The Aravalli Mountain Range is a northeast-southwest trending orogenic belt in the northwest part of India and is part of the Indian Shield that was formed from a series of cratonic collisions.Mishra, D.C.; Kumar, M. Ravi. Proterozoic orogenic belts and rifting of Indian cratons: Geophysical constraints. Geoscience Frontiers. 2013 March. 5: 25–41. The Aravalli Mountains consist of the Aravalli and Delhi fold belts, and are collectively known as the Aravalli-Delhi orogenic belt. The whole mountain range is about 700 km long.Mckenzie, N. Ryan; Hughes, Nigel C.; Myrow, Paul M.; Banerjee, Dhiraj M.; Deb, Mihir; Planavsky, Noah J. New age constraints for the Proterozoic Aravalli–Delhi successions of India and their implications. Precambrian Research. 2013 November. 238: 120–128. Unlike the much younger Himalayan section nearby, the Aravalli Mountains are believed much older and can be traced back to the Proterozoic Eon. They are arguably the oldest geological feature on Earth. The collision between the Bundelkhand craton and the Marwar craton is believed to be the primary mechanism for the development of the mountain range.
The precise evolutionary processes responsible for the Aravalli Mountain Range remains controversial today, and there are diverse theories put forward for the tectonic history.
Geology of the Aravalli Mountains
The Aravalli Mountain Range features a horst-like structure and consists of a series of Proterozoic rocks that are intensely deformed and metamorphosed.Verma, P.K.; Greiling, R.O.. Tectonic evolution of the Aravalli Orogen (NW India): an inverted Proterozoic rift basin?. Geol Rundsch. 1995 August. 84: 683–696.
General formation Three main subdivisions of rocks constitute the stratigraphy of the mountain range, with the Archean Bhilwara Gneissic Complex basement being the lowest strata, followed by the overlying lower Aravalli Supergroup and the upper Delhi Supergroup. The northern part of the mountain range only consists of the Delhi Supergroup, and this has given to its name of ‘North Delhi Belt. On the southern side, however, both the Aravalli and Delhi supergroups are present. The mountain range is bounded by the Eastern and Western marginal faults, where the former is also termed as the Great Boundary Fault. +General geological formation of the Aravalli MountainsDelhi SupergroupAjabgarh Group (=Kumbhalgarh Group)Carbonate, mafic volcanic and argillaceous rocksAlwar Group (= Gogunda Group)Arenaceous and mafic volcanic rocksRaialo Group Mafic volcanic and calcareous rocksAravalli SupergroupJharol GroupTurbidite facies and argillaceous rocksDebari GroupCarbonates, quartzite, and pelitic rocksDelwara GroupArchean basementBanded Gneissic Complex (BGC)Schists, gneisses and composite gneiss Quartzites
Archean Bhilwara Gneissic Complex basement The Bhilwara Gneissic Complex basement is about 2.5 Ga old. It is made up of a group of metamorphic and igneous rocks that are mainly amphibolite to granulite grade, tonalitic to granodioritic gneisses and intrusive granitoids with a minor amount of metasedimentary and metavolcanic rocks.Kaur, Parampreet; Zeh, Armin; Chaudhri, Naveen; Gerdes, Axel; Okrusch, Martin. Archaean to Paleoproterozoic crustal evolution of the Aravalli mountain range, NW India, and its hinterland: The U-Pb and Hf isotope record of detrital zircon. Precambrian Research. 2011 March. 187: 155–164.Lente, B. Van; Ashwal, L.D.; Pandit, M.K.; Bowring, S.A.; Torsvik, T.H.. Neoproterozoic hydrothermally altered basaltic rocks from Rajasthan, northwest India: Implications for late Precambrian tectonic evolution of the Aravalli Craton. Precambrian Research. 2009 January; 170: 202–222. The basement is categorized into two subdivisions: the Sandmata Complex and the Mangalwar Complex.Rao, V. Vijaya; Prasad, B. Rajendra; Reddy, P.R.; Tewari, H.C.. Evolution of Proterozoic Aravalli Delhi Fold Belt in the northwestern Indian Shield from seismic studies. Tectonophysics. 2000 June. 327 (1–2): 109–130.Sinha-Roy, S.; Malhotra, G.; Guha, D.B.. A transect across Rajasthan Precambrian terrain in relation to geology, tectonics and crustal evolution of south-central Rajasthan. In: Sinha-Roy, S., Gupta, K.R. (Eds.), Continental Crust of NW and Central India. Geological Society, India. 1995. 31: 63–89. The Sandmata Complex refers to gneisses and granitoids while the Mangalwar Complex refers to the metasedimentary and metavolcanic series which are considered to be metamorphosed older granite-greenstone belt.
Aravalli Supergroup On top of the Archean basement, the Aravalli Supergroup overlies with clear unconformities separating the two strata. The Aravalli Supergroup is divided into three groups: lower Delwara Group, middle Debari Group, and Upper Jharol Group.Meert, Joseph G.; Pandit, Manoj K.. The Archean and Proterozoic history of Peninsular India: tectonic framework or Precambrian sedimentary basins in India. In: Mazumder, R. & Eriksson, P. G. (eds), Precambrian Basins of India: Stratigraphic and Tectonic Context. Geological Society, London. 2015 March. 43: 29–54 The lower and middle groups shared similar lithology, where both groups are dominated by carbonates, quartzite, and pelitic rocks, thus suggesting a shelf depositional environment. Turbidite facies and argillaceous rocks are prominent in the upper Jharol group, thus suggesting a deep marine depositional environment. The depositional age of these sequences ranges approximately from 2.1 to 1.9 Ga.
Three major episodes of deformation and metamorphism were involved in the Aravalli Supergroup, including foldings, shearing, kink bands and crenulations etc. Metamorphic grades of the rocks generally range from greenschist facies to amphibolite facies.
