Abstract
Impact-generated shock waves can modify the remanent magnetization preserved in target rocks, yet their effects remain poorly constrained. Here we examine how shock waves modify rock magnetization by analysing unshocked granitoids and diorites, and shock-affected monomict breccia and impact melt rock of the Paleoproterozoic Dhala impact structure in India. Microscopic, thermomagnetic and hysteresis analyses were used to identify magnetic minerals and their domain states. Remanent magnetization and demagnetization experiments were performed to evaluate shock effects on the palaeomagnetic behaviour of impact-generated and unshocked target rocks. The unshocked rocks contain strong and stable magnetization carried by titanomagnetite. In contrast, the monomict breccia carries titanomagnetite and titanohematite and shows extremely weak and unstable magnetization, consistent with shock-related grain-size reduction and microfracturing. Impact melt rocks display intermediate behaviour, with titanomagnetite, titanohematite and pyrrhotite as magnetic carriers. These results show that shock can substantially reduce crustal magnetization. The results help to explain weak magnetic signatures at terrestrial and planetary impact structures, even in the presence of an ambient magnetic field.
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Introduction
Understanding the effects of impact-generated shock waves on the magnetic properties of the target lithology is essential for explaining the shock demagnetization and low magnetic intensities observed over many Martian craters and in shock-affected regions along the peripheries of major impact basins on Mars (e.g., Hellas, Argyre, and Isidis). Here, the low magnetic intensity may be attributed to several processes, including shock demagnetization1,2, the non-acquisition of thermal remanent magnetization after dynamo cessation3, excavation of crustal material that is more strongly magnetized than the underlying mantle4,5, or cooling in a reversing dynamo field6,7. According to the present state of knowledge, an impact can either demagnetize or remagnetize the target material and the impactites, depending on the absence or presence of an ambient magnetic field1,5,8 generated by the dynamo operating within the planetary body. This argument has been used to place constraints on the timing of dynamo cessation on Mars5,9,10,11.
Near the center of the Martian impact basins formed after dynamo cessation, both thermal and shock processes may have contributed to demagnetization. Contrary to the Martian setting, an ambient magnetic field on Earth may impart a stable thermal remanent magnetization (TRM) to the rocks near the crater center. TRM is acquired as these rocks cool down after being heated beyond the Curie temperature due to impact. For example, the Ries crater in Germany exhibits a negative magnetic anomaly at its center, which is attributed to the cooling of its ~400 m thick melt sheet during a period of reversed geomagnetic polarity. Similarly, the Manicouagan crater in Canada, with an impact melt sheet over 1 km thick, displays a positive central anomaly12.
Notably, at the major Martian basins, shock demagnetization extends much farther, sometimes up to their periphery, thus affecting a larger volume of rocks3,13. These rocks experienced low shock temperatures, yet the mechanisms responsible for their demagnetization remain unclear. Similarly, monomict breccia, the most common impactite at the Dhala impact structure, experienced low shock temperatures. Even though the target granitoid rocks are ferrimagnetic14, the structure features a low-intensity magnetic anomaly15. Thus, the cause of the low-intensity magnetic anomaly at the Dhala impact structure is critical to investigate. It will provide clues to understanding the shock demagnetization processes active at the periphery of the major Martian impact basins. Therefore, here we study the magnetic properties of the unshocked target rocks and impactites at the Dhala impact structure, India (see geological setting in “Methods”).
Our analyses show that shock modifies the ability of rocks to acquire and retain stable remanent magnetization, with the lowest remanence occurring in the shocked monomict breccias. In contrast, the impact melt rocks and unshocked lithologies preserve comparatively stable magnetic signals. These results demonstrate that shock processes alone can substantially reduce crustal magnetization in the presence of an ambient magnetic field. This provides a mechanism for subdued magnetic signatures in impact crater regions where thermal resetting is limited. This framework helps explain magnetic lows at terrestrial craters and offers new insight into how the weak magnetic signatures characteristic of large Martian basins may arise from shock demagnetization in their peripheral zones, where thermal effects are minimal and an ambient magnetic field may still have been present.
Results
Outcrop and microscopic features
Impact melt rock forms six reddish-orange to pale brown-coloured outcrops around the CEA (Fig. 1, see geological setting in Methods). They present millimeter to centimeter size, circular to elliptical vesicles, some of which are filled by secondary minerals such as agate, quartz, and chlorite (Fig. 2a). The impact melt rocks exhibit an aphanitic to fine-grained texture and comprise predominantly lithic clasts, along with mineral and melt clasts. The primary mineral constituents include quartz, orthoclase, plagioclase feldspar, sericite, zircon, and opaque minerals. The shock metamorphic features, such as ballen-textured quartz and planar deformation features, are common (Fig. 3a, b). Titanomagnetite is the most common opaque phase, followed by titanohematite, observed in a few samples (Fig. 3c). There are two distinct types of magnetite: cubic, likely formed due to slower cooling in the target rocks and included in impact melt16, and skeletal magnetite, indicative of rapid cooling and crystallization from impact melt.
