1 Introduction

The timing and nature of the collision between the Indian and Eurasian plates is a controversial and debated topic in geology (Rowley 1998). Generally, the collision timing is bracketed between 65 and 50 Ma (Cai et al. 2011; Searle and Treloar 2019). Although, the timing of the early collision has been constrained between ~59 Ma to as young as ~34 Ma (Aitchison et al. 2002; Ali and Aitchison 2008) or ~25 Ma (van Hinsbergen et al. 2012; Najman et al. 2017). Criteria like cessation of marine facies, the first arrival of the Kohistan–Ladakh arc (KLA) on the Indian Plate, and the relative position of India and Asia with time are relied upon to constrain initial collision (Najman et al. 2010). Apart from collision timing, recent studies have postulated a two-stage collision between the Indian–Eurasian plates. In the first stage, possibly the Indian Plate collided with the KLA along the Indus suture at ~50 Ma (Khan et al. 2009) or at ~85 Ma (Chatterjee et al. 2013). And in the second stage, with the Eurasian Plate along the Shyok suture zone at ~40–34 Ma. However, the idea of collision in two stages was contested by some workers (Najman et al. 2017; Bhattacharya et al. 2020) with the argument that both Indus and Shyok sutures were closed by 54 Ma. Though a discrepancy in crustal shortening to the tune of 1000 km may result from the wide variation in collisional timing (van Hinsbergen et al. 2012), a general consensus among workers is near ~3000 km of Neotethyan oceanic crust subducted before the initiation of collision (Rowley 2019; van Hinsbergen et al. 2019).

In this backdrop, the molasse (terrestrial) deposits of the Indus Basin along the Indus Suture Zone (ISZ), NW Himalaya, hold orogenic sedimentation records. These records are associated with the uplift and erosion of the southern margin of Asia due to the Indian–Eurasian plates collision since ~55 Ma. Thick sections of Indus Molasses have been well exposed along the ISZ and are mostly syn- to post-collisional, continental sedimentary rock sequences. These sequences constitute sediments (Searle et al. 1990) derived by variable environmental agents such as alluvial fans, fluvial and lacustrine, majorly supplied from the Ladakh batholith and to a minor extent from the Tethyan Himalayas. The literature portrays that Indus Molasses has been studied mostly in light of U–Pb geochronological and isotopic proxies, and very few studies (Yang et al. 2015; Ma et al. 2016) have taken into account the magnetic proxies. Therefore, to give input for the depositional age of Indus Molasses and also to give onset collision and upliftment information, a detailed paleomagnetic study has been planned. Before that, the samples were subjected to detailed rock magnetic analyses, as shown in the present study. The sole objective of this work is to find out whether Indus Molasse sedimentary rocks sufficiently carry reliable and accurate signals of the geomagnetic field throughout geologic time. As rock magnetic techniques are rapid and non-destructive, they are of great importance for studying the diagenetic and authigenic processes experienced by magnetic minerals of sediments/sedimentary rocks. Magnetic minerals are important for recording sedimentary paleomagnetic signals that are used widely in geosciences (Roberts 2015). Thus, rock magnetic study provides first-order information about the sample’s further usefulness in paleomagnetic study to understand the depositional history of Indus Molasses (local) and the upliftment history of the Himalayan mountains (regional) since the Cenozoic.

Therefore, the aim of the present rock magnetic investigations of Indus Molasse sedimentary rocks is to identify the constituent magnetic remanence-carrying minerals, quantify their grain size, detect any kind of digenetic interferences and overall gain knowledge about their suitability for paleomagnetic (magnetostratigraphy) studies. Apart from knowing the sample’s suitability for the intended paleomagnetic (magnetostratigraphy) study, the current rock magnetic approach allowed us to comment on the depositional environment and sediment provenance of Indus Molasse.

