Mantle dynamics, isostasy, and the support of high terrain

Journal article

Dynamic topography is commonly understood to be deflection of the Earth's surface that results from convection of the mantle. Because different authors use the words "dynamic topography" differently, topography designated as dynamic may amount to only a few hundred meters or may exceed 2000 m. For most regions, however, surface heights computed on the assumption that the lithospheric column is in isostatic equilibrium provide good approximations to observed topography. The small free-air gravity anomalies and still smaller isostatic anomalies (<~30-50 mGal) associated with long-wavelength topography suggest that deflections of the Earth's surface induced by flow-induced normal tractions applied to the base of the lithosphere do not exceed ~300 m. Little evidence exists to show that such tractions support more than ~100 m of high terrain in regions like southern Africa, the Rocky Mountains and Colorado Plateau of the USA, eastern Asia, or the Aegean-Anatolian region, where some have argued for many hundreds to >2000 m of dynamic topography. Moreover, simple examples show that if flow-induced stresses maintain a given density distribution, then the resultant surface deflections can be smaller than those that would exist in isostatic equilibrium. In light of confusion over the term "dynamic topography," we offer readers simple tools that we hope will enable them to diagnose the component of topography that results from flow-induced stresses and to distinguish it from topography that is compensated isostatically.

Authors: Molnar, P; England, P; Jones, C

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Journal of Geophysical Research. Solid Earth
• Volume: 120
• Issue: 3
• Pages: 1932-1957
• Publication date: 2015-04-01
• DOI: 10.1002/2014JB011724
• EISSN: 2169-9356
• ISSN: 2169-9313
• Keywords: mantle dynamics; geodynamics; dynamic topography; isostasy