a Impact melt rock with vesicles. b Monomict breccia composed of angular to sub-angular granitoid clasts embedded in a finer clastic matrix. c Outcrop of coarse-grained equigranular granite. d Medium-grained diorite displaying an interlocking texture of plagioclase and hornblende.
a Quartz with Ballen structure in impact melt rock, b Planar deformation features embedded in impact melt rock. c Titanomagnetite (Ti-Mag) oxidized to titanohematite (Ti-Hem) along the margins in impact melt rock. d Monomict breccia with angular clasts of fractured granitoid embedded within a fine-grained matrix. e A clast of monomict breccia consisting of plagioclase (Plg) and quartz (Qtz) grains. f Unshocked granite with plagioclase (Plg) altered to sericite, quartz (Qtz), and biotite (Bio). g Unshocked diorite presenting interlocking texture of plagioclase (Plg) and hornblende (Hbl).
At outcrop scale, the monomict breccia presents brecciated fragments of coarse to medium-grained granitoid clasts embedded within a finer clastic matrix (Fig. 2b). While, at microscopic scale, large granitic clasts, primarily composed of quartz and feldspar, are surrounded by a fine-grained clastic matrix (Fig. 3d). Extensive fracturing is observed within the quartz clasts (Fig. 3e). Modal analysis indicates that the feldspar-to-quartz ratio of the clasts ranges between 60:30 and 40:50, with the remaining 10% consist of accessory minerals such as chlorite, biotite, and various opaque minerals (Fig. 3e). Titanomagnetite is the dominant opaque phase, followed by Ti-hematite in a few monomict breccia samples (Supplementary Fig. 4).
The monomict breccia is surrounded by unshocked granitoid and diorite outcrops (Fig. 1), which exhibit minimal fracturing and brecciation (Fig. 2c, d). The granite is coarse-grained and equigranular, with visible feldspar, quartz, and biotite grains. At the microscale, unshocked granite consists of plagioclase clasts, quartz, and biotite, with sericite alteration derived from plagioclase and quartz (Fig. 3f). The diorite at the outcrop scale displays a medium to coarse-grained texture with interlocking mineral grains (Fig. 2d), while at microscale they present predominantly hornblende, plagioclase, with minor amount of quartz (Fig. 3g).
Thermomagnetic behaviour
Unshocked granite (UG) and diorite (UD) exhibit higher magnetization in their cooling curves, indicating phase transformations or mineral recrystallization during heating (Fig. 4a, b, and Supplementary Fig. 2 and Supplementary Table 3). The Curie transitions at (TC1) between 568 °C – 585 °C represent Ti-poor magnetite in both rock types. The stoichiometric magnetite remains stable throughout the experiment. The curves are irreversible with increased magnetization in the cooling curve at temperatures below TC1. The increased magnetization may be due to the formation of a higher moment phase due to oxidation during heating of the sample16. The monomict breccia samples are dominated by Ti-magnetite, with occasional occurrences of Ti-hematite (Supplementary Figs. 4 and 3g, h)14.
Representative thermomagnetic curves showing variations in the magnetization with temperature for unshocked granite (a), unshocked diorite (b), and impact melt rock (c, d). The heating and cooling curves are shown in red and blue, respectively. Tc1, Tc2, TC3, and Tc4 mark the Curie transition temperatures corresponding to Ti-poor magnetite, Ti-magnetite, pyrrhotite, and Ti-hematite, respectively. See Supplementary Figs. 2 and 3 for thermomagnetic curves of other samples and Supplementary Table 3 for the values of each sample.
The impact melt rock presents up to three distinct Curie transitions, TC2, TC3, and TC4 (Fig. 4c, d, Supplementary Fig. 3 and Supplementary Table 3). TC2, 518 °C–545 °C, is attributed to Ti-magnetite, but with higher titanium content (ulvöspinel 0.05-0.10) than in unshocked granite and diorite17. TC3 (321 °C–351 °C) represents pyrrhotite18,19. While, TC4, 617 °C–633 °C, characteristic of Ti-hematite20 is observed only in I3.12 and I6.18 (Fig. 4d and Supplementary Fig. 3f). Curves of all impact melt rock samples have comparable shapes and are reversible, indicating the stability of the three magnetic carriers during heating in air (Fig. 4c, d and Supplementary Fig. 3).
Bulk domain state
The hysteresis curves of unshocked granite and diorite are narrow (Fig. 5a, b and Supplementary Fig. 1), typical of MD magnetic grains21. In contrast, impact melts exhibit wasp-waisted hysteresis curves, indicating low-coercivity ferromagnetic grains such as SD or PSD magnetite with SP fractions (Fig. 5c, d and Supplementary Fig. 1)19,22,23. Monomict breccia shows pot-bellied and wasp-waisted hysteresis loops14. Remanence of coercivity (Hcr) is lower in unshocked granite (8.53–21.45 mT) and diorite (7.41–23.31 mT), compared to impact melt rock (39.32–109.92 mT) and monomict breccia (27.54–49.38 mT), revealing increased single-domain (SD) content (Fig. 5 and Supplementary Table 2). Bulk coercivity (Hc) trends are in agreement, with lower values in unshocked granite and diorite, and higher values in impact melt rock and monomict breccia.