2 Geological setting

Indus Molasses are syn- to post-collisional sedimentary rocks of ISZ deposited during the Eocene to post-Eocene age. They are products of the collision between the Indian and Eurasian plates, representing a classic example of continent–continent collision. The beginning of molasse sedimentation along the Yarlung Tsangpo Suture Zone (Tibetan part) and the ISZ was regarded as Eocene (Searle et al. 1987). In the north of the ISZ lies the Lhasa–Karakoram block (affinity to the Eurasian Plate) and to the south lies the Tethyan Himalaya of northern India. The Indus Molasses covers a considerable part of ISZ in the form of a ~2000 km long arcuate belt that preserves marine and terrestrial sedimentary records. It is a folded and thrusted sequence of carbonate and clastic formations that unconformably rest over the Ladakh batholith (Clift et al. 2002; Wu et al. 2007). They are exposed along the southern margin of the trans-Himalayan magmatic arc and extend between Kargil (in the east) and South Tibet through Nyoma–Hanle of eastern Ladakh (Bajpai et al. 2004). The Indus Molasse also unconformably overlies the Indian passive margin unit of ophiolitic melanges close to Chilling (Robertson 2000) and south of the Nyoma region near Nidar village. The geological map of the easternmost part of Indus Molasses with sampling locations is depicted (figure 1).

Figure 1
figure 1

Schematic geological map of part of eastern Ladakh Himalaya constitutes the Indus molassic sedimentary rocks modified after Virdi (1986), Maheo et al. (2004) and Bhat et al. (2019). The sampling locations are depicted with red circles along the Nyoma–Rhongo transect of the easternmost Indus Molasse.

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Indus Molasses are best exposed and studied in the Zanskar Gorge region (Clift et al. 2002; Henderson et al. 2010), while other exposures further east in eastern Ladakh still need to be addressed and understood. The Indus Molasse (sedimentary rocks of Indus Basin) has been stratigraphically subdivided by different researchers (Garzanti and van Haver 1988; Searle et al. 1988, 1990; Sinclair and Jaffey 2001) into two major groups, the older is Tar Group and exposed conformably above this is the younger Indus Group. These groups are subdivided into four distinct formations, each latest by Henderson et al. (2010), in an attempt to reassess the stratigraphy of Indus Molasses. Here, the Tar Group has the oldest unit, named Jurutze Formation, overlaid by Sumda and Chogdo formations and the youngest unit of this group is the Nummulitic Limestone Formation, having conformable contact with the older Chogdo Formation. This Nummulitic Limestone Formation of the Tar Group has conformable contact with the base of the Indus Group’s Nurla Formation. This Nurla Formation is overlain by the Choksti and Lower Nimu formations. This Lower Nimu Formation has a faulted contact with the Choksti Formation (Searle et al. 1990; Sinclair and Jaffey 2001; Clift et al. 2002), which was not observed by Henderson et al. (2010), and suggested the contact is not conformable. The youngest sedimentary unit of the Indus Group is the Upper Nimu Formation, which is closely juxtaposed south of the Ladakh batholith. The literature studies and our field observations (figure 2) inform us that the lithology of Indus Molasses is majorly controlled by medium to coarse-grained sandstone, carbonates with intraclasts, siltstones, mudstones and partly phyllites. These sediments were mostly deposited in an alluvial fan, fluvial channel, and fluvial floodplain environments (Supplementary figure S1). Apart from this, usual sedimentary structures and features have also been observed. Small‐scale extensional faulting and compressional deformations have been recognized in Indus Molasses (Searle et al. 1990; Henderson et al. 2010).

Figure 2
figure 2

Field photographs from the Nyoma–Rhongo Indus Molasse section. (a) The fluvial unit overlain by a thick conglomerate horizon, (b) the conglomerate directly overlies the Ladakh batholith, (c) fluvial sandstone overlies the Ladakh batholith, and (d) the cross-stratified character of the thinly bedded fluvial sandstones.

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3 Sampling and methodology

Indus Molasse sedimentary rock samples were collected from fresh and unweathered outcrops following standard oriented sample collection technique using a Brunton compass and spirit leveller. Sampling was performed along the easternmost Nyoma–Rhongo road section of the ISZ, Ladakh Himalaya (Supplementary table S1). A total of 19 oriented samples from four sites were collected, a minimum of 4–5 oriented rock samples per site. All the collected samples have a distinct separation of around 15–20 m within the site depending on the lithology and freshness of outcrops, thus possessing a good representation of the easternmost part of Indus Molasse. Similar sampling approaches are targeted in the middle and western parts of Indus Molasses to understand magnetic mineralogical variations and eventually carry out rigorous paleomagnetic (magnetostratigraphic) studies.

