PUBLICATIONS. Journal of Geophysical Research: Solid Earth. AMS NRM interferences in the Deccan basalts: Toward an improved understanding

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1 PUBLICATIONS RESEARCH ARTICLE Key Points: Magnetic studies in flood basalts Flow direction evaluation in lava flows Interference of induced and remanent magnetization AMS NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic s in flood basalts Stefan Schöbel 1 and Helga de Wall 1 1 GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, Erlangen, Germany Correspondence to: S. Schöbel, Stefan.Schoebel@fau.de Citation: Schöbel, S., and H. de Wall (2014), AMS NRM interferences in the Deccan basalts: Toward an improved understanding of magnetic s in flood basalts, J. Geophys. Res. Solid Earth, 119, , doi:. Received 4 SEP 2013 Accepted 24 MAR 2014 Accepted article online 26 MAR 2014 Published online 22 APR 2014 Abstract The evaluation of flow direction in volcanic rocks is among the most important applications of magnetic s studies. A statistically significant sample set of titanomagnetite-bearing lava flows from the Malwa Plateau, the northern part of the Deccan traps in India, has been investigated for a possible interference of induced and natural remanent magnetization (NRM). The NRM alters the scalar anisotropy of magnetic susceptibility (AMS) parameter and the orientations of the AMS principal magnetic axes, which are crucial for the evaluation of the flow direction. For cleaning of the NRM component, the lava samples have been demagnetized by use of an alternating field (AF) tumbling demagnetizer (peak fields of 100 mt) as previous studies have shown that static AF demagnetization can bias the results. Samples with normal magnetic s demonstrate a redistribution of their principal axes after the demagnetization. The evaluated flow directions show a more differentiated flow pattern of the Malwa area, which seems to fit better into the regional geological setting. In samples with inverse magnetic s, carrying a higher portion of single-domain particles, AMS principal axes remain unchanged after the demagnetization, indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations. According to these results, we propose that the AMS measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers (at least for samples with normal magnetic s) and therefore a more precise and better reflection of the actual mineral. 1. Introduction Magnetic analysis (in particular anisotropy of magnetic susceptibility (AMS)) has been established as a state-of-the-art method to determine lava flow directions and to identify volcanic feeder systems. In addition to the pioneering, mostly field based studies [e.g., Knight and Walker, 1988; Ernst and Baragar, 1992] and also texture and grain shape analyses showed that magnetic and mineral can be principally correlated in lava flows (electron backscatter diffraction/shape-preferred orientation (SPO) studies) [e.g., Plenier et al., 2005; Bascou et al., 2005; Chadima et al., 2009; Fanjat et al., 2012]. The correspondence of rock magnetic parameters to a flow-related mineral has been extensively studied sincethe 1960s (forreview,see Cañón-Tapia [2004]). AMS for rock studies has further been promoted due to the fast and easy acquirement of the magnetic. With modern, fully automatic devices, AMS can be measured in less than 2 min with a very accurate precision, and therefore, this method is successfully used as routine analyses, e.g., for evaluation in featureless, massive basalts [e.g., Staudigel et al., 1992; Glen et al., 1997; Raposo, 1997]. However, as the magnetic susceptibility is a complex parameter, information about the origin and the contributing minerals is indispensable as the interpretation of magnetic s in terms of their geological significance is not always straightforward. For a reliable interpretation, concerns are necessary about the flow regime [e.g., Cañón-Tapia, 2004, and references therein], field-dependent effects in Ti-rich oxides [de Wall et al., 2004; de Wall and Nano, 2004; Vahle and Kontny, 2005], the contribution of magnetic interactions [e.g., Stephenson, 1994; Cañón-Tapia, 1996; Muxworthy et al., 2004; Gaillot et al., 2006; Fanjat et al., 2012], and the origin and significance of inverse magnetic s [e.g., Potter and Stephenson, 1988;Jackson, 1991; Rochette et al., 1992]. Despite these extensive AMS studies, there is still a controversial opinion on the influence of a preexisting remanent magnetization (induced or natural remanent magnetization) on the magnetization acquired during magnetic measurement. Stacey [1961, 1963] was the first who suggested that an AMS can be produced by an alternating field. He already proposed that an applied alternating field (AF) could cause SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2651

2 changes in the domain alignment of multidomain (MD) grains, which is reflected in the AMS. Experimental studies by Bathal and Stacey [1969], Violat and Daly [1971], and Kapicka [1981] showed that a field-induced AMS in rocks can indeed result from an alternating field. In Stacey and Banerjee [1974] showed that the intrinsic susceptibility in magnetic grains is much higher parallel to the domain walls than perpendicular to them, and therefore, changes in the domain alignment have to be reflected in the magnetic. Experiments on artificial samples containing dispersed γfe 2 O 3 and magnetite particles by Potter and Stephenson [1988, 1990a, 1990b] indicate that the shape of the impressed susceptibility ellipsoid strongly depends on the particle size and on the applied field (AC or DC magnetic field). These results have been compromised in a theoretical model by Stephenson and Potter [1996]. More recent experimental studies were done by Henry et al. [2007] and Jordanova et al. [2007]. By the stepwise demagnetization of various kinds of rock types, they show how significant the influence of a preexisting magnetic remanence on the AMS can be. This was also shown by Schöbel et al. [2013]. They showed in an experimental study on basalts the strong influence of tumbling and static demagnetizing magnetic fields on the AMS ellipsoid. They argued that a static demagnetizing field will impress an additional anisotropy on the sample, while tumbling demagnetization will remove the magnetic remanence at least in the MD grains. However, even these studies clearly demonstrate that the magnetic susceptibility can be significantly biased by a remanent component, studies evaluating the impact for magnetic analysis on a larger data set and the implications for evaluation of magma flow directions are missing, as the former studies only showed the possible interferences of remanent and induced components for individual specimens. In this study, we will demonstrate the significance of such interferences for the evaluation of magma flow s for the Deccan basalts of the Malwa Plateau in India, which have general consequence on the interpretation of magnetic s in titanomagnetite (TM)-bearing basalts. This is achieved by comparison of AMS measurements and deduced magma flow direction before and after alternating field demagnetization and accompanied magnetomineralogical studies. Until now all constrains on flood basalts to identify lava flow directions based on the magnetic did not consider a possible interaction of remanent and induced magnetization (e.g., Ernst and Baragar [1992] on the Mackenzie dike swarm, Glen et al. [1997] and Tamrat and Ernesto [1999] on the Parana and Etendeka Large Igneous Province (LIP), Cañón-Tapia and Coe [2002] on the Columbia River flood basalts, Callot and Geoffroy [2004] on the Siberian LIP, and Plenier et al. [2005] and Fanjat et al. [2012] on the Kerguelen LIP). In order to obtain a statistic relevant database, almost 900 specimens from 179 samples were evaluated. The Deccan basalts with typical tholeiitic composition [e.g., Mahoney et al., 2000] are TM bearing and show a well-defined paleofield vector acquired during cooling of the flows. Furthermore, flood basalts are usually characterized by thick lava flows with internal laminar flow over long distances. They are therefore favorable for such a study. The paper reviews the recent status of knowledge of AMS in lava flows with special emphasis on flood basalts and then discusses the interactions of induced and remanent magnetization. We will show that removal of the natural remanent component will result in a better approximation of lava flow directions in this northernmost part of the Deccan Large Igneous Province (LIP). 2. AMS in Flood Basalts AMS measurements in flood basalts have a decisive advantage toward the most other volcanic settings, due to the unusual amount of lava which is poured from the source within a short time interval. Lava flows in LIPs tend to be thicker, and inflation is common due to high efflux rates [Self et al., 1996]. Considering the length and the extent, the transportation of the lava had to occur in thermally isolated channels and the central zone of the flow is therefore dominated by channeled lava flow. Thus, the flow regime in lava tubes is mainly laminar, especially in the inner parts of the tube. Laminar flow is one basic requirement for a reasonable analysis of the flow direction. Also, the vast size and the fast eruption rate of the lava flows enable sampling over a large area with more or less constant paleofield directions Origin of AMS The magnetic susceptibility of rocks is a product of many factors, where every mineral/component contributes to the total susceptibility. Concerning basalts, Fe-Ti oxides are usually the main magnetic contributor to the AMS. According to Tarling and Hrouda [1993], about 1 vol % of a ferrimagnetic component is sufficient to dominate all magnetic properties of the rock and in basalts (k typically in the range of 10 2 SI) SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2652

