Gravity anomalies and flexure of the lithosphere at mountain ranges

Abstract

The Bouguer gravity anomaly over the Himalayan, Alpine, and Appalachian mountains is characterized by a generally asymmetric gravity “low”, which spans the mountains and associated foreland basins. The minimum of the gravity “low” is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology. In addition, the Alps and Appalachians are associated with a generally symmetric gravity “high” that is unrelated to the topographic relief. Together the gravity low and high form a characteristic positive‐negative anomaly “couple”. The steep gravity gradient between the positive and negative couple correlates with the Insubric line and its equivalents in the Alps and the Brevard Zone in the southern Appalachians. This gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface (buried) and surface (topographic) loading. The magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography. By combining these loads the flexure of the lithosphere and the associated Bouguer gravity anomaly are calculated for different assumed values of the elastic thickness Teof continental lithosphere. The best fitting value of Tefor the Himalayas is 80–100 km, for the Alps 25–50 km, and for the Appalachians 80–130 km. The model calculations suggest that the main contribution to the gravity couple in the Alps and Appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads. However, in the case of the Himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low. When subsurface loads do exist therefore, we would not expect a close correlation between mountain topographic relief and depth to the M discontinuity as required by classical models of isostasy. Rather, the M discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere. The surface and subsurface loads, inferred from the gravity anomaly, play a major role in the development of mountains and foreland basins. The emplacement of large subsurface loads represents the primary event in the Alps and Appalachians, while in the Himalayas the emplacement of the surface load is the primary event. The emplacement of the subsurface load probably marks the initiation of foreland basin development. The regional characteristics of the basin are controlled by the primary load, while local variations are controlled by the secondary, thrust sheet loads. Subsequent movement of either load will produce migration of the basin depocenter. The origin of the subsurface load is not clear, but its association with the insubric line in the Alps and the Brevard Zone in the Appalachians suggests that it may be related to the “obduction” of crustal blocks/flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate.