Stable Isotope Paleoaltimetry in Orogenic Belts The Silicate Record in Surface and Crustal Geological Archives

Abstract

In this chapter, we discuss the use of variations in the oxygen and hydrogen isotope compositions of hydrous silicates to paleoaltimetry studies. Currently, there are numerous isotopic (lacustrine, palustrine, and pedogenic carbonates, hydrous silicates, fluid inclusions) and paleofloral (leaf physiognomy, stomata density) proxies that provide information on climatic and topographic histories of mountain belts. Compared to lacustrine and paleosoil carbonates that frequently provide detailed and temporally extensive paleoaltimetric records, the use of hydrous silicates (such as smectite, kaolinite, chert, as well as metamorphic minerals that grow in the presence of meteoric waters) in paleoclimate and paleoaltimetry studies has important additional advantages. First, hydrous silicates provide both a hydrogen and oxygen isotope record. These two isotope systems can be used to evaluate climate change, but are also useful in understanding evaporative effects on meteoric waters prior to mineral formation. Since evaporation can cause isotopic changes that otherwise would correspond to changes on the order of kilometers of surface uplift, it is critical in any isotopic paleoaltimetry study within continental interiors to examine both the hydrogen and oxygen isotope record. Second, hydrogen isotope ratios of micas in deep-seated extensional shear zones often reflect the composition of meteoric waters that infiltrated these rocks during the late stages of orogenesis. As such, it is possible to use the combined oxygen, hydrogen and geochronological record of synkinematic micas in extensional shear zones as a paleoaltimeter. This information when combined with high-precision geochronology and paleoaltimetry studies of intermontane basins formed above these shear zones allows for the development of an integrated structural, sedimentologic, and surface elevation history of an evolving orogen.