3 the share of diamagnetic and paramagnetic minerals (e.g., plagioclase, olivine, pyroxene, and hornblende) to the AMS can be neglected. The most influential parameters on the AMS in basalts are the magnetocrystalline anisotropy and the shape anisotropy of Fe-Ti oxides. However, the anisotropic distribution and the configuration of magnetic domains in ferromagnetic (in a broad sense) grains can also tribute to the AMS. The magnetocrystalline anisotropy can be defined as the energy necessary to deflect the magnetic moment in a crystal from the easy to the hard direction [Moskowitz, 1991]. Due to the high lattice symmetry in TM, the magnetocrystalline anisotropy often plays a minor role. However, the single-domain effect [Potter and Stephenson, 1988] predicts very high magnetocrystalline anisotropies for small particles. Also, the influence of magnetocrystalline anisotropy will increase in multidomain grains, when the magnetic domain size exceeds a certain size (20 μm) [Moskowitz, 1991]. For the AMS in basalts, the shape anisotropy of Fe-Ti oxides plays usually a major role. Ferromagnetics (in a broad sense) possess a magnetic charge on the grain surface. The magnitude of shape anisotropy depends on the shape form of the grain and the saturation magnetization. The easy axis of magnetization is hereby along the long axis of the grain. In lava flows TM crystallizes skeletal, interstitial xenomorphically, or almost euhedral dependent on their crystallization history. According to Hargraves et al. [1991], the anisotropic distribution of ferromagnetic (in a broad sense) particles can lead to magnetic interactions of individual grains when the particles are closely spaced (distribution anisotropy). However, the significance of magnetic interactions on the bulk AMS is still a matter of debate [e.g., Stephenson, 1994; Grégoire et al., 1995, 1998; Cañón-Tapia et al., 1996; Muxworthy et al., 2004; Gaillot et al., 2006; Fanjat et al., 2012]. On basis of magnetocrystalline, shape and distribution anisotropy Hargraves et al. [1991] postulates that the AMS in pristine igneous rocks is a direct or indirect reflection of preexisting silicate. Generally, the Ti-Fe oxides reflect the late phase of magma flow just before or after flow has stopped [e.g., Park et al., 1988]. Therefore, the Fe-Ti oxides crystallize within the residual magma volumes remaining after the earlier silicates (mainly plagioclase, minor pyroxenes, and olivine) have formed. The lath-like plagioclases are aligned with their long axes more or less parallel to the flow, resulting in a silicate framework, which shape is directly influenced by the direction of the flow. The Fe-Ti oxides crystallize within this silicate framework, and thus, their alignment reflects the direction of the lava flow. Alternatively, Cañón-Tapia et al. [1996] proposed a model where the local shear plane, defined by individual fluid elements, controls the distribution of the ferromagnetic (in a broad sense) component. The distribution of every particle of all grain sizes is a function of the deformation of each fluid element, and this distribution controls the orientation of the principal AMS axes. Furthermore, secondary processes such as mineral growth during hydrothermal alteration and tectonic fracturing/deformation can have significant influence on the magnetic signal of a rock. Studies by Ellwood [1981], Walderhaug [1993], Just et al. [2004], and de Wall et al. [2010] demonstrate how secondary effects can influence the magnetic properties and alter the initial directions of the principal magnetic axes. However, the influence of the configuration of magnetic domains in ferromagnetic (in a broad sense) grains on the magnetic has not been considered in the very most AMS studies, although various studies (see section 1) showed that a remanent magnetization can have a tremendous impact on the AMS Magnetic Fabric Geometries Most studies on basalts show that the magnetic lineation (kmax) is usually aligned parallel to the flow direction and the magnetic foliation (kmax + kint) parallels the flow plane [e.g., Cañón-Tapia et al., 1996; Tamrat and Ernesto, 1999; Herrero-Bervera et al., 2002; Bascou et al., 2005]. The axis representing the direction of the lowest magnetic susceptibility kmin has to be therefore normal to the flow boundary. In such a case, the lava flow exhibits a normal magnetic. Unfortunately, this trivial linkage is not always given. Due to the complexities of lava flow rheology and magnetic behavior, various different s (normal, inverse, oblique, complex) can occur. Based on the orientation of the magnetic foliation in respect to the flow plane, normal, inverse, and oblique magnetic s can be distinguished: 0 29 normal, oblique, and inverse magnetic s. Besides, samples showing no distinct cluster of their principal axes SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2653

4 Figure 1. Idealized normal and inverse magnetic in respect to the flow axes as it is explained by the SD effect for a N-trending, horizontal lava flow. A normal magnetic is characterized by a low fraction of SD particles, while inverse magnetic s can be caused by a high portion of SD grains. The respective principal magnetic axes are displayed in a lower hemisphere Schmidt net: square = kmax; triangle =kint; circle =kmin. suggest a more complex correlation of the magnetic to the flow geometry and are impractical for flow analysis. The common approach to explain the phenomenon of inverse s was introduced by Potter and Stephenson [1988]: the so-called single-domain (SD) effect. The SD effect predicts that inverse magnetic s can occur due to the present of iron-bearing carbonates, tourmaline, cordierite, goethite, where the maximum low-field magnetic susceptibility is normal to the long axis of the grain [Rochette et al., 1992]. In ferrimagnetic samples, inverse s are related to a high portion of SD TM. For SD grains, the applied field does not increase the net magnetization (SD particles are magnetically saturated) but changes its orientation [Jackson, 1991]. Therefore, a weak field (as applied during AMS measurements) perpendicular to the long axis of an elongated SD grain causes the magnetization to rotate slightly toward the field direction in order to minimize the total energy, and thus, the susceptibility is not equal to zero (δm/δh 0). If the field is applied parallel or antiparallel to the long axis of the SD grain, there is no angular gradient in the energy balance to rotate the magnetization, and therefore, the susceptibility is zero (δm/δh =0)[Jackson, 1991]. In summary, in magnetic SD grains the magnetic susceptibility is maximum perpendicular to the long axis and zero parallel to it [Potter and Stephenson, 1988], which gives rise to a strong magnetic anisotropy. If multidomain (MD) and SD TM are aligned with their long axes parallel to the flow direction, inverse magnetic s can be solely explained by the SD effect (Figure 1). The ratio of SD to MD grains is herby crucial. Since most basalts contain a mixture of all domain sizes and therefore a variable fraction of SD grains, the threshold value of SD grains forcing a normal magnetic to become inverse is hard to evaluate. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2654