Representative hysteresis curves of the unshocked granite (a), unshocked diorite (b), and impact melt rock (c, d) showing variation in magnetization with applied field. e Day plot74,75,81 showing the bulk domain behavior by plotting the ratio of remanent magnetization (Mrs), and saturation magnetization (Ms) against the ratio of remanence of coercivity (Hcr) and coercive force (Hc). The SD, MD, and PSD stand for single domain, multi-domain, and pseudo-single domain, respectively. The ratios for monomict breccia are taken from Tiwari et al.14 See Supplementary Table 2 for the values of each parameter and Supplementary Fig. 1 for the hysteresis curves of the rest of the samples.
The Day-Dunlop plot (Fig. 5e) reveals that the MD component is highest in the unshocked rocks, >85%, followed by the impact melt rock, 60–40%, and monomict breccia, 60–0%. The impact melt rocks plot in the PSD field, which likely reflects averaging of multiple magnetic populations, e.g., inherited titanomagnetite, fine-grained magnetite, and pyrrhotite. The monomict breccia and the unshocked rocks plot along the SD–MD mixing line, with more than 40% SD component (Fig. 5e). Although dominated by MD-like behavior, the hysteresis parameters of the unshocked rocks are also consistent with prolate vortex states24.
Susceptibility and remanent magnetization
Unshocked rocks present the lowest values of frequency-dependent susceptibility, 0.29% ≥χfd% ≥1.06%, preceded by monomict breccia (7.26% ≥ χfd% ≥8.82%); while the values are highest, 2.01% ≥ χfd% ≥ 47.55%, in impact melt rocks (Supplementary Table 4). Low χfd value in unshocked rocks is owed to MD grains, which is in agreement with the hysteresis results. In contrast, high χfd values in impact melt rocks exceed the 15% threshold typical of SP behavior25,26, and in some cases, reach the range reported for extremely fine magnetic particles27,28. This is coherent with the wasp-waisted hysteresis of the impact melt rocks23.
Unshocked granite and diorite have comparable bulk magnetic susceptibility (χ), ranging from 0.24 × 10−3 to 1.49 × 10−3 SI (Supplementary Table 1). The monomict breccia presents the lowest, 0.13 × 10−3 ≥ χ ≥ 0.34 × 10−3 SI. The impact melt rocks have the highest susceptibility, 0.66 × 10−3 ≥ χ ≥ 1.86 × 10−3 SI.
NRM intensities in the unshocked granite and unshocked diorite are comparable. The former ranges from 1.61 to 10.52 A/m and the latter from 1.50 to 10.37 A/m (Fig. 6 and Supplementary Table 1). In contrast, the NRM intensity of the monomict breccia is two orders of magnitude lower, 5.0 × 10−3 to 1.7 × 10−2 A/m (avg. = 9.0 × 10−3 A/m). NRM intensities in impact melt rocks fall in between the unshocked rocks and monomict breccia, ranging from 3.0 × 10−2 A/m to 6.5 × 10−1 A/m (avg. = 2.2 × 10−1 A/m).
Plot showing the variation in Natural Remanent Magnetization (NRM) and Koenigsberger (Q) ratio with magnetic susceptibility. Cross symbols show mean values, and dashed ellipses represent two-dimensional 95% confidence ellipses based on the covariance of the log-transformed variables (green: unshocked granite and diorite; blue: monomict breccia; red: impact melt).
Unshocked rocks have much higher Koenigsberger (Q) ratios than the impactites. The unshocked granite and diorite have a high (Q) ratio, 118.2 to 702.3 (Fig. 6, Supplementary Table 1), indicating that magnetization is largely controlled by remanence. Among the impactites, the impact melt rocks have slightly higher Q ratios, 1.3 to 14.6, than the monomict breccia, 0.35 to 1.50. In the impactites the magnetization, in some cases, is controlled by induced magnetization (Q < 1), and in other cases, 1 ≤ Q < 10, by a combination of remanent and induced magnetization18,29. Two unshocked samples (UG3, UG7) present anomalously high Q ratios (1310.0 and 875.3), unusually strong NRM intensities (44.6 and 88.1 A/m, nearly an order of magnitude greater than other unshocked specimens), and unstable demagnetization behaviour, consistent with lightning strike overprints. They were excluded (see “discarded” in Supplementary Table 1)30,31.
