Understanding of seismicity and seismotectonics of Delhi and adjoining areas is essential as these areas lie in the seismic zone IV and are geologically confined to the Delhi Fold Belt (DFB), juxtaposed to the Himalayan Frontal Thrust Fold Belt. Owing to the set-up, seismicity in this area is ascribed to the Himalayan Thrust System and activation of DFB Fault Systems. Considerably improved instrumental seismic monitoring in this area and data analysis had resolved three regions of pronounced seismicity that lie close to Sonepat, Rohtak and western part of the NCT Delhi, attributed to activation of various portions of the fault systems of the DFB. Based on seismic telemetry network data, the seismicity pattern analysis revealed that the Mahendragarh Dehradun Sub-Surface Fault (MDSSF) and Delhi Sargodha Ridge (DSR) are the two major zones of structural importance for the nucleation of seismicity in this region. These revelations were corroborated with the fault plane solution of the earthquakes. The dominant mechanism in nucleation of seismicity in DFB is the thrust with minor strike slip. The seismicity and seismotectonics of Delhi and adjoining areas endemic to activation of DFB is reviewed and presented in this paper.

Clays associated with the Precambrian unconformity-related (sensu lato) uranium mineralization that occur along fractures of Rohini carbonate, Bandai sandstone and clay-organic rich black carbonaceous Gorakalan shale of the Sonrai Formation from Bijawar Group is significant. Nature and structural complexity of these clays have been studied to understand depositional mechanism and palaeoenvironmental conditions responsible for the restricted enrichment of uranium in the Sonrai basin. Clays ( chlorite > illite > smectite mineral assemblages, whereas, Solda Formation contains kaolinite > illite > chlorite clays. It has been found that the former mineral assemblage resulted from the alteration process is associated with the uranium mineralization and follow progressive reaction series, indicating palaeoenvironmental (cycles of tropical humid to semi-arid/arid) changes prevailed during maturation of the Sonrai basin. The hydrothermal activity possibly associated with Kurrat volcanics is accountable for the clay mineral alterations.

Deccan basalt contains primary silicates rich in Ca, Mg, and Fe ions, suitable for CO₂ sequestration. Previous, basalt-water-CO₂ interaction studies were focused on other than Deccan basalt types. However, such studies on the Deccan basalt are rare. Thus, present basalt-CO₂ water saturated interaction modelling under hydrothermal-like conditions was carried out on the Deccan basalt of the Mandla area to understand apposite pCO₂ and time parameters. Modelling results were further validated by experiments run in a laboratory time framework. Present results show negative entropy (ΔS) and enthalpy (Δ_fH) that suggest feasibility of plagioclase, pyroxene and magnetite dissolution. Obtained negative Gibb’s free energy (Δ_fG), Δ_fH and ΔS values for calcite, dolomite and magnesite indicate spontaneous reaction, whereas, positive Δ_fG and negative Δ_fH and ΔS values of the siderite suggest non-spontaneous and opposing reactions. Calcite is the first carbonate mineral to form, but, at a faster rate. Magnetite dissolution begins after a time lag (not initiated along with the plagioclase and pyroxene). X-ray Powder Diffraction results of post-experiment residues revealed formation of calcite, aragonite, ankerite, huntite, siderite, smectite, chlorite, smectite/chlorite mixed layers and chabazite. Scanning electron microscopic images show tiny calcite crystal growth over the larger calcite crystal and incipient-disordered calcite with imperfections on crystal faces. Thus, basalt carbonation is mainly controlled by time, but temperature, pCO₂ and pH played sub-ordinate role. Largely, thermodynamic models do not agree well with the experimental results as numerical models indicate larger carbonate growth. Additionally, transition state theory based models work well to predict dissolution rates for most of the minerals, but, they overpredict growth of the secondary minerals.

Clay minerals associated with intra-volcanic bole horizons of varied colours and thicknesses contain montmorillonite, halloysite and kaolinite, show distinct microstructures and microaggregates. In kaolinite, Fe³⁺ ions substitute for Al³⁺ at octahedral sites. Most of these clays are dioctahedral type, show balance between net layer and interlayer charges. The interstratified illite — smectite (I/S) mixed layers containing variable proportions of montmorillonite. Illite contains sheet-like, well oriented microaggregates. The parallel stacks of chlorite sheets show chlorite/smectite (C/S) mixed layers. Progressive enrichment of Fe and depletion of Al ions with the advancement of kaolinization process is observed. High order of structural and compositional maturity observed in these bole clays, indicate long hiatus between the two volcanic episodes. It is observed that the bole horizons suffered rigorous weathering, approximately 5 fold higher than the respective parent lava flows under the alternate wet and dry spells of climatic changes. Translating the production time estimates of clays associated with the 21 bole horizons, found in the eastern Deccan volcano-sedimentary succession, it is assessed that the minimum time required for their formation is approximately 7 my. Clay minerals that occur across the stratigraphic sequence show cyclic changes in the climate, favouring longer duration of volcanic activity. Late Maastrichtian Lameta beds post-date Deccan volcanism to 70 Ma or earlier to this. Considering the formation of bole clays in terms of time, it is possible that the volcanic activity started much earlier in the late Maastrichtian, and continued after 65 Ma or even later. Thus, the clay stratigraphy based estimates lend support to a prolong duration.