SPECIAL SECTION: INTRAPLATE SEISMICITY Comparison of the deep crustal structure and seismicity of North America with the Indian subcontinent Walter D. Mooney1, V. Vijaya Rao2,*, P. R. Reddy2, Gary S. Chulick1,3 and Shane T. Detweiler1 1US Geological Survey, Middlefield Road, Menlo Park, California 94025, USA 2National Geophysical Research Institute, Hyderabad 500 007, India 3University of Wisconsin, Whitewater, Wisconsin 53190, USA Geology and tectonics We summarize geophysical information from North America and India, and find many similarities in the deep crustal structure of the two continents. From this, North America we infer that similar processes have operated, particularly in the accretion and stabilization of the cratonic re- The core of the North American continent is composed of gions. Seismic images and other data suggest that plate several Archean cratons including the Superior, Slave, tectonic processes have been active, both in the North Nain, Hearne, Wyoming and Rae (Figure 1). These cratons American and Indian shields, since the late Archean. were welded together during Paleoproterozoic collisions and Precambrian orogens and high-pressure granulite ter- have been coherent since 1.7 Ga5. Plate tectonic processes, rains of both regions have developed nearly identically believed to have been active since the Archean, have dic- through time since about 2500 Ma. Such similarities tated the evolution of this continent. The present-day con- invite direct comparisons of deep crustal structure. tinental boundaries evolved during the Paleozoic era, and Here we present results from work in North America including: maps of crustal thickness, maps of average the two main Paleozoic features of the continent are the P-wave velocity of the crystalline crust, maps of average western Cordillera and the Appalachian fold belt in the west and east respectively. Pn (sub-Moho) velocity, and statistical analysis of crustal properties. The oldest rocks within the present-day Western Cordillera are Precambrian, and the Cordillera now extends from Mex- ico to Alaska6 (Figure 1). The Archean and Proterozoic rocks THE seismic structure of the crust and upper mantle provides of the Cordillera were truncated by late Proterozoic rifts critical information regarding the composition and evolution (related to the break-up of a late Proterozoic supercontinent), of the lithosphere. Studies have been conducted on a global which formed a carbonate-dominated stable continental mar- basis in a wide range of geologic and tectonic environments. gin lasting from the Cambrian to the Early Carboniferous. These environments include: shields, platforms, orogens, This margin has been active since 300 Ma, first with a series rifts, and highly-extended crust. These studies have aided of accretions that formed such structures as the Sierra our understanding of the geologic and tectonic processes 1–4 Nevada mountains, and then since 25 Ma with the extension responsible for the evolution of the crust . In addition, that formed the Basin and Range Province6. seismic profiles can reveal much along active crustal fault The Appalachian fold belt of eastern North America zones, where seismic hazard is high. consists of the eroded core of a Paleozoic mountain chain that Until recently, past geophysical studies in India have been extends some 3000 km from the southeastern United States mainly confined to crustal investigations, and therefore this to Newfoundland (Figure 1). The Appalachian mountains also comparative review will focus on the crust. We summarize contain a record of a number of arc–continent, and continent– velocity–depth structure in different geological settings continent collisions, as well as numerous rifting events7. as deduced from refraction studies, but also to illustrate the finer structural variations depicted in the reflectivity char- acter. In addition, we discuss the seismicity of the two con- India tinents. The Indian subcontinent is a mosaic of several Archean cratonic blocks including the Dharwar, Bhandara, Singhbum, Southern Granulites, Bundelkhand and Mewar crustal8,9 (Figure 2). The Indian Shield has evolved as a result of *For correspondence. (e-mail: [email protected]) the interaction of these crustal blocks which are separated CURRENT SCIENCE, VOL. 88, NO. 10, 25 MAY 2005 1639 SPECIAL SECTION: INTRAPLATE SEISMICITY Figure 1. Generalized map of the cratons and major structural features of North America (after Hoff- man5). YS, Yellowstone; LV, Long Valley. Ages of formation are given in the key at right. by rifts, suture zones, and fold belts. Several of these suture margins. Some of these basins are covered with younger zones have been reactivated during various geological periods flood basalts. The Crozet/Kergulean