Consistent with these trends, the mass-normalized isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) intensities exhibit systematic lithology-dependent variations (Supplementary Fig. 5). Average values of SIRM1000mT (saturation isothermal remanent magnetization) and ARM decrease from unshocked rocks (6.7 × 10−1 and 1.03 × 10−2 Am2 kg-1) to impact melts (1.11 × 10−2 and 3.65 × 10−5 Am2 kg−1) and monomict breccia (4.18 × 10−3 and 2.18 × 10−6 Am2 kg−1), respectively. These results indicate a progressive reduction in remanence acquisition capacity from unshocked rocks to monomict breccia, with unshocked lithologies exhibiting the highest ARM/SIRM ratios, impact melts showing intermediate ratios, and monomict breccia the lowest.
Alternating field and thermal demagnetization results
Unshocked granite and diorite show a low-coercivity antiparallel component that is removed by 10 mT of AF demagnetization (Fig. 7a–d; Supplementary Fig. 6 and Supplementary Table 5). This secondary component is present in most unshocked samples and is interpreted as a low-coercivity secondary overprint rather than a viscous remanent magnetization (VRM), as its direction deviates from the present geomagnetic field. Further demagnetization isolates an intermediate-coercivity, stable component whose magnetization intensity decays gradually (red symbols in Fig. 7a–d). Of the eight unshocked sites, three yielded moderately clustered and distinct ChRMs (Fig. 8; Supplementary Fig. 6 and Supplementary Table 5), while the remaining sites did not provide stable directions. By contrast, monomict breccia shows unstable behavior during both AF and thermal demagnetization, with no consistent ChRM isolated across specimens (Fig. 7e–h and Supplementary Fig. 7).
Demagnetization behaviour of representative samples is shown using Zijderveld diagrams and lower-hemisphere stereographic projections. a–d Alternating field (AF) demagnetization of unshocked target rocks (granite, UG, and diorite, UD). e, f AF demagnetization and (g, h) thermal demagnetization of monomict breccia samples. i, j AF demagnetization and (k, l) thermal demagnetization of impact melt rock samples. Solid and open symbols in the Zijderveld plots denote projections onto the horizontal and vertical planes, respectively. Stable components are shown in red. All plots were generated using PuffinPlot software v.1.4.176. Additional plots and data are provided in Supplementary Figs. 6–8 and Supplementary Tables 5 and 6.
Equal-area lower-hemisphere stereographic projections of characteristic remanent magnetization (ChRM) directions for (a) unshocked target rocks and (b) impact melt rocks. Solid and open symbols represent ChRM directions with positive and negative inclinations, respectively. Dashed/solid ellipses represent the α95 confidence limits for the mean ChRM. Mean directions were computed and plotted using PuffinPlot software v.1.4.176. Additional plots and data are provided in Supplementary Tables 5 and 6 and Supplementary Figs. 6 and 8.
Most impact melt rock samples display a stable component during both alternating-field (AF) and thermal demagnetization, without any secondary overprint (Fig. 7i–l; Supplementary Fig. 8 and Supplementary Table 6). Orthogonal plots show linear decay trajectories toward the origin, while stereoplots illustrate vector clustering. A few specimens (e.g., 10.1, 11.1, 13.1) exhibit a weak secondary overprint that is not VRM. After removal of this component, further demagnetization isolates a stable component, similar to that in the samples lacking the secondary overprint. The mean ChRM direction of the impact melt rock has a declination of 169°, an inclination of 38.2°, with a Fisher precision parameter k = 9.8 and an α95 of 9.3° (Fig. 8; Supplementary Fig. 8 and Supplementary Table 6). The modest scatter in the ChRM directions of individual melt rock specimens likely reflects magnetic heterogeneity within the melt arising from variations in mineralogy and domain state (Fig. 8).
Discussion
To understand the effects of hypervelocity impact-generated shock waves on the magnetization, we compare the magnetic properties of the unshocked target rocks with the monomict breccia and the impact melt rocks. Our results reveal that in the unshocked rocks, MD grains of low-Ti magnetite carry magnetic signals and lead to intermediate magnetic susceptibility, high NRM intensity, and stable paleomagnetic directions (Figs. 6, 7a–d, 8 and Supplementary Fig. 6).
The impact melt rocks feature the highest magnetic susceptibility, intermediate NRM intensities, and stable paleomagnetic directions (Figs. 6, 7i–l and Supplementary Fig. 8). Joshi et al.32 suggested that at least some of the Ti-magnetite grains in the impact melt rocks are derived from the target rocks. Similar to the Ti-magnetite in the monomict breccia, they would also have experienced shock waves that caused domain defects and domain size reduction33,34. However, the high temperatures within the impact melt would have annealed any shock effects35,36, and these magnetic grains subsequently acquired thermal remanent magnetization during cooling, along with other PSD grains that crystallized as the impact melt solidified.