All the samples were drilled, cut and ground to standard cylindrical specimens (2.5 cm in diameter and 2.2 cm in length) and subjected to bulk magnetic susceptibilities (k) measurements using AGICO’s KLY-5A (Czech Republic) susceptibility apparatus at a frequency of 976 Hz, housed at Paleomagnetism Laboratory of Wadia Institute of Himalayan Geology (WIHG), Dehradun. Those samples that yield higher magnetic susceptibility values are then considered for the detailed rock magnetic analyses (two samples/site) since they could sufficiently carry magnetic mineralogical information (Supplementary table S2).

In the laboratory, the petrographic and detailed rock magnetic measurements involving thermomagnetic analysis, magnetic hysteresis loops, acquisition of isothermal remanent magnetization (IRM) and coercivity spectra (backfield demagnetization) have been carried out. Petrographic analyses were carried out on thin sections of the same samples that were used for rock magnetism using a polarising microscope at the Sedimentology Laboratory of WIHG, Dehradun. The rock magnetic measurements were performed in air on powdered samples selected based on magnetic susceptibility measurements. These measurements were carried out using the Advanced Variable Field Translation Balance (AVFTB) instrument at the Paleomagnetism Laboratory of CSIR-National Geophysical Research Institute (NGRI), Hyderabad, India. The rock magnetic data and curves were visualized using the RockMagAnalyzer software by Leonhardt (2006).

4 Results

A vital aspect in rock magnetic studies is the presence of magnetic remanence carrying minerals in a rock and how the rocks acquire magnetism. This knowledge helps us to consider the rock units to understand the palaeogeography and tectonic evolution of the region. Here, the sample’s rock magnetic characteristics were studied to determine the magnetic mineralogy, grain sizes, domain states, and overall magnetic behaviour. We present the detailed rock magnetic results for the first time from the easternmost Indus Molasse sedimentary rocks, Ladakh Himalaya, India.

4.1 Petrography

In general, the Indus molassic sandstones are medium to coarse-grained, poorly sorted and arkosic in nature. They are mainly composed of plagioclase, alkali feldspar, biotite, muscovite, quartz, lithic fragments, and opaque grains in minor proportion (figure 3). Plagioclase has been altered to clay and secondary sparry calcite. In some places, euhedral plagioclase exhibits well-developed oscillatory zoning. Biotites are ubiquitous and show evidence of fracturing and deformation. Similarly, orthoclase grains are highly fractured. Quartz shows embayed grain boundary; in rare instances, some bipyramidal quartz has also been observed. The opaque grains present (figure 3) are sparsely distributed in the thin section and could mainly constitute the magnetites.

Figure 3
figure 3

Photomicrographs of Indus Molasse samples showing (a) zoned plagioclase, (b) dominance of fresh plagioclase feldspar, (c) presence of biotite, (d, e) presence of lithic clast and (f) opaque minerals. Orthoclase: Or, Quartz: Q, Plagioclase: Pl, Biotite: Bi, Lithic clast: Lc, Opaque: Opq.

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4.2 Thermomagnetic curves

The obtained thermomagnetic curves for the selected samples are shown in figure 4, where the sample’s susceptibility in response to increasing temperature is observed. The samples were heated from room temperature to 700°C (heating cycle) and then cooled back to room temperature (cooling cycle). The arrows placed near the heating and cooling cycles (figure 4) suggest the progression of heating and cooling during the experiment. The thermomagnetic curves of samples show the cooling cycles lie below the heating cycles and do not follow each other; they are irreversible indicating that the magnitude of susceptibility is affected by the thermal treatment. Among the analyzed samples, some show single and straight trajectories of the heating cycles that decay around a temperature window of 520–600°C (figures 4A, B, G and H). In other sample’s heating cycles (figures 4C, D, E and F) show a distinct decay in susceptibility with increasing temperatures slightly around 320–350°C and majorly around 520–580°C. The thermomagnetic curves of samples IM1.1, IM1.3, IM4.3 and IM4.4 (figures 4A, B, G and H) show straight and steady decay in susceptibility in the heating cycle and yield a single and stable Curie temperature (Tc) of ~580°C, the cooling cycles follow the heating cycles but with decreased magnitude. This Tc likely corresponds to a stable and predominant ferrimagnetic mineral, magnetite. Samples IM2.4 and IM2.5 (figure 4C and D) display a distinct decrease in susceptibility in the heating cycle, initially around 320°C (Tc1) and then show a major loss in signal at around 520°C (Tc2).

Figure 4
figure 4

Thermomagnetic (magnetic susceptibility, k vs. temperature, T) curves for samples. Arrows show the progression of heating (heating cycle) from room temperature to a maximum of 700°C and back to room temperature (cooling cycle). Sample numbers are shown in the boxes.

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The Tc1 corresponds to the presence of an unstable and weakly magnetized pyrrhotite phase (Dekkers 1989) that alters to a comparatively stable magnetic phase of Ti-poor magnetites (Tc2) regarded as secondary magnetite (of titanomagnetite solid solution series) that adds up to originally existing primary magnetite in samples. Samples IM3.1 and IM3.4 (figure 4E and F) also depicted an initial and gentle fall in susceptibility signal at around 350°C (Tc1) and later steeply decreased around 520°C (Tc2). The Tc1 in this sample could be due to the accessory presence of greigite mineral (Snowball and Thompson 1990; Snowball 1991; Roberts 1995) and later transformed due to heating in a relatively stable magnetic phase of Ti-poor magnetite, again regarded as secondary magnetite. The details about the samples Tc and the associated magnetic mineralogy are provided in table 1. Thermomagnetic curves of the samples are complex but reveal various components responsible for magnetic remanence in samples as indicated by mineralogical transformations during heating cycles (figure 4). Thus, the magnetic mineralogy of Indus Molasses is majorly controlled by magnetite (Tc = ~585°C), secondarily by Ti-poor magnetites (Tc = ~520°C), and accessorily by pyrrhotite (Tc = ~320°C) and greigite (Tc = ~350°C).

Table 1 The detailed rock magnetic characteristics of representative Indus Molasse samples.
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4.3 Magnetic hysteresis loops

Magnetic hysteresis loops in the maximum applied magnetic field of 1T were obtained for the selected samples after paramagnetic corrections (figure 5), and details of the hysteresis parameters are provided in table 1. The magnetization saturation for all the samples is visible below 300 mT and the paramagnetic contribution towards susceptibility is negligible, suggesting the susceptibility is mainly governed by ferrimagnetic minerals. All the analyzed samples displayed consistent and identical s-type hysteresis loops in the ‘pot-bellied’ shape that was thinly closed. Such thin nature of loops may be due to fine to medium-grained magnetic particles. Also, the low coercive magnetic grains in samples govern this shape of loops.

Figure 5
figure 5

Hysteresis loops of typical pot-bellied shape for the samples of Indus Molasses. The hysteresis loops are after the paramagnetic corrections. Sample numbers are shown in the boxes.

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The hysteresis parameters obtained from hysteresis loops were processed and the values of saturation magnetization (Ms), saturation remanent magnetization (Mrs), coercivity force (Bc) and coercivity remanence (Bcr) were noted. The schematic Day plot (Day et al. 1977) of the remanence ratio (Mrs/Ms) against the coercivity ratio (Bcr/Bc) has been developed to obtain information about the domain state of magnetic minerals and thus their average grain size (figure 6). The present study samples have a Mrs/Ms ratio ranging from 0.10 to 0.19 and a Bcr/Bc ratio ranging from 1.91 to 3.07. The Day plot distinguishes the single domain (SD), pseudo-single domain (PSD), and multidomain (MD) magnetic grains and thus serves as a better measure of the magnetic mineral’s grain size.

Figure 6
figure 6

The bivariate Day diagram for Indus Molasses: SD, PSD and MD are single-domain, pseudo-single domain, and multidomain fields for magnetic grains, respectively. The black circles represent samples that majorly belong to the PSD state. Mrs/Ms and Bcr/Bc ratios were calculated after the paramagnetic correction.

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The Day plot results reveal that most of the magnetic remanence carriers have grain size falls within the PSD region, possibly due to an admixture of SD and MD particles. Thus, the magnetic grain size can be revealed to be fine to medium-sized. The hysteresis properties of the present study samples are majorly commanded by the stable and fine-grained magnetic phase of magnetite and the influence of accessory phases seems negligible.

4.4 IRM acquisition curves and coercivity spectra

Magnetic mineralogy is a mixture of differential magnetization saturation levels with high and low coercivity mineral phases. The acquisition of IRM and the coercive force primarily gives knowledge of magnetic mineralogy. Here, samples were subjected to these analyses to modify and/or strengthen the outcomes of the above rock magnetic measurements.

