by Dr Jaideep Kaur Tiwana, Geologist at Geological Survey of India
The Aravalli–Delhi Mobile Belt (ADMB) is a structural archive of major Precambrian orogenic events that shaped the northwestern Indian subcontinent. The Aravalli Supergroup, located in the southern part of the ADMB, represents a Paleoproterozoic metasedimentary succession that unconformably overlies the Archean Banded Gneissic Complex. Geologically, it comprises quartzites, phyllites, dolomites and mica schists, deposited in environments ranging from shallow marine shelves to deeper basinal settings. Major base metal deposits of lead, zinc, and copper are primarily hosted within the Paleoproterozoic Aravalli-Delhi Fold Belt, specifically within metasedimentary sequences. Understanding the belt’s tectonic history is crucial for guiding exploration and mining. This article presents information pertaining to folded metasedimentary succession around the Dungarpur region exhibiting pronounced tectonic complexity recorded through composite foliations, mineral lineations, superposed folds and well-developed interference patterns.
The sequence records polyphase deformation labelled as AF₁ , AF₂ , AF₃ and AF₄ , accompanied by greenschist to amphibolite facies metamorphism, making it crucial for understanding early basin evolution and Proterozoic tectonics. The first-generation AF₁ folds are predominantly rootless, isoclinal, and recumbent. The second-generation AF₂ folds are upright to steeply inclined, whereas the late-stage AF₃ deformation is manifested in the form of recumbent folds and sharp-hinged kink bands.
D₁ deformation and development of S₁ foliation
The first deformation phase (D₁) is marked by the development of a penetrative S₁ foliation (Figure 1), which is axial planar to tight, isoclinal AF₁ folds. It is characterized by rootless folding (Figure 2) and well-developed L₁ mineral lineations parallel to the F₁ fold hinges. Intense layer-parallel shortening during D₁ led to the complete or partial transposition of primary bedding.
D₂ deformation: AF₂ folds and crenulation cleavage
The D₂ deformation phase produced spaced crenulation cleavage (S₂) (Figure 3) and steeply inclined (Figure 4), tight to close folds exhibiting variable plunge but generally displaying NE–SW-oriented axial planes (Figure 5). AF₂ folds refolded both the S₁ foliation and earlier quartz veins which were emplaced along planar S₁ fabric (Figure 6). At several locations, these quartz veins are subjected to boudinage, particularly within the limb domains, reflecting high-strain partitioning. These structures collectively indicate a progressive and continuous ductile deformation regime.
D₃ phase and late brittle overprints
The third-generation AF₃ folds modify earlier structural fabrics and fold geometries developed during AF₁ and AF₂. These folds typically appear as broad, open recumbent folds with shallow plunging fold axes and nearly horizontal axial planes (Figure 7). Their development is thought to be driven by horizontal compression, possibly combined with vertical flattening or gravitational readjustment during the waning stages of orogenesis. In contrast to the intense, tightly packed folds of earlier phases (D₁ and D₂), the D₃ phase produced gentler, more open structures. Sharp-hinged kink bands with sub-horizontal axial planes are commonly observed on the limbs of megascopic AF₂ folds (Figure 8). Kinking reflects a brittle overprint on an otherwise ductile deformation history and is most prominently developed in high-strain domains, where earlier foliations are tightly spaced.
Another late-stage deformation event, designated as AF₄, is manifested through the development of NE–SW-oriented fracture cleavage (S₄) and jointing. S₄ often develops orthogonal to F₂ fold axes and can locally accentuate pre-existing fold geometries, thereby enhancing the doubly plunging appearance of earlier F₂ folds. The development of these brittle structural features suggests a late-stage tectonic evolution of the Proterozoic basin, reflecting the gradual shift from deep and ductile to more brittle deformation during the waning phases of the orogeny.
Earth’s past in slice: Structural tale from petrography
The deformation history of the Aravalli Supergroup is equally well-preserved at the microscale. The D₁ deformation phase is represented by a well-developed S₁ foliation, defined by the parallel alignment of muscovite, biotite, and chlorite in the pelitic layers (Figure 9). This foliation appears as a continuous, planar fabric, occasionally wrapping around porphyroblasts of garnet and magnetite, indicating syn-tectonic growth. Evidence of dynamic recrystallization along S₁, particularly in quartz-rich domains, is observed as undulose extinction. The subsequent D₂ deformation phase is recorded by the development of a prominent crenulation cleavage (S₂) that appears as a series of spaced microfolds or kink-like features (Figure 10). In areas of strong crenulation, the earlier S₁ fabric becomes discontinuous or obliterated within fold hinges. Folded inclusion trails within porphyroblasts further support the overprinting relationship between D₁ and D₂.
Earth origami and structural memory
One of the striking structural features in this terrain is the presence of coaxial, hook-shaped folds, where the first two folding phases (AF₁ and AF₂) exhibit Type 1 interference patterns (as defined by Ramsay). Early-generation recumbent, tight to isoclinal AF₁ folds are refolded by AF₂ (Figure 11), and appear more reclined, especially within the limb domains of the AF₂ folds. The S₁ schistosity associated with AF₁ is often crenulated or transposed by a later axial planar fabric (S₂), yet the orientation of fold axes remains broadly consistent, representing a classical signature of progressive coaxial deformation (Figure 12). The superposition of successive folding phases generates complex interference patterns, including doubly plunging (Figure 13), non-planar, non-cylindrical folds (Figure 14) and Type 1 dome-and-basin geometries. These structural features are closely associated with the Aravalli orogeny and likely record the tectonic fabric formed during the accretion of the Aravalli metasediments over Archean basement.
Metallogeny and global connections
The structural complexity preserved within the Aravalli Supergroup records a rich deformational history and shows its direct implications for base metal mineralization. Geological and geochronological studies across famous deposits (Zawar, Rajpura–Dariba, and Rampura-Agucha) indicate that the initial zinc–lead–copper sulfide mineralization likely occurred during the early stages of basin development, making it syngenetic with sedimentation. These stratiform ore bodies were later affected by successive deformation events, particularly those associated with the D₁ and D₂ phases, which played a key role in remobilizing and concentrating the mineralization along axial-planar cleavages, fold hinges and shear zones. The D₂ deformation enhanced the structural permeability and created favorable sites for fluid flow and ore localization. Later tectonothermal overprints, like those linked to the Grenvillian orogeny (~1.0 Ga), further recrystallized and upgraded these deposits. An early syngenetic phase followed by structural and metamorphic reworking highlights a significant connection between deformation and the region’s metallogenic evolution.
The Aravalli folding story is not restricted to the Indian Shield. Its orogeny aligns with the assembly of the Nuna (Columbia) supercontinent. The subsequent Grenvillian orogeny likely left a thermal imprint and subtly reworked these earlier Aravalli structures, reflecting the tectonic processes associated with the formation of Rodinia.
The images here convey the intricate structures of the Aravalli Fold Belt. History is recorded in folds, cleavages and the slow warping of rock through time. Every structural fabric, hinge and overturned limb carries a story etched by pressure, heat and motion, which geologists are striving to unravel—one fold at a time.
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