Our frequency-dependent susceptibility measurements reveal SP particles in impact melt rocks. The rapid cooling, which produced the skeletal grains32, could have formed these SP particles. The MCB-10 borehole37 revealed ~70 m thick impact melt rock deposits, which most probably cooled within years at the margins, but took centuries near the center. This estimate is based on comparison with cooling times of the ~200 m thick Manicouagan melt sheet38. The SP particles impart high susceptibility values to the impact melt rocks. The dominance of SP over PSD particles explains the intermediate Q-ratios and NRM intensity (Fig. 6). SP grains contribute to magnetic susceptibility, but do not sustain long-term remanence23. Stable remanence in impact melt rocks is carried by PSD grains39.
The coercivity of remanence (Hcr) and bulk coercivity (Hc) are higher in impactites compared to unshocked rocks. In impact melt rocks, this increase in coercivity can be attributed to the formation of high coercivity phases such as pyrrhotite and occasional hematite. Pyrrhotite may have formed due to hydrothermal activity during the early stages of cooling of impact melt rocks. It enhances coercivity while contributing little to remanence. Instead, the persistence of stable directions at high unblocking temperatures and high coercivities shows that Ti-magnetite carries the ChRM (Fig. 7i–l and Supplementary Fig. 8). Titanohematite occurs occasionally along Ti-magnetite grain margins in both impact melt and monomict breccia and is absent in the unshocked target rocks. Its occurrence is best explained by localized oxidation of Fe–Ti oxides, consistent with reports from other impact melt sheets40. The monomict breccia presents magnetic properties that contrast with those of the unshocked and impact melt rocks, showing lower magnetic susceptibility, extremely low NRM intensities, and unstable paleomagnetic directions (Figs. 6, 7e–h and Supplementary Fig. 7). The cause of these differences lies in the formation of the monomict breccia, which is discussed below.
Previous studies suggest that the shock waves fractured and brecciated the target granites to form monomict breccia14,41. During brecciation, the MD Ti-magnetite in unshocked granite developed fractures and domain defects, leading to a decrease in apparent domain size and an increase in coercivity35. Shock-induced defects suppress the ability of grains to carry remanence, leading to lower NRM and susceptibility relative to the unshocked rocks. In agreement with this, other studies on naturally and experimentally shocked rocks suggest that microfracturing and domain wall pinning induced by shock can cause MD grains to mimic SD-like behavior2,8,36,42,43. In general, SD grains are paleomagnetically more stable than MD grains44. Thus, the SD-like behavior of the Ti-magnetite in monomict breccia does not explain unstable paleomagnetic directions.
Large-scale randomization due to mixing in monomict breccia is precluded because Tiwari et al.14 showed that the monomict breccia samples collected 10–20 m apart present well-developed magnetic fabric and comparable orientation. They concluded that the monomict breccia experienced no displacement or mixing. However, at the millimeter scale, individual clasts in the monomict breccia are randomly oriented (Fig. 2d). This small-scale physical randomization is sufficient to partially cancel the magnetic vectors of ~10 cm³ specimens, and lead to the low NRM and unstable paleomagnetic directions in monomict breccia.
Experimental studies have revealed loss of NRM due to low shock pressures of ~2 GPa and under hydrostatic compression of ~1.24 GPa2,8,45,46,47,48,49,50. Shock demagnetization mechanisms differ in different domain states. In MD grains, compressive stresses decrease the total volume of domains with spontaneous magnetization aligned parallel to the compression axis48,51. This irreversible domain realignment results in a measurable reduction in the NRM intensity and susceptibility. In SD grains, the shock wave strains the domains, increasing the magnetostrictive energy beyond the magnetocrystalline anisotropy, thereby reducing the remanent magnetization and susceptibility1,52,53,54.
In summary, the low NRM and unstable paleomagnetic directions in the monomict breccia likely result from a combination of millimeter-scale clast randomization, which reduces net remanent magnetization, and shock-induced domain-level modifications, where fractures and domain defects in MD Ti-magnetite weaken remanence despite SD-like hysteresis behaviour.
Several lines of evidence argue against hydrothermal alteration as the cause of demagnetization in the monomict breccia. First, in monomict breccia, ARM and IRM data show very low remanence efficiency (Supplementary Fig. 5), with SIRM intensities and ARM/SIRM ratios consistently an order of magnitude lower than in impact melts and unshocked lithologies. Thus, monomict breccia is unable to acquire remanence. Second, Ti-magnetite is the primary magnetic carrier in monomict breccia, and is occasionally altered to Ti-hematite along its grain boundaries (Supplementary Fig. 4)14. Third, the Giant Quartz Veins at Dhala represent three phases of regional hydrothermal activity55, that affected both unshocked lithologies and impactites. Hydrothermal activity generally produces low-intensity stable chemical remanent magnetization40,56,57. However, the monomict breccia features unstable directions.
Thus, the absence of stable directions in monomict breccia, together with only minor alterations in Ti-magnetite, indicates that hydrothermal activity did not substantially affect the magnetic mineralogy and paleomagnetic characteristics.