The IRM-acquisition curves of the samples and their respective coercivity spectra are shown (figure 7), indicating the IRM signal went up at the beginning in response to the applied magnetic field. About 90% of the IRM curves have attained saturation by a field of 300 mT in most of the samples, which suggests the presence of low coercive magnetic minerals. These coercivity spectra (figure 7) support the contention that low coercive magnetic minerals are present, since the coercivity values are below 50 mT in all the samples. It is thus speculated that these samples contain soft magnetic minerals as the main magnetic remanence carriers. The elementary conclusion that can be drawn from IRM and coercivity analyses is that samples have saturated to 90% by 300 mT in IRM, and coercivity values below 50 mT are due to the predominance of soft and less coercive magnetic minerals like magnetite.

Figure 7
figure 7

IRM acquisition curves and in the inset are their respective coercivity curves (backfield IRM). Sample numbers are shown in the boxes.

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5 Discussion

5.1 Inferred magnetic mineralogy

The magnetic mineralogy associated with the sedimentary rocks of Indus Molasse is mainly attributed to ferrimagnetic magnetites (primary and secondary) and Ti-poor magnetites (belongs to a titanomagnetite solid solution series). These minerals not only have a dominant presence in samples but also have an association with other unstable and poorly present iron sulfides (pyrrhotite and greigite). The thermomagnetic curves are irreversible and indicate that pyrrhotite and greigite underwent thermal alteration during heating and were later transformed into magnetite (secondary) completely and added up to the primary magnetite. This neoformation of magnetite (secondary) during the heating cycles is variable with high thermal stability. The thermomagnetic curves of samples depict no secondary peak during the cooling cycle, which evidences magnetite’s sole predominance in the samples.

Pyrrhotite exhibits minimal presence in samples, identified by blunt loss of susceptibility in the heating curve around 310–330°C (which is Tc of pyrrhotite) in samples IM2.4 and IM2.5 (figure 4), which later completely transformed to a stable magnetic mineral phase. The first report of pyrrhotite in the Himalayas as the secondary remanence carrier was from the Manang area of Nepal (Appel et al. 1991) and later by Appel et al. (1995), Rochette et al. (1994), Schill et al. (2001), etc., in other parts of the Himalaya. These studies are in line with the current study results of pyrrhotite occurrence in the Himalaya and their decomposition into a newer and more stable magnetic phase of ferrimagnetic magnetite (secondary).

Another accessory magnetic mineral present in samples is greigite, an authigenic ferrimagnetic mineral associated with early digenesis of the sedimentary rock. A relatively small change in susceptibility in a heating cycle is observed at ∼350°C, after which susceptibility majorly dropped at ~520°C. This indicates the presence of a relatively small and unstable amount of greigite (IM3.1, IM3.4; figure 4) that decomposed to Ti-poor magnetites. The greigite gets completely decomposed in response to increased heating; hence, the cooling cycle shows only the presence of Ti-poor magnetite in samples (figure 4; Frank et al. 2007). Several proposed pathways for the greigite mineral formation require the content of sulfide and iron available at the time of diagenesis (Roberts and Weaver 2005). The presence of pyrrhotite and greigite in samples preserves magnetic remanence records that were acquired during final deposition/lithification and thus reliably renders the geomagnetic field variations (if any).

The dissolution and neoformation of magnetite (i.e., the secondary magnetite) have been identified in the current study, confirmed by the absence of pyrrhotite and greigite in the cooling cycles of thermomagnetic curves. The hysteresis parameters revealed that the pyrrhotite and greigite bearing samples show PSD behaviour, and the neoformed Ti-poor magnetites and magnetites also behaved similarly, i.e., falling in the PSD range. A remarkable work with magnetites was done by Dunlop (1995), who suggested that most magnetite grains that carry paleomagnetic remanence fall in the PSD range. Analogous to this, the dominance of PSD magnetite in the current study and their rock magnetic characteristics suggests that Indus Molasses carry sufficient magnetic remanence. Thus making them suitable for paleomagnetic and geotectonic-related studies (regional tectonics, structural geology, geodynamics, etc.). The obtained domain states of magnetic minerals using a Day plot are valid for pure (Ti-) magnetite bearing samples only, and no or poor contribution of accessory minerals (here pyrrhotite and greigite) is recorded.

