya would have promoted a significant decline in the mean 87Sr/86Sr of Himalayan rivers, consistent with the observed evolution of (87Sr/86Sr)sw that shows a marked decrease in slope at 16 Ma. While GH rocks above the MCT have 87Sr/86Sr near 0.74–0.76 (France-Lanord et al., 1993), oLH carbonate strata (the dominant oLH source of dissolved Sr) are much less radiogenic and range from 0.709 to 0.714 (Singh et al., 1998). Exhumation rates of more radiogenic GH slowed at approx-imately 17 Ma (Najman et al., 2009). An increase in the relative contribution of oLH weathering sources of Sr is capable of low-ering the mean river 87Sr/86Sr sufficiently to produce the observed decrease in the rate of growth of the marine 87Sr/86Sr from ∼6.8 ×10−5 Ma−1 to ∼2.7 × 10−5 Ma−1 after 16 Ma (Fig. 6, see Supple-

Fig. 6. Strontium isotope evolution of the Neogene ocean from 23 to 10 Ma. The model curve is initialized with an “excess flux” of radiogenic Sr from Himalayan rivers with 87Sr/86Sr = 0.715 at 24 Ma. From 24 to 16 Ma the Himalayan Sr flux grows at a rate of 1.3 ×10−7 yr−1, while its isotopic composition evolves at a rate of 1.2 ×10−9 yr−1. From 16 to 10 Ma, the flux grows at 1.1 ×10−7 yr−1 while the rate of change of the 87Sr/86Sr of Himalayan rivers is −0.8 × 10−9 yr−1. Other inputs, including the world’s rivers, hydrothermal, and diagenetic fluxes, are held constant (see Supplementary Data 3 for details). Modeled Himalayan river isotopic ratios are consistent with data from Neogene paleoriver archives from the Himalayan foreland. Linear fits to the model output from 23 to 16 and from 16 to 10 Ma are 6.1 ×10−6 yr−1 and 2.7 × 10−6 yr−1 respectively, consistent with data from ODP 747A (Oslick et al., 1994).

mentary Data 3 for model details). The 87Sr/86Sr of the Ganges–Brahmaputra declined slightly from 16 to ∼11 Ma, and then began to increase dramatically (Derry and France-Lanord, 1996), consis-tent with exhumation of the oLH beginning ∼16 Ma and the iLH ∼11 Ma. The model results for Os and Sr are fully consistent, and demonstrate that there is a plausible link between a ma-jor shift in Himalaya bedrock exhumation at ∼16 Ma and the evolution of Os and Sr in the Neogene oceans. The combina-tion of our model calculation, and our inferred initiation of ero-sion of the Tal Group and related strata beginning near 16 Ma, explain the ∼5 m.y. offset between the shift in the marine Os and Sr isotopic records at ∼16 Ma and the influx of older LH source materials at ∼11 Ma in Nepal (DeCelles et al., 1998a;Chesley et al., 2000).

Various geochemical, mineralogical, and sedimentological in-dices that reflect intensity of chemical weathering and erosional flux of sediment, derived from cores in three oceans adjacent to the Himalayan system, provide independent evidence for an abrupt change in the nature and intensity of weathering processes at ∼16 Ma (Clift et al., 2008). Changes in Himalayan orography asso-ciated with southward shifting of the locus of thrusting at 16 Ma could have led to these changes through a number of different processes, including exposure of new source materials for both weathering and erosion, spatial and temporal changes in precipi-tation patterns, and associated changes in the locus of erosion.

6. Implications for thrust belt evolution and oLH emplacement in northern Indian

Our results may have important implications for recent mod-els of the tectonic evolution of the north Indian Himalaya. Existing models show a variety of timing scenarios for uplift and erosion of the oLH, including the Tal Group. These range early collisional shortening immediately following ∼50 Ma to as late as ∼5 Ma.