Further supporting evidence comes from the impact melt rocks. Here, Ti-magnetite grains are altered to Ti-hematite along the margins (Fig. 3c), and some samples (I1, I2.8, I3.13, and I3.31) contain hydrothermal pyrrhotite (Fig. 3c, d and Supplementary Fig. 3). Despite these secondary phases, most samples yield stable ChRM directions that are comparable throughout the six impact melt outcrops. Thus, as with the monomict breccia, hydrothermal alteration did not affect the remanent magnetization of the impact melt rocks.
Our current understanding suggests that impact cratering and shock waves can remagnetize and demagnetize rocks depending on the presence or absence of an ambient field. Impact melt rocks record the ambient field during impact and control magnetic anomalies. For example, Manicouagan and Ries impact structures are characterized by thick coherent melt sheets that dominate their paleomagnetic signals, producing magnetic anomalies consistent with the geomagnetic polarity at the time of impact, positive over Manicouagan, negative over Ries12,58.
At older or smaller terrestrial craters, impact melt sheets are often not preserved. The scaling laws predict that even small, simple craters can generate melt layers59, and impact melt has been observed in lunar craters as small as 170 m in diameter60. However, the preservation potential of impact melt at small terrestrial craters is much lower due to erosion and limited melt volumes. Thus, the magnetic anomalies may be controlled by shocked target rocks. One such example is the Santa Fe impact structure in the USA (crater diameter 6–13 km), where shock demagnetization has been attributed to plasma shielding of the ambient field61. This led to the momentary absence of the ambient field when the rocks were shocked, as proposed by the authors. However, this explanation remains speculative at this stage, as plasmas have also been shown to amplify magnetic fields62,63.
In the case of impact craters on Mars, which formed after the cessation of the dynamo, the melt rocks cool down in the absence of an ambient field, leading to low magnetic anomalies. However, weak magnetization at Martian craters can also arise through several processes that operate even when a dynamo-generated magnetic field is present. Low magnetic anomalies at Martian impact basins have been explained by several processes, including excavation of strongly magnetized crust that exposes deeper, weakly magnetized material4,5 and cooling through a reversing dynamo, which can produce subdued anomalies despite an active field6,7. Shock demagnetization has also been invoked to explain magnetic lows at Martian basins2.
Our results provide an additional perspective on these interpretations. They show that weak magnetization can also arise in peripherally shocked regions, even when an ambient magnetic field is present, a process that complements previously proposed mechanisms focused mainly on melt-sheet behavior near the centers of large basins. Our findings, therefore, help constrain how magnetization may be reduced outside the melt sheet, particularly in the peripheral regions where shock effects dominate, and thermal annealing is limited.
The principal magnetic carriers on Mars are magnetite, hematite, and pyrrhotite, which occur in a range of domain states64,65 and are also present at Dhala. The Dhala structure is ~11 km in diameter, smaller than Martian impact basins that can be magnetically characterized from orbital data, with the largest examples such as Hellas, Argyre, and Isidis exceeding 1000 km in diameter. In large basins, the crust is interpreted to be completely demagnetized within ~0.8 crater radii by a combination of thermal and shock effects, while beyond the rim (>1.4 radii), shock effects dominate, producing partial demagnetization13. Interior basin temperatures would have been far higher than those of the Dhala impact melt, likely melting and annealing shock-modified grains. Our results are, therefore, most relevant to the peripheral zones of larger basins, where shock-induced changes to domain structure and clast-scale randomization could survive the relatively lower shock temperatures. These results may also be relevant for smaller Martian impact structures whose magnetic signatures are not yet resolved from existing orbital satellite data5.
Conclusions
This study investigated the rock magnetic properties of unshocked target rocks, monomict breccia, and impact melt rocks at the Dhala impact structure. The unshocked lithologies present the highest values of NRM and Q-ratio, and intermediate magnetic susceptibility. Impact melt rocks present intermediate values of NRM and Q-ratio and stable paleomagnetic vectors. They feature the highest magnetic susceptibility. Such magnetic behaviour is attributed to PSD and SP grains. Some PSD grains were inherited from the pre-impact target lithologies, while the SP particles crystallized during rapid cooling of the impact melt.
The monomict breccia exhibits much lower values of NRM and Q-ratio and unstable paleomagnetic vectors. Shock caused micro-fracturing and domain defects in magnetic minerals, producing SD-like hysteresis behaviour in originally MD grains and weakened their ability to carry remanence. Clast-scale randomization leads to cancellation of magnetic vectors at the specimen scale. Together, these processes rendered the low NRM intensity and unstable directions and produced the apparent shock demagnetization of the monomict breccia. Thus, the subdued magnetic signatures at impact structures cannot be attributed to a single process or the absence of an ambient field. Shock demagnetization may have contributed to the weak magnetization observed in the peripheral regions of large basins and in smaller impact craters on Earth and other planetary bodies. This study demonstrates that shock may demagnetize rocks even in the presence of an ambient magnetic field.