5.2 Grain size effects and depositional environment

There are visible correlations between the particle size of sedimentary rocks and grain size of constituent magnetic minerals (Oldfield 1994; Zhang et al. 2001). The variations in magnetic properties usually are due to the differential grain size of magnetic minerals. The Indus Molasses has stable and consistent mineral magnetic characteristics. Hence, grain-size dependency is limited. Samples have fine- to medium-sized magnetic grains and are majorly dominated by the PSD state of magnetite. This implies not much variation in magnetic grain size and thus, sediment input to the ISZ is constrained from the local origin. Small seasonal streams and slope erosion could be the major agents of sediment supply for, at least, the easternmost part of the Indus Molasse. Stable and consistent magnetic mineralogy with observed sedimentary structures upholds the idea of ISZ marking the exact line of collision between the Indian and Eurasian plates.

The magnetite is chemically adequately strong during the sediment transport to survive, although these particles experience some superficial oxidization. The substitute occurrence of iron sulfides (greigite and pyrrhotite) in Indus Molasses indicates that the sediments transportation was less and the possible source area is nearer, which is also supported by petrographic evidence. The study area (easternmost Indus Molasse) was tectonically least active, and no major tectonic uplift and/or subsidence were experienced during their deposition and lithification. These interpretations are in accord with the results of Searle et al. (1990) and Aitchison et al. (2007), who suggested a tectonically quiescent period after the initial phase of thrusting between the Indian–Eurasian plates around the early Eocene.

Changes in magnetic parameters in sedimentary rocks can be caused by several factors, some of which can be attributed to the sediment depositional environment. Since the variations in rock magnetic properties of the studied samples are very less thus informs stable, consistent, and faster sedimentation. Uniform grain size and stable magnetic properties may further indicate a relatively steady sediment supply in the easternmost Indus Molasse basin. For extending the knowledge gained from the current study which is greatly based on rock magnetic characteristics, a detailed magnetostratigraphic approach is required. This will further permit to comment on the rate of sedimentation since the early to mid-Cenozoic times. Such an attempt will also allow detailed insight into the lithostratigraphic evolution of the Indus basin, Ladakh Himalaya.

In the photomicrographs (figure 3), the opaque grains are identified as magnetite grains of euhedral to subhedral textures and occur alongside other magnetic minerals (pyrrhotite and greigite). As a whole petrographic observations support the rock magnetic results.

5.3 Sediment provenance

With the India–Eurasia collision during ~50–55 Ma, the intense reworking of pre-existing lithology brought down the material to the Indus basin and may offer knowledge of the thermal evolution of the ISZ. The occurrence of pre-collisional marine sediments in ISZ (Searle et al. 1990) marks their paleo-Tethyan affinity.

One of the several sediment provenance methods is an examination of rock magnetic properties, which helps identify the sediment-source relation. In the present study, the resultant rock magnetic properties are consistent, it appears that this part of Indus Molasses (easternmost Indus Molasses) has been relatively stable and less possibly affected by any tectonic activity. Here, the magnetic mineralogy is dominantly governed by magnetites (primary = already existing in the rocks and secondary = neoformed by decomposition of unstable magnetic minerals) and accessorily carried by magnetic minerals such as greigite and pyrrhotite (iron sulfides). The lack of variations in rock magnetic properties is associated with quick sediment deposition in the easternmost part. The inferred magnetic minerals showed the effects of some chemical changes due to heat, which resulted in a small range of mineralogical transformations/alterations that slightly affected the magnetic mineral assemblages of the Indus Molasses. These slight changes may be associated with weak remagnetization (secondary magnetization) imposed in them with time by small-scale diagenesis (Lowrie and Heller 1982; Karlin and Levi 1983).

The magnetic mineralogy shows consistent rock magnetic characters and the occurrence of greigite (authigenic mineral) in samples, indicating that the sediment source for the easternmost part of the Indus Molasses is local. The north-lying Ladakh batholith is best possibly the source of detrital material since it falls close to the Indus Molasse basin. The sediments derived from such a close vicinity were less transported, deposited, finally lithified, and turned into sedimentary rock units of Indus Molasse. Early syn-collisional sediments were feasibly deposited directly upon an uplifted and eroded surface that developed over the Ladakh batholith (south of the Eurasian Plate); these sediments were entirely derived from the north, with the Ladakh arc and Karakoram terrane being the most likely sources (Zhou et al. 2020).