5 Next to normal and inverse magnetic s, Rochette et al. [1992] argued that additional types, intermediate s, may occur when SD und MD grains are mixing. One of those intermediate s predicts a swap of kmax and kint axes while the kmin axis stays perpendicular to the flow plane. This kind of intermediate is hard to recognize as the magnetic foliation is still parallel to the flow plane. Therefore, every specimen which has an apparent normal magnetic must be suspected to possess an intermediate instead. However, the existence of intermediate s due to the mixing of MD and SD grains is not physically proved, and such a differentiation is not handy for a practical use. We therefore suggest that the discrimination of magnetic s should be based on the relation of the magnetic foliation to the flow plane and not on the orientation of the individual principal axes. Beside the SD effect other mechanisms are also discussed to result in inverse magnetic s: 1. Hargraves et al. [1991]andStephenson [1994] postulated that magnetic and rock may differ when interactions between ferrimagnetic grains occur. Cañón-Tapia [1996] showed in a 3-D model, how this can lead to inverse magnetic s. In a recent study Fanjat et al. [2012] suggested magnetic interactions as reason for abnormal s in lava flows from the Kerguelen Archipelago. 2. Rochette et al. [1991] observed in dikes that inverse s may be generated due to hydrothermal activity, as clusters of euhedral magnetite are aligned parallel to the fluid migration direction. 3. Merle [1998] showed that permutations of the magnetic axes can be explained by local stress changes in the lava flow due to the position of sampling or the preexisting topography. 4. Archanjo et al. [2002] proposed that interstitial magnetite grains can crystallize both along and perpendicular to the lineation given by the silicate framework producing abnormal s. 5. In a theoretical study, Cañón-Tapia and Chávez-Álvarez [2004] focused on the particle movement in dikes and the influence on the AMS. Their assumptions are based on the equations of Jeffery [1922] which predict a cyclic movement of ellipsoidal bodies in a viscous fluid. However, recent studies suggest that in natural conditions with higher shear strains this cyclicity vanish and the elements are stabilized closely parallel to the flow/shear plane [e.g., Arbaret et al., 2013; Jezek et al., 2013]. 6. It is also a conceivable approach that inverse magnetic s can occur due to the presences of magnetite dendrites. Dendritic growth of magnetite indicates rapid cooling. If the long axis of those skeletal magnetites is normal to the cooling surface, the magnetic lineation would be perpendicular to the flow plane. The influence of dendritic TM on the AMS has been shown by Shaar and Feinberg [2013] in a recent study. 7. Schöbel et al. [2013] observed changes in the magnetic after the application of demagnetizing fields. They propose that a strong NRM can alter the magnetic. As the alignment of the domain walls can influence the direction of the principal magnetic axes, it is likely that a remanent magnetization can also cause inverse magnetic s NRM Acquisition and Coercivity of Titanomagnetite-Bearing Basalts The NRM acquisition in lava flows is of complex nature. On the crystal scale this process is mainly a function of temperature, composition, grain size, and grain shape. All these factors take control on the coercivity of the minerals. Above the Curie temperature, the ferromagnetics (in a broad sense) cannot hold a remanent magnetization and behave paramagnetic. When cooling below the Curie temperature, but above the blocking temperature, the magnetic moments in the SD particles are still able to flip among the easy axes. With decreasing temperature, the magnetic moments become fixed with a statistical preference in direction of the applied magnetic field. Due to long relaxation times for SD grains and high Curie temperatures, the TRM for low-ti magnetites is stable at room temperature in SD and pseudosingle-domain (PSD) particles. The acquisition of a NRM in larger grains with a higher number of magnetic domains is more complex, and there is no theory that predicts the relaxation time as a function of the grain size. However, prior to demagnetization, the alignment of the domain walls in MD grains are mostly dominated by magnetocrystalline and shape anisotropy. Furthermore, a possible TRM and viscous remanent magnetization may also contribute to the arrangement of the domain walls. It is to assume that with increasing grain size, the acquisition of a TRM becomes more ineffective and the influence of viscous magnetization increases. However, deuteric exsolution in the TM can reduce the effective magnetic grain size drastically. Large MD grains can be partitioned in several smaller SD districts. This can increase the ability to preserve the TRM. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2655

6 Figure 2. (a) The Deccan Large Igneous Province and the correlated major rift systems. The working area as indicated by a rectangle covers the western part of the Malwa Plateau. Map modified after modified from Roy [1962] and Mahoney et al. [2000]. Underlying satellite image belongs to the Microsoft Corporation. (b) Digital elevation model (DEM) of the working area with indicated sites based on the data provided by the ASTER G-DEM project (METI and NASA). The Ti content will have a significant influence on the ability to preserve a TRM. Primarily, an increase in Ti will reduce the Curie and blocking temperatures and therefore reduce the relaxation time. Secondly, the Ti substitution reduces the magnetostatic energy in the TM. On the one side this will weaken the stability of a metastable domain alignment in MD grains and lead to lower coercivities of those grains. On the other side the effective magnetic domain size can be reduced, as the critical grain size for SD particles increases, which will result in higher coercivities. 3. Geological Setting For this key study, lava flows from the Malwa Plateau, the northern part of the Deccan traps (Figure 2a) in India, have been investigated. With an area of ~80,000 km2 and an average height of 500 m above mean sea level (amsl), the plateau represents an important but until now almost unaccounted part of the Deccan Large Igneous Province. The Malwa traps are mainly overlying rocks from the Neoproterozoic Vindhyan Supergroup and in its western and southern parts Archean and Proterozoic rocks of the AravalliBundelkhand craton. The plateau is bordered to the south by the Narmada valley, which is part of a graben structure related to the Narmada-Son Lineament. The prominent tectonic structure has intensely been studied by geological and geophysical survey [e.g., Kaila and Rao, 1986]. This rift system has a long-termed geological history with several events of reactivation (for a review, see Chamyal et al. [2002]). The lava flows of the Malwa Plateau reach from the 74 E to the 79 E longitude and from 22 to 25 of northern latitude (Figure 2a). The thickness of the lava sequence varies significantly. In the very south of the plateau, close to Mhow, a few mounts reach up to 860 m amsl and form the highest peaks of this province. Here the lava pile is summing up to a total thickness of about 710 m. In the more distal areas the lava thins out to a few tens of meters. In the central plateau the thickness of the lava flows is hard to evaluate, as there is a lack of reliable geophysical data. The Malwa Plateau consists of mainly simple lava flows, with thickness varying between 10 and 20 m. Due to the low relief, poor exposes, and the partially strong vegetation, it is difficult to trace the flows over a longer distance, but it seems that the thickness of the flows stays more or less constant on respectable distances. Usually, the flow sequences are almost horizontal but can regionally show a slight tilt, which is most likely due to eruptive tectonic processes. Bondre et al. [2004] described the morphology and the emplacement of the Deccan Trap flows with simple lava flows displaying three zones: a crustal, a central, and a basal zone, whereby the central lower part of the flow seems to be suitable for AMS samples, as recommended in other studies [e.g., Bascou et al., 2005]. At the very bottom and the upper part of lava flows, turbulent or inhomogeneous flow is most likely, and thus, these parts are not appropriate for sampling. In thin lava flows, SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2656

7 laminar homogenous flow is rare. Thin pahoehoe flows, e.g., tend to prograde in a spectrum of directions, which results in difficulties when it comes to identify a representative uniform flow direction. Sampling was conducted along a 100 km N-S and a 100 km E-W transect along the western and southern edges of the Malwa Plateau (Figure 2b). Around 900 specimens of 179 samples, taken from 112 sites have been evaluated for rock magnetic measurements. The oriented hand samples were taken with the help of magnetic compasses. 4. Sampling and Methods Samples were cored for 10.8 cm 3 standard cylinders. For each flow a minimum of eight specimens were extracted. The initial AMS was measured for all specimens. The AMS measurements were conducted by a Kappabridge (KLY-4S; Agico). AMS can be described as a symmetric second-rank tensor with three principal axes (kmax kint kmin). The axes form an ellipsoid and are best displayed in a lambert azimuthal equalarea projection, the Schmidt net. The mean magnetic susceptibility kmean is the arithmetic mean of the principal susceptibilities: kmean ¼ ðkmax þ kint þ kminþ=3 The degree of anisotropy of the AMS ellipsoid can be expressed by the corrected anisotropy P [Jelínek, 1981]: n h io 1=2 P ¼ exp 2ðln kmax ln kmean Þ 2 þ 2ðln kint ln kmean Þ 2 þ 2ðln kmin ln kmean Þ 2 The geometric form of the AMS ellipsoid is described by the shape factor T [Jelínek, 1981]: T ¼ ½2ðln kint ln kmin Þ= ðln kmax ln min Þ 1Š In order to get constraints on the TM composition, temperature-dependent susceptibility measurements ( 195 to +700 C) were done by a temperature controlling device (CS-2; Agico) for the KLY-4S. To evaluate field-dependent effects, specimens were measured using a field of 100 and 300 A/m. Furthermore, the bulk susceptibility was measured in 21 different fields (2 450 A/m). Grain and domain size information is obtained from hysteresis experiments done by an alternating gradient force magnetometer (MicroMag Model 2900; PMC) at the Department of Geosciences of the University of Bremen. To discriminate between inverse stable and inverse unstable s, specimens with inverse magnetic s were demagnetized by a static alternating field. The terms stable and unstable magnetic were introduced by Schöbel et al. [2013]. They describe the behavior of the principal magnetic axes due to static demagnetization. Specimens with stable magnetic s will show no variations in the orientation of the principal magnetic axes after the static demagnetization. In contrast, specimens with unstable magnetic s show a redistribution of the principal magnetic axes after the application of a static magnetic field (peak field of 100 mt). The magnetic lineation will get herby aligned parallel to the applied field, while the other principal axes follow the geometry of the applied field. For this discrimination, specimens were statically demagnetized in three orthogonal directions by an AF demagnetizer (MI AFD 300; Magnon), whereby the last direction was applied parallel to the z direction (axial direction) of the specimen. Note that this discrimination of stable and unstable inverse magnetic s is not particular necessary to achieve lava flow directions, as specimens bearing an inverse magnetic should be excluded from the lava flow direction evaluation. To evaluate the influence of the natural remanent magnetization on the magnetic, specimens with normal and inverse (unstable + stable) magnetic s were demagnetized by a tumbling AF demagnetizer (High Field Shielded Demagnetizer; Molespin). Tumbling demagnetization was conducted with peak fields of 100 mt. The NRM has been measured with a superconducting rock magnetometer ( ; LC2G Enterprises) at the Magnetiklabor Grubenhagen from the Leibniz Institute for Applied Geophysics, Hannover. Samples have been stepwise demagnetized (peak field of 100 mt), and characteristic remanent field vector has been determined by applying the routine data evaluation and quality control (Zyderfeld evaluation [Zijderveld, 1967]; principal component analysis, PCA [Kirschvink, 1980]). SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2657

8 5. Mineralogical, Textural, and Remanence Characteristics of Discriminated Fabric Types Out of 179 samples (898 specimens), 81 samples (471 specimens) show a normal magnetic, 46 samples (204 specimens) an inverse magnetic, 30 samples (134 specimens) an oblique magnetic, and 22 samples (91 specimens) an undefined magnetic. For further analysis samples with oblique and undefined magnetic s were excluded Composition of Titanomagnetites Analysis of temperature and field dependency of magnetic susceptibility was done on a statistically significant number of samples to get information on the TM composition for the discriminated types. The Curie temperature decreases with substitution of Ti, and thermomagnetic curves can give important information on stability and exsolution of TM in subaerial lava flows [e.g., Kontny et al., 2003]. A higher Ti content in TM reduces the magnetostatic energy, and in MD grains this will affect the stability of the domain walls. Field variation effects do not occur in pure magnetite but will increase with rising Ti content due to increased nonlinear behavior between applied AC field and magnetization [Jackson et al., 1998; de Wall, 2000;de Wall and Nano, 2004; Hrouda, 2002]. Also, the magnetic domain size, temperature, and magnetic anisotropy may influence the nonlinear behavior of induced magnetization in TM [Vahle and Kontny, 2005]. In this study we used the k bulk(hd) factor to evaluate the magnitude of field dependence as introduced by de Wall [2000]: k bulkðhdþ ð% Þ ¼ k bulk300a=m k bulk30a=m =kbulk300a=m 100 For evaluation of the TM composition, the normalized k bulk is plotted versus the increasing field and resulting curves are compared with the course of temperature-dependent magnetic susceptibility as shown for some examples in Figure 3a. While the thermomagnetic curves give information on the variability of TM composition within a sample, the k bulk(hd) value gives an average measure of TM composition and grain size as well as its contribution to the bulk susceptibility. The varying values of the k bulk(hd) parameter ( 1% to 35%) in the Malwa basalts reflect their heterogeneity in TM composition. The k bulk(hd) values for the respective types are shown in a histogram (Figure 3b). The histogram reflects that samples with unstable magnetic s are characterized by very low k bulk(hd) values, as over 83% of those samples show a k bulk(hd) between 1 and 0%. Samples with stable magnetic s show higher k bulk(hd) values as almost 50% of the samples show k bulk(hd) values over 10%. Samples with normal magnetic s usually show intermediate k bulk(hd) values. The thermomagnetic curves give indication to roughly three types of magnetic carriers: high-ti TM (Curie temperature at C), intermediate-ti TM which may oxidize to magnetite during further heating (Curie temperatures at C and C), and low-ti TM, usually associated with a phase of intermediate-ti TM (Curie temperatures at C and C). Due to scanning electron microscope (SEM) and energy-dispersive X-ray analysis, pyrrhotite (Curie temperature of 320 C) can be excluded as a main magnetic carrier. Except for the samples containing pure magnetite, thermomagnetic curves are usually not reversible. The results of field dependency and temperature-dependent susceptibility measurements are compiled in Figure 3a for four representative samples. The figure correlates the inverse stable magnetic to the presence of high-ti TM, which causes high k bulk(hd) values. On the contrary, specimens with inverse unstable magnetic s usually possess due to the presence of low-ti TM very low k bulk(hd) values. Such a clear correlation could not be recognized for specimens with normal magnetic s. In addition, Table 1 compiles the results for the discriminated types and gives information about the occurrence of low-/intermediate-/high-tm for the respective type. Specimens with unstable inverse magnetic s (100% of all specimens) possess a low-ti TM component but rather seldom a high-ti TM phase (25%). This is reflected in the mean k bulk(hd) values. Specimens with stable inverse magnetic s usually bear high-ti TM (89%) and show higher k bulk(hd) values. Specimens with normal magnetic s do not follow such distinctive trends SEM Analysis Due to different cooling history, Fe-Ti oxides can have a variety of grain sizes and textures. Euhedral TM feature cubic crystal growth (Figure 4a) but subhedral minerals are also common. Very small TM crystals are usually surrounded by a fine matrix (Figures 4c and 4d). Their distribution is generally controlled by larger SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2658

9 Figure 3. (a) Relationship of field-dependent effects, thermomagnetic curves, and stability of inverse magnetic s shown for four representative samples of the Malwa basalts. (b) Histogram of the k bulk(hd) values for the respective magnetic types. silicate minerals (Figure 4d). The presence of dendritic and skeletal crystals (Figure 4b) indicates a rapid growth due to fast cooling [e.g., Bryan, 1972]. Skeletal crystallization produces not only MD grains but also a large amount of PSD/SD particles. The skeletal crystals are usually very complex structures and are either continuous or separated in segments. As separated segments are aligned close to each other, magnetic interactions are likely and can result in complex magnetic behavior [e.g., Shaar and Feinberg, 2013]. In this study skeletal and dendritic crystals are present in normal and in inverse magnetic s and are therefore not a critical indicator for the respective type. Ilmenite (Il)/TM exsolution lamellae (Figures 4e and 4f) can be common in some specimens. They can account as a clear evidence for high-temperature oxidation of TM. Il/TM exsolution occur deuterically during cooling of subaerial basalts between 500 and 900 C [Dunlop and Özdemir, 1997]. Vahle and Kontny [2005] showed that exsolution lamellae in magnetites (when frequent) can have a significant impact on the AMS, as they Table 1. The Percentage of High-/Medium-/Low-Ti Magnetites Occurrences as Evaluated by Thermomagnetic Measurements and the Mean k bulk(hd) Values for Normal and Inverse Magnetic Fabrics a AMS Fabric Type n High-Ti TM Occurrence (%) Medium-Ti TM Occurrence (%) Low-Ti TM Occurrence (%) Mean k bulk (Hd) Normal Stable Inverse Unstable Inverse a High-Ti TM (Curie T: C). Medium-Ti TM (Curie T: C). Low-Ti TM (Curie T: C). SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2659

10 Figure 4. Backscattered electron images of characteristic features of the ferromagnetic (in a broad sense) phase. TM can crystallize in various shapes: (a) euhedral, (b) skeletal, (c) subhedral, and (d) tiny and slender. TM/Il exsolutions appear (e) simple shaped or (f) complex with fine Il lamellae. reduce the degree of anisotropy. Furthermore, exsolution may reduce the magnetically effective grain size as exsolution lamella may be frequent and narrowly spaced. The correlation of size and texture of TM to the respective type is not trivial. Especially normal magnetic specimens can show a wide spectrum of TM grain shapes and sizes with various compositions. However, samples with stable inverse magnetic s often show a high fraction of very small TM crystals (< 1 μm), which are surrounded by a dark, almost glassy matrix. Their distribution and alignment can be a function of larger silicate minerals or a complex cooling history. Tiny crystals (< 1 μm) are either aligned in chains or what looks randomly distributed. It has been noted that samples with unstable inverse magnetic s usually do not feature a high fraction of this small TM phase. In specimens with inverse magnetic s TM/Il exsolution lamellae seem to appear more frequently Magnetic Hysteresis The magnetic domain size has a fundamental influence on the magnetic remanence. The analysis of the magnetic hysteretic behavior can give information about the magnetic domain sizes. The Day plot SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2660

11 Figure 5. (a) Hysteresis loops of four specimens showing normal/stable inverse magnetic s. (b) Day plot [Day et al., 1977] for specimens with normal and stable inverse magnetic s. [Day et al., 1977] is generally used for a discrimination of the magnetic domain sizes, even though many previous studies showed that almost all relevant samples yield hysteresis ratios in the array for PSD particles [e.g., Raposo, 1997; Archanjo et al., 2000; Callot et al., 2001; Zhu et al., 2003; Callot and Geoffroy, 2004; Raposo et al., 2004; Chadima et al., 2009]. As specimens with normal and stable magnetic s show vast differences in their magnetic behavior, magnetic hysteresis measurementsweredonefortherespective types (total of 23 specimens). Peak fields of 0.3 T were used for the low loop and 1 T for the high-loop measurements. However, during the hysteresis run, specimens with normal and stable inverse s showed a similar behavior, which is expressed by similar hysteresis loops (Figure 5a). All measured specimens showed herby typical PSD hysteresis loops [e.g., Tauxe et al., 2002]. This can also be realized in the Day plot (Figure 5b), where specimens with normal and stable inverse magnetic s plot in the PSD box. Therefore, a discrimination of the s type on basis of the hysteresis parameters is not possible. This is most likely due to the mixture of SD/PSD and MD particles in the natural samples Coercivity and Stability of Remanence In comparison to the intensely studied main sections of the Deccan traps in the Western Ghats [e.g., Vandamme et al., 1991; Vandamme and Courtillot, 1992; Chenet et al., 2007, 2008; Jay et al., 2009], SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2661

12 Table 2. Median Values of Some Remanence Parameters for the Respective AMS Fabric Types AMS Fabric Type n NRM Intensity (A/m) Median Destructive Field (mt) Residual Magnetization at 100 mt (%) All Normal Stable Inverse Unstable Inverse paleomagnetic studies on the Malwa basalts are scarce and focus on limited sections in the southern part of the plateau [Bhalla and Rao, 1974; Rao and Bhalla, 1981; Khadri and Nagar, 1994; Khadri, 2003] as other parts of the Malwa lie in more remote areas. In the frame of this study, the characteristic remanent magnetization (ChRM) for a total of 182 specimens has been evaluated. Measurements show that the Malwa basalts generally hold a high NRM intensity (median of 182 specimens: 5.5 A/m) and a stable NRM as typical in TM-bearing basalts. Comparing the remanence characteristics of samples with normal and inverse s, it is hard to define a clear trend on basis of the type. This is interpreted as a consequence of the mixed domain state (SD, PSD, MD) in all specimens. The median values for the respective types are listed in Table 2. The histograms of the NRM intensities and the median destructive field (MDF) values are shown in Figure 6. In all AMS s a strong variation of parameter is realized, e.g., for the MDF which varies between 4 and 71 mt in specimens with normal, between 3 and 64 mt for specimens with stable inverse, and between 6 and 63 mt for specimens with unstable inverse s, respectively. The AF demagnetization curves with respect to the types are shown in Figure 7. The specimens of the groups show hereby similar variations. All types show a large scatter in the behavior of the magnetic remanence intensity with increasing demagnetizing fields. Figure 6. Histograms of the (a) NRM intensity and the (b) MDF values for the respective magnetic types. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2662

13 Figure 7. AF Demagnetization curves for the respective types. The plots show the behavior of the remanent magnetization intensity with increasing demagnetizing field. Specimens with stable inverse magnetic s tend to possess higher NRM intensities (Figure 6a). In those specimens the remanent magnetization is more stable than in normal and unstable inverse s as indicated by slightly higher MDF values (Figure 6b and Table 2). The higher MDF values indicate a higher portion of not demagnetized ferromagnetic (in a broad sense) minerals (SD particles or high coercive minerals) at demagnetizing fields of 50 mt. However, specimens with stable inverse magnetic s are not characterized by a higher residual magnetization after 100 mt AC demagnetization (Table 2) Paleofield Direction in Malwa Lava Flows The ChRM could be deduced for 153 specimens by use of the principal component analysis (PCA) [Kirschvink, 1980]. After some test runs, the specimens were AF demagnetized with an average of 10 steps in fields up to SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2663

14 a) N b) N c) N n=182 n=153 n=519 Figure 8. Stereographic, equal-area projections of (a) the NRM and (b) the ChRM directions of individual specimens from this study. (c) Evaluated mean remanent magnetic directions for 519 lava flows from former studies as summarized by Vandamme et al. [1991] (recalculated for Mumbai). Full squares in upper hemisphere (normal polarity) and open squares in lower hemisphere (reverse polarity). Fisher mean vectors [Fisher, 1953] with 95% confidence intervals are indicated as stars. 100 mt. The paleomagnetic studies reveal a paleofield direction acquired during the drift of the Indian Plate with mean directions of 343/ 36 for specimens with a normal magnetization and 158/50 for specimens with a reverse magnetization (NRM: 356/ 20; 150/68). As in this study the interference of a remanent direction on the AMS is of more importance than a precise deduction of the virtual geomagnetic pole; the measured/deduced remanent directions of all specimens were included in Figures 8a and 8b. Note that these projections are showing the data for individual specimens all over the study area, which might explain the large scatter of data. However, the measured/deduced NRM/ChRM directions are in general agreement with the remanent directions of previous paleomagnetic studies (Figure 8c). Typical demagnetization behavior for characteristic specimens is shown in vector diagrams (Figure 9, Zyderfeld plots) [Zijderveld, 1967]. The most specimens show a secondary low coercive direction isolated between 5 and 10 mt in the direction of the present-day Earth s magnetic field. After removal of Figure 9. Six examples of orthogonal projection of alternating field demagnetizations of representative samples. Filled circles in orthogonal projections correspond to the N-S/E-W planes, open circles to the N-S vertical planes. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2664

15 Table 3. Median Values of the Scalar Magnetic Susceptibility Parameters for AMS Fabric Types a Fabric Type n (Samples) n (Specimens) kmean (SI) P T Total E Normal E Stable Inverse E Unstable Inverse E a Measurements in AC field of 100 A/m prior to any demagnetization. this component, the original magnetization and sometimes other secondary components are still present. The cylinders 152-3_2 and 165-2_1-1 are examples of specimens showing a single-characteristic component with normal polarity. The characteristic component heads toward the diagram center and reaches the origin after a certain demagnetization step. The magnetization for specimen 165-2_1 is hard, and a field of 100 mt is insufficient to remove the magnetization completely. This hard remanent magnetization can indicate a small fraction of high coercive (titano)hematite. The specimens 11 1and5 1 possess a reversed polarity. Both specimens show a slight curvature after the removal of a viscous magnetization. This second component is removed at a field of 20 mt and 30 mt, respectively. Some specimens are not suitable to record information of the ancient magnetic field as there is no confident result obtained after the removal of the secondary components. The cylinders 59-1_4 and are shown as an example for these poor quality specimens. 6. AMS Results Before and After Demagnetization 6.1. Initial AMS The magnetic characteristics presented in this section were determined from measurements at 100 A/m prior to any demagnetization steps. The is therefore named initial AMS. The median scalar values of the initial AMS are shown in Table 3. Shape factor T versus corrected anisotropy P (Jelinek plot) and kmean versus corrected anisotropy P for the respective types are shown in Figure 10. The overall mean susceptibility for all specimens is with 2.00E 02 SI typical for basalts. Samples with normal and inverse magnetic possess similar median values, but the latter show a broader scatter of their mean susceptibilities. However, as the standard variation for all types is very high, the mean susceptibility on its own cannot be accounted as a significant criterion to distinguish between the magnetic s. The corrected magnetic anisotropy P of the AMS ellipsoid is rather low (overall median of 1.6%) as it is usual for lava flows. However, samples with stable inverse magnetic s have higher anisotropies (3.0%) as compared to normal magnetic s (median 1.5%). Constantly, very low anisotropies are observed in samples with inverse unstable magnetic s (median 0.3%). Although all types exhibit a broad scatter of the T parameter, there is a general tendency to triaxial-oblate shapes of the AMS ellipsoid as the median values for the T parameter range from to Especially specimens with inverse magnetic s (stable + unstable) trend with increasing anisotropy to a more oblate form of the AMS ellipsoid. The compilation of all initial AMS measurements indicates three distinct submaxima defined by horizontal and vertical orientations of all AMS principal axes (kmax + kint + kmin): maxima in horizontal NNW-SSE, horizontal WSW-ENE, and a steep (sub)vertical direction (Figure 10). The individual analysis of normal and inverse stable/unstable magnetic s reveals a similar pattern in the distribution of the principal axes (Figure 10). As normal magnetic s are defined due to the parallel alignment of magnetic foliation and flow plane, the dense (sub)vertical cluster of kmin axes is characteristic. The mean vector and confidence ellipsoids of kmax indicate a preferred tendency in NNW-SSE direction. Considering the vast extent of the sampling area, this overall trend is surprisingly consistent. Stable inverse magnetic s feature an even better definition of their principal axes. Here, in comparison to normal magnetic s, kmax and kmin axes are swapped, while the kint axes remain stable. Therefore, the kmax axes tend to be very steep and the kmin axes cluster tightly in NNW-SSE direction. The Schmidt net compilation of unstable inverse magnetic s shows a more dispersed distribution of the principal axes. The kmin and kmax axes form a NNE-SSW trending great circle, while the mean vector for the kmin axes tends in WNW-ESE direction. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2665

16 Figure 10. Principal magnetic axes of all specimens and for the respective s types are displayed in lower hemisphere Schmidt nets: squares = kmax; triangles = kint; circles = kmin. The mean tensor (filled symbol) and the 95% confidence ellipses are shown as calculated according to Jelínek [1978]. Shape factor T versus corrected anisotropy P (Jelinek plot) and kmean versus corrected anisotropy P are given for the respective type Change of AMS After Demagnetization The distribution of principal axes and the main scalar magnetic parameters of normal and inverse s before and after demagnetization (Molspin tumbler in peak fields of 100 mt) are compared in the following. The study includes the data of 548 specimens (424 normal, 86 stable inverse, and 38 unstable inverse) which allows for a statistical evaluation of the data set Scalar Parameter Median values are displayed in Table 4, and characteristic differences become visible in the box and whisker diagrams (Figure 11). Also, the scalar parameters for all specimens (548 specimens) are shown in Figure 12. In those plots the scalar AMS parameters are sorted according to the magnitude of the initial values and are displayed as decreasing curves. The values of the respective specimens after tumbling demagnetization are added in green. This kind of visualization displays the variability of the initial parameters and shows the change of every individual specimen in respect to the initial values after tumbling demagnetization. Almost all specimens experienced deviations of scalar magnetic parameters from their initial values after tumbling demagnetization. Volume susceptibility is usually enhanced after AF demagnetization in all SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2666

17 Table 4. Median Values of the Scalar Magnetic Susceptibility Parameters (Initial/After Tumbling Demagnetization) for the Respective AMS Fabric Types kmean (SI) P T Fabric Type n (Specimens) Initial Tumbler Initial Tumbler Initial Tumbler Total E E Normal E E Stable Inverse E E Unstable Inverse E E types. The average increase for the median considering all specimens is 13.4%. The absolute rise of kmean is higher for specimens already possessing a high initial volume susceptibility. However, the relative rise of kmean is quite constant regardless to the initial values. The demagnetization has also a significant impact on the corrected magnetic anisotropy P and the shape factor T. In the following the changes of those parameters due to the demagnetization are described in respect to the type: 1. Normal magnetic. Specimens with normal magnetic s usually show strong variations in P due to demagnetization. The median value of P for all specimens decreases from 1.5% to 1.2% which is realized for most specimens. However, specimens with very low initial anisotropies (< 1.01) will rather show a slight increase. Tumbling demagnetizing has a substantial influence on the shape form of the AMS ellipsoid. These mutations are reflected in the shape factor T. Despite almost constant mean values of T before and after demagnetization (T: 0.11 versus 0.16) parameters change significantly for individual specimens, which can be realized in Figure 12. The median calculated change of the T parameter (ΔT) for all individual specimens with normal magnetic s is 0.24, which is very high. It seems that initially prolate s are becoming more oblate, while oblate s become more prolate. 2. Stable inverse magnetic. Specimens with a stable magnetic show very consistently an increase of the magnetic anisotropy after tumbling demagnetization. In fact P was enhanced in all but one out of 86 specimens after the demagnetization. The median value shows a rise from 2.6% to 4.3%. Specimens with stable inverse magnetic s show a consistent trend in the evolution of T. Although the mean values of Tare constant before and after demagnetization (T: 0.17), the shape factor changes for individual specimens (Figure 12). After demagnetization the AMS ellipsoids tend to a more oblate shape. This is especially realized for s with an initial prolate shape. The relative change of T is with ΔT =0.11smallerthanforothertypes. 3. Unstable inverse magnetic. Specimens with unstable magnetic s are characterized by very low initial magnetic anisotropies. Although showing almost constant median values for P (1.007 versus 1.008), tumbling demagnetization will increase the variance of the median. The behavior of the T parameter is quite similar to specimens with normal magnetic s. Despite almost similar mean T values (T: 0.23 versus 0.27) before and after demagnetization, the individual specimens also show significant changes for individual specimens. Here ΔT is with 0.22 quite high. Similar to the normal magnetic s, it seems that initially prolate s becoming oblate, while oblate s become prolate due to the tumbling demagnetization Orientation of AMS Principal Axes After Demagnetization The individual principal magnetic axes of the respective types prior and after tumbling demagnetization are shown in Figure 13. The behavior of the AMS axes is described in the following in respect to the type: 1. Normal magnetic. The compilation of specimens with normal magnetic shows a drastic redistribution of the principal axes after demagnetization. The initial cluster of kmax axes in NNW-SSE direction vanishes. Also, the former preferential WSW-ENE orientation of kint axes is no more existent after tumbling demagnetization. The steep maxima of kmin axes remain after the demagnetization but become more dispersed. 2. Stable inverse magnetic. Specimens with inverse magnetic s were statically demagnetized prior tumbling demagnetization. Specimens with stable inverse magnetic s show no significant redistribution of the principal axes after the demagnetization experiments. As the axes remain stable during static demagnetization, they also show no redistribution due to tumbling demagnetization. As before kmin and kint axes are defining a WSW-ENE orientated great circle and the kmin axes are showing a strong maxima in NNW-SSE direction. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2667