P.M. Myrow et al. / Earth and Planetary Science Letters 417 (2015) 142–150 149

The early balancing effort by Srivastava and Mitra (1994) shows Himalayan development as a function of deformation of a single layer-cake stratigraphy, such that Tal-equivalent rocks near the top of this stratigraphy were uplifted and eroded semi-continuously throughout all Himalayan shortening. Célérier et al. (2009a, 2009b)shows the oLH as the southernmost toe of the Tethyan fold-thrust belt, with uplift and erosion of the unit in the Eocene and Oligocene. Similarly, Ahmad et al. (2000) suggests that the oLH was the dominant exposed part of the eroding orogenic wedge at ∼30 Ma; thus, initial erosion of the oLH would precede 30 Ma. Webb and co-workers show stratigraphic connection between the oLH and Tethyan rocks to the north, but restrict initial uplift of the oLH rocks to ∼14 Ma and initial erosion of these rocks to ∼5 Ma(Webb et al., 2011b; Webb, 2013).

Stasis in the Eocene to early Miocene 187Os/188Os seawater record (Fig. 4), followed by a shift to increasing values at ∼16 Ma, is consistent with many lines of evidence to suggest transfer of movement from the MCT to faults further south (e.g., Tons Thrust) to cause onset of weathering of Os-rich rock. Thus, oLH erosion did not start prior to the Miocene (cf. Srivastava and Mitra, 1994; Ahmad et al., 2000; Célérier et al., 2009b), nor was it so late as the Miocene–Pliocene transition (cf. Webb, 2013). Our model, calling for a tectonic shift and onset of weathering at 16 Ma is supported by apatite fission track and zircon (U–TH)/He data that indicate up-lift of the LH on a system of thrusts south of the MCT, no later than ∼15 Ma (Deeken et al., 2011). As mentioned earlier, an 11 Ma shift in provenance likely represents onset of exposure of older lower Lesser Himalayan strata following erosion through the younger up-per Lesser Himalayan strata in the LH zone.

If correct, the near-synchronous changes in the Neogene 187Os/188Os and 87Sr/86Sr record of seawater, and the linked tectonic–ge-ochemical model described herein to explain these changes, at 16 Ma would provide the timing of uplift that could be incorpo-rated into future models and tested using various thermochrono-metric techniques.

7. Conclusions

Our stratigraphic and geochemical data have important implica-tions for the inferred uplift and erosional history of the Himalaya, including a potential explanation for marked secular changes in the isotopic geochemistry of the oceans. Lithostratigraphic and paleoenvironmental reconstructions indicate that Neoproterozoic to Cambrian strata formed a northward-deepening succession of strata that extended from the craton to the Tethyan Himalaya to the north. A proximal marine facies belt, which includes fossilifer-ous phosphatic shale and evaporite, is extensively exposed south of the Main Central Thrust well onto the craton, and along strike from north-central India to Pakistan. More distal deposits of the Tethyan belt consist of deltaic to shelf deposits. These Neoproterozoic to Cambrian strata present in the Lesser Himalaya (LH) were presum-ably eroded during uplift of the Himalaya during Cenozoic collision of India with Asia. We postulate that unroofing of these cover rocks along the Himalayan front began at ∼16 Ma, and would have re-quired an average exhumation rate of 1.9 mm/yr, consistent with published estimates of Cenozoic Himalayan denudation rates. First-order, geochemical modeling of the effects of erosion of the strata in this succession, and erosion patterns in general, indicate that these changes were likely responsible, in part, for secular changes in the Neogene 187Os/188Os and 87Sr/86Sr record of seawater. This has major implications for models concerned with global silicate weathering and CO2 consumption during the Cenozoic (e.g., Torres et al., 2014).

Furthermore, exhumation at 16 Ma was part of a significant tectonic shift in the evolution of the Himalayan orogeny, which resulted from a southward advance in the position of the active

thrust belt. Given the potential influence of this shift on global ocean chemistry for the rest of the Cenozoic, it should be a critical component of future models regarding Himalayan system evolu-tion.

Acknowledgements

We gratefully acknowledge the input from three excellent anonymous reviewers, and the guidance provided by editor An Yin. This research was supported by NSF grant EAR-1124518 to PMM and EAR-1124303 to NCH. NRM was supported by a UT Austin Jackson Postdoctoral Fellowship and a Yale Flint Postdoctoral Fel-lowship.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2015.02.016.

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