The nature of the Indus molassic rocks has been well examined by researchers (e.g., Garzanti et al. 1987; Searle et al. 1990; Clift et al. 2002; Henderson et al. 2010) and suggested two sources (1) Indian passive margin and (2) Ladakh batholith. The present study results agree with the literature work of Kumar (2008) on the magnetite series of granites in Ladakh batholiths, suggesting that the Indus molassic sediments were dominantly sourced from the Eurasian margin, i.e., Ladakh batholith. Also, the current study results are in agreement with the literature work of Sangode et al. (2013), suggesting the sediment inputs to the Leh Valley basin of the Indus River were from the near-source region. Similarly, Trivedi et al. (1982) reported the accessory presence of the magnetite and titano-magnetite in Gaik granites of Ladakh batholiths, thus satisfactorily supporting our contention of genetic link between Indus Molasse and Ladakh batholiths. Rather at present no direct confirmable indications of Indus Molasses’s Eurasian affinity (Ladakh batholiths) are available, and inputs from the Indian Plate cannot be denied. To examine this possibility, more robust work on similar lines to the present study should be conducted from other parts of the Indus Molasses and rocks of Ladakh batholiths.

Two ferrimagnetic iron sulfides (pyrrhotite and greigite) that are important to paleomagnetism analyses can carry substantial magnetization signals, and both are present in the Indus Molasse samples. Pyrrhotite and greigite are important minerals to consider when evaluating sedimentary basin’s thermal history (Akatakpo and Geissman 2022) like the Indus Molasse basin. The pyrrhotite in samples is locally attributed to the degradation of the pre-existing rock, in which the high temperature and stable ferrimagnetic phase get accumulated, i.e., magnetite. Likewise, greigite is an authigenic sedimentary mineral that forms during sediment accumulation by precipitation or recrystallization of pre-existing lithology. Such a significant occurrence of pyrrhotite and greigite with dominant magnetite in the mineral assemblage is indicative of the preservation of stable paleomagnetic directions (Kars and Kodama 2015; Roberts 2015; Yang et al. 2022) and may also be useful for strength estimations of the geomagnetic field (paleointensity). Eventually, we demonstrate that these easternmost Indus molassic sedimentary rocks preserve reliable records of the geomagnetic field and thus are suitable for paleomagnetic (magnetostratigraphy) and paleointensity studies.

6 Conclusions

We investigated the rock magnetic properties of samples from the easternmost Nyoma–Rhongo transect of Indus Molasse, Ladakh Himalaya. The results of thermomagnetic measurements indicate that the dominant magnetic mineral is magnetite with a range of Tc between 574 and 592°C. The hysteresis measurements indicate that samples mostly exhibit a pseudo-single domain (PSD) state of magnetic minerals with fine to medium grain size. The IRM acquisition curves approach to saturation by ~300 mT, with coercivity values below 50 mT, confirms the magnetic minerals in samples are soft and low-coercive. This predominant, high thermal stability and low-coercive magnetic mineral is regarded as magnetite (and Ti-poor magnetite), and other accessory magnetic minerals present are pyrrhotite and greigite (iron sulfides). Consistent and stable rock magnetic results in the present study indicate that the sedimentary rocks of the Indus Molasse might have lithified over well-developed palaeogeography with minor post-lithification variations.

The Indian–Eurasian plates collision is a global event of geoscientific interest that seems wrongly attributed to the widely accepted time (~50–55 Ma), but the on-site collision appears to have lagged ~20 Ma (Aitchison et al. 2007), thus proposing a requirement of critical paleogeographic reassessment. To solve this discrepancy in the exact timing of collision, an attempt at a paleomagnetic (magnetostratigraphy) study in combination with a detailed geological mapping could be vital. The present study validates the usefulness of Indus Molasses for the paleomagnetic (magnetostratigraphy) study.

We finally remark that the ambient environment that existed during the formation of Indus Molasse was likely stable with small-scale tectonic modulations (localized, i.e., in the easternmost part of Indus Molasse in ISZ) within the large-scale deformational regime (regional, i.e., Himalayan Orogeny).