18 k mean [SI] initial normal tumbler normal initial stable inv. tumbler stable inv. initial unstable inv. tumbler unstable inv P initial normal tumbler normal initial stable inv. tumbler stable inv. initial unstable inv. tumbler unstable inv T initial normal tumbler normal initial stable inv. tumbler stable inv. initial unstable inv. tumbler unstable inv. Figure 11. Box and whisker plots of the scalar AMS parameters, before and after tumbling demagnetization for the respective s types. The central box covers the middle 50% of the data values. The ends of the whisker are set at 1.5*IQR (interquartile range) above the third quartile (Q3) and 1.5*IQR below the first quartile (Q1). If the minimum or maximum values are outside this range, then they are shown as outliers. The positive outliers for the kmean of stable magnetic s are not included in the diagram. 3. Unstable inverse magnetic. Specimens with unstable inverse magnetic s show a significant redistribution of the principal axes due to the static AC field. As the magnetic axes become aligned according to the geometry of the applied field during the prior static demagnetization, the axes shift back close to their initial stage after the tumbling demagnetization. However, the initial NNE-SSW trending great circle defined by the kmax and kint axes becomes more blurred. Also, the preferred trend of the kmin axes in WNW-ESE direction is more unclear after the tumbling demagnetization. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2668

19 Figure 12. The scalar magnetic parameters for the specimens measured before and after tumbling demagnetization (n: normal magnetic = 424; n: stable inverse magnetic = 86; n: unstable inverse magnetic = 38). Initial values are shown by the black line. Values for the respective demagnetized specimens are displayed in green. Specimens are sorted in respect to the initial values Comparison of Lava Flow Directions for Initial and Demagnetized AMS For the evaluation of lava flow directions, the mean vector for each sample has been defined, whereby only samples with distinct clustering of the respective principal axes (kmax axes for normal magnetic, kmin axes for inverse magnetic s, and kmax + kint axes for girdle distribution) were considered. The evaluated flow directions are displayed as bidirectional trends, which consider only the strike of the determined flow directions while inclination is hereby neglected. Such a simplification is legitimate in this flood basalt setting with horizontal lava flow. The flow axes for all evaluated sites are displayed in Figure 14 in a digital elevation model, based on data provided by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (G-DEM) project (Ministry of Economy, Trade, and Industry (METI) and NASA). As the spacing for sites is very narrow in some areas, for nearby sites with comparable results the data are SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2669

20 Figure 13 SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2670

21 Figure 14. DEM with the evaluated bidirectional flow trends for (a) samples with normal and inverse magnetic s before demagnetization and (b) for samples with normal magnetic s after tumbling demagnetization. (c) The sites in the circled area are displayed in a columnar profile. compiled in rose diagrams. The figure presents flow directions evaluated prior to (a) and after (b) demagnetization for comparison. The map with presentation of initial AMS (Figure 14a) can be considered as classical approach for AMS interpretation where normal and inverse magnetic (stable + unstable) samples are displayed prior any demagnetization. The alternative interpretation presented in Figure 14b is based on flow directions evaluated from normal magnetic s after tumbling demagnetization. For this presentation, data of inverse magnetic s have been excluded as the stable behavior after demagnetization might indicate that the AMS is still biased by remanence interferences (see discussion in section 7). Prior any demagnetization, the evaluated flow directions show predominantly NNW-SSE direction. This is realized in normal and in inverse s as shown in the compilation of all data in the inset shown in Figure 14a. The individual sites indicate disturbances of this clear pattern in the southwest and southeast of Dhar. Here the evaluated flow directions show a preferred NE-SE trend or add an ENE-WSW direction, respectively. Noticeable is the high occurrence of inverse magnetic s in the SE (on the basal slope of the Narmada valley; SE of Mhow) and in the west of the study area (south of Banswara). These areas show strict alignment of directions arriving from inverse magnetic s. After tumbling demagnetization the general trend of the evaluated flow directions changes significantly. Regarding the individual sites, the change in flow trend after demagnetization is most prominent in the southwestern part of the study area and also east of Mhow. In the southwestern area the flow directions change from NNW-SSE to NE-SW. This trend is characteristic and also realizable when all evaluated flow directions (shown in the inset in Figure 14b) are compared. In the east of Mhow the AMS now indicates a W-E flow direction, which is almost perpendicular to the previous evaluated flow directions. This region, where the lava pile reachesathicknessofabout710m,isfurtherevaluated, and sections southeast and southwest of Mhow are combined in a columnar profile as shown in Figure 14c. Especially in the lower and highest units of the profile, the flow directions changes drastically after demagnetization. Figure 13. The individual principal magnetic axes of the respective types before and after demagnetization displayed in a lower hemisphere Schmidt net: squares = kmax; triangles = kint; circles = kmin. The corresponding mean density contour plots and mrd (multiples of a random density) values are displayed on the right. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2671