Methods
Geological setting
The Dhala impact structure (Fig. 1), located in the Shivpuri district of Madhya Pradesh, India, is a Paleoproterozoic impact crater, 1.7–2.5 billion years old, with an original diameter of about 11 km66. The eroded remnant of the structure has a present-day diameter of ~4.5 km14. The target lithologies at the Dhala structure are part of the Archean crystalline basement of the Bundelkhand craton, which comprises older tonalite-trondhjemite-granodiorite (TTG) gneisses (2.7–3.6 Ga), intruded by diorites and granites of ~2.5 Ga67. Two to three phases of hydrothermal activity between ~1.4 and 2.0 Ga formed the giant quartz veins along the preexisting fractures55. This was followed by the intrusion of tholeiitic dykes ~1.1–2 Ga68. Three major deformation phases have affected the area, the first two resulting from compressive tectonic regimes which led to the folding of TTG gneisses (~3.7–2.7 Ga), and the third deformation occurred during granitic intrusion from 2.56 to 2.44 Ga69.
The stratigraphy of the impactites at the Dhala structure is based on Mohar Cauldron Borehole drill data and outcrop exposures37. The impactite lithologies include pseudotachylitic breccias, monomict breccia, impact melt rocks, and suevites. Monomict breccia and impact melt rocks are exposed and studied here, while suevite and pseudotachylite breccia are recorded in borehole37. Among the two, monomict breccia is far more extensively and frequently exposed, with more than 200 outcrops forming a ring-like pattern that defines the outermost limit of the present-day crater rim14. Many of the reddish-brown monomict breccia outcrops show a low, rounded, and elongated morphology with convex profiles70. Monomict breccia is suggested to have formed in situ due to the fracturing and brecciation of the target granite14.
Six outcrops of impact melt rock are exposed at Dhala, all in the northern and western parts of the structure, likely due to the erosion of the rest. In the excavation stage, the impact melt flowed eastward in a semi-molten, lava-like state and was subsequently emplaced within the crater as crater-fill impactites32. The impact melt remained semi-molten, with temperature below ~1500 °C, with some pre-impact Ti-poor magnetite grains still present32.
The post-impact lithologies feature Dhala formation and Sumen sandstone, equivalent to the Semri and Kaimur groups of the Vindhyan Supergroup, respectively. The Dhala Formation is primarily composed of sandstone, siltstone, shales, and conglomerates, with the dominant minerals being poorly sorted angular quartz clasts, feldspar, biotite, and sericite. Sumen Sandstone is partially covered by a laterized conglomerate at the top71. These sediments are exposed at a geomorphic mesa-like structure, called the Central Elevated Area (CEA). The CEA is ~418 m above mean sea level, covering an area of ~ 5 km2.
Sampling and palaeomagnetic analyses
To analyze the microscopic and magnetic properties of the impact melt rocks, monomict breccia, and unshocked target rock, samples were collected during two field campaigns. Fractured and weathered outcrops were avoided. For impact melt rocks, 34 oriented cylindrical cores were drilled, each measuring 1 inch in diameter and 4–5 inches in length, from 6 paleomagnetic sites (Fig. 1) using a portable hand drill. The cylinders were oriented using a magnetic compass before being removed from the impact melt rocks. Oriented block samples of monomict breccia and unshocked target rock were collected from 4 and 8 paleomagnetic sites, respectively. The blocks were drilled in the lab to retrieve cylinders, which were then cut into specimens, 2.54 cm in diameter and 2.2 cm in length. Unshocked target rocks do not exhibit extensive fracturing, brecciated clasts, or vesicles, making them easily distinguishable from impactites in the field. Larger outcrops have more than one sampling site to average out local variations (Supplementary Table 1).
The temperature-dependent magnetization (M-T) analysis was conducted using an Advanced Variable Field Translation Balance at the Paleomagnetism Laboratory, CSIR–NGRI, Hyderabad, having a sensitivity of 5 × 10−5 Am2. This technique measures the magnetic dipole moment (M) as a function of temperature (T), either in a zero field or an applied field. M-T curves are used to determine the Curie temperature (Tc) using the 1st derivative method72 to know the stability of the principal magnetic carriers and the phase changes during heating and cooling (Supplementary Table 3).
Hysteresis loops were obtained using a Vibrating Sample Magnetometer (Model EV7-VSM; ADE Technologies, USA) with a sensitivity of ~1 × 10−9 Am2, under a maximum applied field of 1.75 T at the Advanced Centre for Material Science, IIT Kanpur. Paramagnetic correction was performed by subtracting the linear portion of the hysteresis loop, observed after the ferromagnetic component saturates19. The correction interval varied depending on lithology: for impact melt rocks, it was applied above ~1124 mT, and for unshocked rocks above ~640 mT. HystLab software, version 1.1.273, was used to calculate the coercive force (Hc), remanent magnetization (Mrs), and saturation magnetization (Ms). The remanence of coercivity (Hcr) was determined using the curve-shifting method19. The ratios Hcr/Hc and Mrs/Ms were plotted on the Day plot74,75 to estimate the relative proportions of single domain (SD), multi-domain (MD), and pseudo-single domain (PSD) (Supplementary Table 2).
High-frequency (χhf, 4.6 kHz) and low-frequency (χlf, 0.46 kHz) magnetic susceptibility of selected samples of unshocked rocks and impactites were measured to determine frequency-dependent susceptibility (χfd%) using a dual-frequency MS2B Sensor (Bartington Instruments Ltd.) with a sensitivity of 1 × 10−6 SI (Supplementary Table 4). χfd % was calculated using the equation:
Bulk magnetic susceptibility (χ) of unshocked target rock was measured using a Kappabridge MFK-1A, which has a sensitivity of 2×10-8 SI at the Paleomagnetism Laboratory, CSIR–NGRI, Hyderabad, while χ data for impact melt rock and monomict breccia have been taken from earlier published reports (Supplementary Table 1)14,32.
Natural remanent magnetization (NRM) was measured using a spinner magnetometer (Mu-Spin, Kodama Scientific Instruments, Japan) with a sensitivity of 10−11 Am2 before demagnetization. A subset of impact melt specimens (n = 11), monomict breccia (n = 5), and unshocked target rocks (n = 8) were used for isothermal remanent magnetization (IRM) and anhysteretic remanent magnetization (ARM) analyses. IRM was induced with an ASC Scientific Impulse Magnetizer (IM-10-30, USA) in stepwise fields up to 1000 mT (increments of 10–200 mT) at room temperature, and ARM was induced with a Magnon AFD 300 alternating-field demagnetizer using a 100 mT peak alternating field and a 100 µT DC bias. The resulting IRM and ARM were measured with the same Mu-Spin magnetometer.
Specimens from impact melt (n = 23), monomict breccia (n = 5), and unshocked target rocks (n = 15) were subjected to alternating-field demagnetization (AFD) using a three-axis AGICO LDA 3A demagnetizer (AGICO, Czech Republic) in eight steps up to 100 mT. Thermal demagnetization was carried out on representative specimens of impact melt (n = 4) and monomict breccia (n = 5) with an MMTD 80 A thermal demagnetizer (Magnetic Measurements Ltd., UK) in 14 steps from 100 to 700 °C. Natural remanent magnetization (NRM), isothermal remanent magnetization (IRM), anhysteretic remanent magnetization (ARM), alternating field demagnetization (AFD), and thermal demagnetization (TD), and dual frequency magnetic susceptibility measurement were conducted at the Rock Magnetic Laboratory, Department of Geology, Savitribai Phule Pune University (SPPU), Pune. Zijderveld diagrams were generated in PuffinPlot (v1.4.1)76, which was also used for principal component analysis (PCA) to determine characteristic remanent magnetization (ChRM) directions, and for calculating Fisher statistics, k (a measure of the concentration of directions about the mean direction) and α95 (the 95% confidence cone), for site-mean directions assuming a Fisher distribution77.
The Koenigsberger (Q) ratio, defined as the ratio of remanent magnetization to induced magnetization78, was calculated using the formula:
where 37.56 A/m represents the present-day geomagnetic field intensity at the Dhala structure.
Data availability
All data analyzed in this study, including raw paleomagnetic measurements, thermomagnetic data, hysteresis data, and all datasets used to produce the figures and tables in the main manuscript and Supplementary Information, have been deposited in Figshare and are publicly available at79: https://doi.org/10.6084/m9.figshare.30851126.
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Acknowledgements
The SERB/ES/2020301 (DST, India), IITK/ES/2019372 (IIT Kanpur, India), and STC/ES/2025349 supported this study. A.K.P. extends sincere gratitude to the Department of Science and Technology, New Delhi, Government of India, for their generous support in the form of an INSPIRE fellowship (IF190993). We thank the Department of Earth Sciences, Indian Institute of Technology Kanpur, for providing the optical microscopy and the working facilities. The authors acknowledge the open access funding provided by the One Nation One Subscription (ONOS) initiative of the Government of India.
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A.K. Pandey: Conceptualization, fieldwork and sampling, sample analysis, data interpretation, writing original draft, review, and editing. A. Agarwal: Conceptualization, data interpretation, review and editing, funding acquisition. G. Joshi: Fieldwork and sampling, review and editing. S.J. Sangode: Sample analysis, review, and editing. M. Venkateshwarlu: Sample analysis, review, and editing.
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Pandey, A.K., Agarwal, A., Joshi, G. et al. Shock demagnetization in an ambient magnetic field at the Dhala impact structure, India. Commun Earth Environ 7, 145 (2026). https://doi.org/10.1038/s43247-025-03164-6
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DOI: https://doi.org/10.1038/s43247-025-03164-6
