A GRAVITY MODEL OF THE NORTH EURASIA CRUST AND UPPER MANTLE: 1. MANTLE AND ISOSTATIC RESIDUAL GRAVITY ANOMALIES

Abstract

A numerical density model of the North Eurasia crust is constructed and its gravity effect is calculated. This model includes variations in the thickness and density of the sedimentary cover and solid crust, derived from generalization of seismic and geological data and specified on a 1o<img src="latex008.gif" alt="times">1o grid within (30°-75° N, 14° W-195° E). Residual mantle anomalies are calculated by removing the anomalous model field from the observed gravity field. The mantle anomalies are reliably separated into two components accounting for the effects of different factors: (1) The regional component does not correlate, in a first approximation, with crustal structures and reflects large-scale structural heterogeneities of the Eurasia lithosphere, supposedly related to its thermal regime. Intense positive anomalies characterize northern and central Eurasia, and negative anomalies are observed in Western Europe and Southeast Asia. The regional component is consistent with the shear wave velocity distribution obtained from seismic tomography data. (2) The local component of the mantle gravity field has wavelengths shorter than 2000-2500 km and evidently correlates with specific tectonic structures. Maximum positive anomalies with amplitudes exceeding 100 mGal characterize some structures of the East European platform (Baltic Shield and Voronezh Massif) and East Siberia (Tunguska syneclise). A chain of negative mantle anomalies is clearly traced west of the Tesseyre-Tornquist line (Pannonian basin-Rhine graben-Massif Central). In Central Asia the most prominent zone of negative mantle anomalies is located southwest of Lake Baikal, approximately in the Khamar-Daban Range area. These anomalies are likely to be related to intrusion of an anomalously hot and light mantle. The intense negative mantle anomalies observed along the eastern boundary of Eurasia are associated with backarc seas. A new map of the isostatic gravity anomalies is calculated throughout the territory studied. As distinct from previous studies, real data on the crustal structure, including variations in the thickness and density of the sedimentary cover and solid crust, were used for its calculation. The use of these data instead of the traditional Airy scheme has led in many cases to a revision of notions concerning the isostatic state of the crustal structures. In particular, as compared with previous maps, significantly reduced amplitudes of the isostatic anomalies are obtained for the South Caspian, Tien Shan and Urals regions. E). Residual mantle anomalies are calculated by removing the anomalous model field from the observed gravity field. The mantle anomalies are reliably separated into two components accounting for the effects of different factors: (1) The regional component does not correlate, in a first approximation, with crustal structures and reflects large-scale structural heterogeneities of the Eurasia lithosphere, supposedly related to its thermal regime. Intense positive anomalies characterize northern and central Eurasia, and negative anomalies are observed in Western Europe and Southeast Asia. The regional component is consistent with the shear wave velocity distribution obtained from seismic tomography data. (2) The local component of the mantle gravity field has wavelengths shorter than 2000-2500 km and evidently correlates with specific tectonic structures. Maximum positive anomalies with amplitudes exceeding 100 mGal characterize some structures of the East European platform (Baltic Shield and Voronezh Massif) and East Siberia (Tunguska syneclise). A chain of negative mantle anomalies is clearly traced west of the Tesseyre-Tornquist line (Pannonian basin-Rhine graben-Massif Central). In Central Asia the most prominent zone of negative mantle anomalies is located southwest of Lake Baikal, approximately in the Khamar-Daban Range area. These anomalies are likely to be related to intrusion of an anomalously hot and light mantle. The intense negative mantle anomalies observed along the eastern boundary of Eurasia are associated with backarc seas. A new map of the isostatic gravity anomalies is calculated throughout the territory studied. As distinct from previous studies, real data on the crustal structure, including variations in the thickness and density of the sedimentary cover and solid crust, were used for its calculation. The use of these data instead of the traditional Airy scheme has led in many cases to a revision of notions concerning the isostatic state of the crustal structures. In particular, as compared with previous maps, significantly reduced amplitudes of the isostatic anomalies are obtained for the South Caspian, Tien Shan and Urals regions.