hotspot (115 Ma) in since the Proterozoic. The Aravalli, Satpura, Delhi and the east and the Reunion hotspot (66 Ma) in the west also Eastern Ghats are the important Paleo-Mesoproterozoic influenced the evolution of the continent, resulting in the fold belts of the Indian shield. extrusion of the Rajmahal and Deccan flood basalts The Indian subcontinent is primarily made up of Pre- which now cover ~20% of the surface of the Indian sub- cambrian rocks. The Dharwar craton (3400 Ma) is a typical continent (Figure 2). Archean greenstone-granite-gneissic terrain as observed elsewhere in the world. The Narmada–Son lineament (NSL) Comparison of seismicity patterns is the dividing line between the northern and southern cratons, and extends to a distance of 1600 km in the NE–SW direction, Earthquakes are one of the primary indicators of present- from the west coast to the northeastern margin of the Indian day plate boundaries. The western margin of the North Shield. It is regarded as a Paleoproterozoic plate boundary American plate is clearly identified by the presence of which reactivated as a rift during subsequent periods. The earthquakes (Figure 3). Likewise, the present northern less deformed intra-cratonic Vindhyan, Cuddapah, Chat- plate boundary of the Indian shield (Himalayan ranges) is tisgarh and Bastar basins are formed during Mesoprotero- also clearly identified by seismicity (Figure 3). Intracon- zoic period at the paleo-plate boundaries (Figure 2). tinental seismicity observed in the Indian shield along the The Indian continent was a part of Gondwanaland during mobile belts (Figure 2) indicates the locations of paleo-plate the Paleozoic era. Present-day continental margins are formed boundaries (sutures) as well. These paleo-boundaries have during the Mesozoic rifting in the east and west with the deep-seated faults which are being reactivated due to stress separation of Australia, Antarctica and Africa/Madagascar accumulation. Eighty per cent of intraplate earthquakes occur during 130–80 Ma period. The Himalayan mountains are at these paleo-plate boundaries10. Many of these paleo–plate evolved during the Eocene by the collision of the Indian boundaries also exhibit high heat flow which raises the plate with the Eurasia. brittle-ductile transition. Petroliferous basins from the Mesozoic to the present are The seismicity associated with the Narmada–Son lineament situated in between cratons and also along the continental (NSL; Figure 2) includes the Jabalpur earthquakes (21 May 1640 CURRENT SCIENCE, VOL. 88, NO. 10, 25 MAY 2005 SPECIAL SECTION: INTRAPLATE SEISMICITY Figure 2. Generalized tectonic map of India. Abbreviations are, Early-late Proterozoic fold belts: AFB, Aravalli Fold Belt; DFB, Delhi Fold Belt; SMB, Satpura Mobile Belt; EGMB, Eastern Ghat Mobile Belt; BPMB, Bhavani Palghat Mobile Belt; NSL, Narmada Son Lineament; CIS, Central Indian Suture. Pro- terozoic Basins: CuB, Cuddapah Basin; Ba, Bastar Basin; ChB, Chattisgarh Basin; VB, Vindhyan Basin. Phanerozoic Basins: GB, Godavari Basin; MB, Mahanadi Basin; BB, Bengal Basin; CB, Cambay Basin; KB, Kutch Basin. Metamorphic activities: a, 2500 Ma ; b, 1800 Ma, c, 1100 Ma; d, 550 Ma. Figure 3. Comparison of seismicity in North America and India. Earthquakes shown are from 1973 to present, 5.0 <= M <= 9.0 and were of shallow (<33 km) depth. Note that seismicity clearly delineates tectonic plate boundaries in western North America and northern India (Himalayas). CURRENT SCIENCE, VOL. 88, NO. 10, 25 MAY 2005 1641 SPECIAL SECTION: INTRAPLATE SEISMICITY Figure 4. Comparison of available crustal structure data for North America and India. In regions where data density is poor, extrapolation techniques are used to infer crustal properties. Also, one can infer properties of the crust in data-poor regions that are of similar tectonic origin to other, better- studied areas. 1997, M = 6.0 and 17 May 1903, M = 6.0), the Son-valley deep crustal reflection data13,14. The main advantages of earthquake (2 June 1927, M = 8.5) and the Satpura earthquake such studies are: (i) the exact time and position of the (14 March 1938, M = 6.2). These events could be attri- seismic sources are accurately known; (ii) there is no reliance buted to reactivation of faults situated at the edges of different on passive (earthquake) sources that may occur infrequently sub-blocks located all along the 1600 km lineament. Other and/or have an unsuitable spatial distribution; (iii) the re- seismic zones are associated with mega-shear zones located cording geometry of the seismic investigation may be along the Eastern Ghat mobile belt (e.g. the Ongole earth- planned in relation to the specific geologic or geophysical quakes, 12 October 1959, M = 6.0, and 27 March 1967, target. M = 5.4) and western