22 7. Discussion and Interpretation Magnetic geometry in the TM-bearing lava flows of the Malwa Plateau is sensitive to demagnetization, and an interference of remanent and induced low-field magnetization has to be inferred. Former studies from other key areas have demonstrated that laboratory remanence acquisition (induced remanent magnetization) can impress an artificial AMS [e.g., Potter and Stephenson, 1990a, 1990b]. The present study on a larger area of the Malwa Plateau with a statistically significant data set gives now evidence that even a NRM can interfere with the magnetic analyses as performed during routine AMS studies and can bias magnetic interpretation in terms of flow evaluation. The prerequisites for such interferences are ideally realized in the Deccan setting as the basalts bear a strong and stable NRM. As the results indicate a different response of normal and inverse s to NRM demagnetization the possible mechanism of interaction of induced with remanent magnetization will be discussed separately in the following Origin of Discriminated AMS Fabric Types Discrimination of normal and inverse magnetic is done on basis of the alignment of the principal magnetic axes in respect to geological relevant structures (Figure 1). A clear-cut correlation of the AMS types to a relevant grain size or composition of the ferrimagnetic contributors is hard to archive because of the strong variability in such lava flows. The Day plot seems to be inappropriate to distinguish between normal and inverse magnetic s. SEM analyses also reveal that the type is not exclusively correlated to the grain size, as specimens with normal magnetic s can exhibit various TM shapes and textures. Demagnetizing parameters, namely, the MDF and the residual magnetization at 100 mt, are also not suitable as significant criteria. However, temperature-controlled susceptibility curves reveal that specimens with stable inverse s typically contain high-ti magnetite, while specimens with unstable inverse s tend to have low-ti magnetites. This is also reflected in the k bulk(hd) values. As a high Ti content reduces the magnetostatic energy of TM grains and therefore reduces the stability of the domain walls in MD grains, these findings seem to be in contradiction. However, grains with lower magnetostatic energy will have an enlarged critical grain size for single-domain particles. Therefore, a high Ti content could lead to a higher fraction of SD particles [see Butler and Banerjee, 1975; Chadima et al., 2009], what will result in a higher probability to have an inverse magnetic. The direction of the AMS is then mainly a function of the SD particles. As peak fields of 100 mt are not high enough to affect all SD grains demagnetization will not alter the direction of the magnetic principal axes. As it is usual for natural rocks, those specimens will contain a fraction of MD/PSD grains, which will also show a high Ti content. The reduced magnetostatic energy and the relatively unstable domain walls of the MD/PSD fraction are therefore responsible for the field-dependent behavior. It is therefore quite likely that specimens with stable inverse s are indeed characterized by a high portion of SD TM, even there is no approval from the hysteresis measurements. In contrast, specimens with unstable inverse s presumably do not possess an unusual high amount of SD grains. The origin of their inverse must be related to other mechanisms (see section 2.2) Impact of NRM on Normal Magnetic Fabrics After tumbling demagnetization specimens with normal magnetic s are usually featuring a redistribution of their principal axes. This reorientation can be minor or quite significant. For discussion we show the principal axes prior and after demagnetization for representative samples in Figure 15. Dek_175 and Dek_53 reflect typical samples with a normal magnetic. In the initial state the magnetic lineations lie shallow in NNW-SSE direction with mean values of 173/19 (Dek_175) and 337/35 Dek_53, respectively. Both directions are subparallel with the mean ChRM directions of 343/ 36 and 158/50, respectively. The kmin axis is almost vertical and represents the pole to the lava flow planes. After demagnetization the kmax and kint axes show a change in their orientation while the kmin axes remain stable. The magnetic lineation in both samples is no longer aligned parallel to the remanence direction as the kmax axes show a welldefined cluster in NE-SW direction. Demagnetization with such magnetic fields (100 mt) has no significant effect on the amount, the grain size, the alignment, or the mineralogy of magnetic contributors, and therefore, the changes of the orientation of the principal magnetic axes must result from modification in the domain wall structure of MD/PSD grains. It seems that an NRM can have a strong influence on the principal AMS axes as the initial magnetic lineation is aligned in direction of the preexisting remanent magnetization. Apparently, the influence of the remanent magnetization is at least partly removed after demagnetization, as SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2672

23 Figure 15. Principal magnetic axes of some representative samples with normal and inverse s before and after tumbling demagnetization displayed in a lower hemisphere Schmidt nets: squares = kmax; triangles = kint; circles = kmin. In addition, the azimuth of the mean remanent directions (158 and 343, respectively) is displayed as a grey axis. the principal axes show a change in their directions. There are some cases where tumbling demagnetization lead to a complete redistribution of the principal axes. This can cause an initial normal magnetic to become inverse after demagnetization as realized in sample Dek_163 (Figure 15c). The initial AMS is normal, and the kmin and kmax axes show a rather loose grouping. After the demagnetization the principal axes underlie a significant change. The magnetic foliation becomes vertical, and the kmin axes get tightly aligned in a shallow NNW direction. This behavior is characteristic and can be realized in a couple of other samples. Interestingly, the directions of the principal axes are almost identical with those of initial inverse magnetic s. The adjustment of the principal axes can be interpreted as followed: The initial AMS is controlled by the MD/PSD fraction. After demagnetization the AMS is dominated by the SD fraction, as the magnetic becomes inverse. Tumbling demagnetization will clean the MD/PSD grains but will not affect the magnetization of the SD grains. As the new NNW direction of the kmin axes is parallel to the remanent magnetization, it seems that not only the alignment of the domain walls in MD/PSD grains but also the preexisting remanent magnetization of the SD fraction can alter the AMS. After the demagnetization the AMS will not be influenced by a remanent magnetization hold by MD/PSD grains but will reflect the remanent direction stored as a TRM in the SD phase Impact of NRM on Inverse Magnetic Fabrics Even though the scalar AMS parameter show aberrations from their initial values after tumbling demagnetization, the principal AMS axes of inverse s remain stable or show only minor deviation from their initial orientation. This is valid for samples with stable as well with unstable magnetic s. Figure 15d shows Dek_116 as a representative example for samples with an inverse magnetic. Especially stable magnetic s show a distinct alignment of the kmin axes in NNW-SSE direction, parallel to the mean remanent direction. In those samples the alignment of the AMS axes is mainly a function of the SD particles. Therefore, demagnetization with 100 mt will primarily affect the MD/PSD grains and the orientation of the AMS axes remains unchanged. A high proportion of SD particles is capable of preserving a strong remanent magnetization and will produce an inverse AMS. The consistent stable and tight alignment of the kmin axes parallel to the remanence vector and the demagnetizing behavior of samples like Dek_163 indicate that a NRM (acquired as a TRM) can have a significant influence on the distribution of the AMS axes. SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2673

24 Figure 16. Three-dimensional DEM map showing the deduced lava flow pattern for the western part of the Malwa Plateau (a) before and (b) after tumbling demagnetization. The insets give an aerial overview of the deduced flow pattern. For Figure 16a samples with normal and inverse (stable + unstable) magnetic s and for Figure 16b samples with normal magnetic s were used Defining Unambiguous Flow Directions Flow directions as presented in Figure 14 are bidirectional. In AMS analysis an unambiguous flow vector can be principally deduced by imbrication structures in the lava flow, which are apparently indicated by the azimuth/dip of the kmin axes [e.g., Knight and Walker, 1988; Cañón-Tapia et al., 1995, 1996]. However, imbrication occurs at the basal and upper part of the flow where the flow regime can be complex and the determination of a flow disadvantageous. We therefore did not sample these parts of the flow, and thus, it is not possible to deduce unambiguous flow vectors on base of the imbrication method. However, as the geological setting in the Malwa region is relatively simple, unambiguous flow direction can be deduced from the regional context. Considering the Narmada rift zone, which is attached in the south to the Malwa Plateau and the thinning of the lava flow sequence toward north, the most presumable scenario of flow is from southerly to northerly direction. According to this the flow, polarities are displayed in Figure 16 for (a) initial and (b) demagnetized AMS. The demagnetized samples reveal that the lava flows show a higher variability in their flow directions. This more differentiated spectrum of flow directions is in good agreement with the local geological setting. Especially the specimens with normal magnetic s along the western brim of the plateau show a redistribution of the magnetic axes after demagnetization. Field observations indicate that the present western border of the plateau represent more or less the maximum extent of the Malwa flows toward west. Starting with a maximum thickness of 710 m for the whole lava pile close to Mhow, the amount and the thickness of the lava flows reduce toward northwest. The lava flow sequence thins out in western direction. Although it is hard to quantify, the lava flows along the western edge are typically finer grained. This may coincidence with the frequent occurrence of stable inverse magnetic s along the western brim (see Figure 14). Those lava flows experienced a faster cooling and are therefore predestinated to possess a higher ratio of SD grains. These observations correspond with the evaluated flow pattern after demagnetization, SCHÖBEL AND DE WALL American Geophysical Union. All Rights Reserved. 2674