ABSTRACT The ~11-km-wide, Paleoproterozoic Dhala impact structure in north-central India comprises voluminous exposures of impact melt breccia. These outcrops are discontinuously spread over a length of ~6 km in a semicircular pattern along the northern, inner limit of the monomict breccia ring around the central elevated area. This study of the magnetic fabrics of impact breccias and target rocks from the Dhala impact structure identified a weak preferred magnetic orientation for pre-impact crystalline target rocks. The pre- and synimpact rocks from Dhala have magnetite and ilmenite as common magnetic phases. The distributions of magnetic vectors are random for most impact melt breccia samples, but some do indicate a preferred orientation. Our anisotropy of magnetic susceptibility (AMS) data demonstrate that the shape of susceptibility ellipsoids for the target rocks varies from prolate to oblate, and most impact melt breccia samples display both shapes, with a slight bias toward the oblate geometry. The average value for the corrected degree of anisotropy of impact melt rock (P′ = 1.009) is lower than that for the target rocks (P′ = 1.091). The present study also shows that both impact melt breccia and target rock samples of the Dhala structure have undergone minor postimpact alteration, and have similar compositions in terms of magnetic phases and high viscosity. Fine-grained iron oxide or hydroxide is the main alteration phase in impact melt rocks. Impact melt rocks gave a narrow range of mean magnetic susceptibility (K m ) and P′ values, in contrast to the target rock samples, which gave K m = 0.05–12.9 × 10 −3 standard international units (SI) and P′ = 1.036–1.283. This suggests similar viscosity of the source magma, and limited difference in the degrees of recorded deformation. Between Pagra and Maniar villages, the K m value of impact melt breccias gradually decreases in a clockwise direction, with a maximum value observed near Pagra (K m = 1.67 × 10 −3 SI). The poor grouping of magnetic fabrics for most impact melt rock samples implies local turbulence in rapidly cooled impact melt at the front of the melt flow immediately after the impact. The mean K 1 for most impact melt samples suggests subhorizontal (<5°) flow in various directions. The average value of K m for the target rocks (4.41 × 10 −3 SI) is much higher compared to the value for melt breccias (1.09 × 10 −3 SI). The results of this study suggest that the melt breccias were likely part of a sheet-like body of sizeable extent. Our magnetic fabric data are also supported by earlier core drilling information from ~70 locations, with coring depths reaching to −500 m. Our extensive field observations combined with available widespread subsurface data imply that the impact melt sheet could have covered as much as 12 km 2 in the Dhala structure, with an estimated minimum melt volume of ~2.4 km 3 .

The ~11-km-wide Dhala impact structure, in north-central India, is located in Archean granitoids of the Bundelkhand craton and is partially exposed beneath Paleoproterozoic sediments belonging to the Vindhyan Supergroup, which place the age of the structure between ca. 2.5 Ga and ca. 1.7 Ga. Field mapping and extensive drill-core observations show that an originally coherent impact melt sheet up to 130 m thick was spread over more than 12 km 2 . Confirmation of the impact origin of the structure is based on the recognition of diagnostic shock metamorphic features in clasts in impact melt rock. In order to better constrain the age of the impact event, a geochronological study was undertaken of the Dhala impact melt breccia, involving 40 Ar/ 39 Ar step-heating and sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating techniques that were applied to surface and subsurface samples. The U-Pb data for two samples yield ages of 2563 Ma and 2553 Ma, which indicate the age of the granitoid basement. The 40 Ar/ 39 Ar experiments resulted in partial plateau ages that indicate that the Dhala impact melt rock was affected by a strong thermal/hydrothermal overprint at ca. 1 Ga. SHRIMP U-Pb ages for two zircon overgrowths indicate a ca. 530 Ma event that could have contributed to the post-impact resetting of the impact melt rock. The results indicate a common problem experienced when attempting to date small- to moderately sized impact events. Further analysis is planned.