Delhi Supergroup The upper Delhi Supergroup overlies on the Aravalli Supergroup with a clear unconformity. This supergroup hosts two main types of rocks: a thick sequence of volcanic rocks that is of continental affinity; and sedimentary rocks that represent fluvial and shallow marine environments and deep marine depositional environment. The depositional age of these sequences is approximately from 1.7 to 1.5 Ga.
In the ‘North Delhi belt’, the Delhi Supergroup is classified into three groups: lower Raialo Group, middle Alwar Group, and upper Ajabgarh Group.Roy, A.B., Jakhar, S.R., 2002. Geology of Rajasthan (Northwest India). Precambrian to Recent. Scientific Publishers (India), Jodhpur, 421 pp. The Raialo Group consists predominantly of mafic volcanic and calcareous rocks. The Alwar Group mainly consists of arenaceous and mafic volcanic rocks. The Ajabgarh Group is dominated by carbonate, mafic volcanic and argillaceous rocks. In the southern part, similar rock types, despite different names, are identified, where they are Gogunda Group (equivalent to Alwar group) and the Kumbhalgarh Group (equivalent to Ajabgarh Group).Gupta, S.N.; Arora, Y.K.; Mathur, R.K.; Iqbaluddin, Prasad B.; Sahai, T.N.; Sharma, S.B.. The Precambrian geology of the Aravalli region, southern Rajasthan and northeastern Gujarat. Geological Survey India. 1997. 123: 262.
Four phases of tectonic evolution The tectonic evolution of the Aravalli-Delhi orogenic belt can be divided into four phases: Bhilwara Gneissic Complex (~ 2,500 Ma) Aravalli Orogeny (~ 1,800 Ma) Delhi Orogeny (~ 1,100 Ma) Post-orogenic evolution (~ 850 – 750 Ma) Two phases of rifting, sedimentation, collision and suturing were documented in the tectonic evolution of the Aravalli-Delhi orogenic belt. During Proterozoic Eon, N-S convergence between the Bundelkhand and Bhandara cratons at the Satpura Mobile Belt, and E-W convergence between the Bundelkhand and Marwar cratons at the Aravalli-Delhi orogenic belt have synchronously occurred in India. This resulted in an overall resultant force of NE-SW convergence of the Aravalli-Delhi orogenic belt, and also led to the arcuate shape of its convergent zone.
Evolution of the Archean basement The Aravalli Mountains basement started with an older sialic crust evolving into extensive granitic batholiths by the emplacement of granitic bodies during the period ca. 3.0 to 2.5 Ga. This subsequently led to rapid cratonization and rapid thickening of crust to about 20–25 km.Choudhary AK, Gopalan K, Sastry CA (1984) Present status of the geochronology of the Precambrian rocks of Rajasthan. Tectonophysics 105: 131–140.Naqvi SM, Divakar Rao V, Hari Narain (1974) The protocontinental growth of the Indian Shield and the antiquity of its rift valleys. Precambrian. Res 1: 345–398. The main crustal source is believed to be old crustal components of the area.Condie, K.C.; Beyer, E.; Belousova, E.; Griffin, W.L.; O’Reilly, S.Y.. U–Pb isotopic ages and Hf isotopic composition of single zircons: the search for juvenile Precambrian continental crust. Precambrian Research. 2005. 139: 42–100. The region subsequently experienced a large scale metamorphic event that granite is partly metamorphosed into gneissic rocks, forming the Archean Basement. These cratonization processes continued together signifying the end of amalgamation of cratonic nuclei that constitutes the development of an early continental crust.
Aravalli Orogeny During the Paleoproterozoic Era, the opening of Aravalli oceanic basin separated the eastern Bundelkhand craton and the western Marwar craton. Sedimentation of the Aravalli Supergroup took place simultaneously with basic magmatism and followed by a gradual subsidence of the Aravalli Basin floor.Mathur, RK; Prasad, B; Sharma, BS; Iqbaluddin. Synsedimentational shoreline volcanism in Aravalli of Rajasthan. Geological Survey India News. 1978.
Soon after the rifting phase ended, the compressional phase took place where the eastern Bundelkhand craton subducted under the western Marwar craton. As collision continued, the subduction zone steepened, leading to the development of an island arc between the two cratons. After collision had proceeded for a certain period of time, the uplift of the Aravalli Supergroup was induced at around 1800 Ma. In the last stage of convergence, the thrust fault further steepened and the colliding blocks eventually become sutured. The suture zone is marked by the Great Boundary Fault.
Delhi Orogeny During the Mesoproterozoic Era, another rifting phase began.Deb, M., Talwar, A.K., Tewari, A., Banerjee, A.K., 1995. Bimodal volcanism in South Delhi fold belt: a suite of differentiated felsic lava at Jharivav, north Gujarat. In: Sinha-Roy, S., Gupta, K.R. (Eds.), Continental Crust of NW and Central India. Geol. Soc. India, Memoir 31, pp. 259–278. At that time, the Bundelkhand-Aravalli-BGC and the Marwar craton lie on the eastern side and western side respectively as the rifting phase separated the Bhilwara Gneissic Complex (BGC) from the Marwar craton. The oceanic basin created in the course of rifting received the Delhi Supergroup sediments.
The compressional phase that followed led to eastward subduction of the western Marwar craton. Continuous subduction of the western block might have created another island arc, and similar to the Aravalli orogeny, further collision between the two blocks with island arc in between gave rise to the development of the Delhi orogeny around 1100 Ma. The suture zone between the two cratons is marked by the Western Marginal Fault and the emplacement of the Phulad Ophiolite Suite in the region.
Post-orogenic evolution
Acid magmatic events