Alison Piasecki

Late Cretaceous gravitational collapse of the southern Sierra Nevada batholith, California

The Sierra Nevada batholith is an ∼600-km-long, NNW-trending composite arc assemblage consisting of a myriad of plutons exhibiting a distinct transverse zonation in structural, petrologic, geochronologic, and isotopic patterns. This zonation is most clearly expressed by a west-to-east variation from mafic to felsic plutonic assemblages. South of 35.5°N, the depth of exposure increases markedly, and fragments of shallow-level eastern Sierra Nevada batholith affinity rocks overlie deeper-level western zone rocks and subjacent subduction accretion assemblages along a major Late Cretaceous detachment system. The magnitude of displacement along this detachment system is assessed here by palinspastic reconstruction of vertical piercing points provided by batholithic and metamorphic pendant structure and stratigraphy. Integration of new and published U-Pb zircon geochronologic, thermobarometric, (U-Th)/He thermochronometric, and geochemical data from plutonic and metamorphic framework assemblages in the southern Sierra Nevada batholith reveal seven potential correlations between dispersed crustal fragments and the Sierra Nevada batholith autochthon. Each correlation suggests at least 50 km of south- to southwest-directed transport and tectonic excision of ∼5–10 km of crust along the Late Cretaceous detachment system. The timing and pattern of regional dispersion of crustal fragments in the southern Sierra Nevada batholith is most consistent with Late Cretaceous collapse above the underplated accretionary complex. We infer, from data presented herein (1) a high degree of coupling between the shallow and deep crust during extension, and (2) that the development of modern landscape in southern California was greatly preconditioned by Late Cretaceous tectonics.

The isotopic structures of geological organic compounds

Abstract Organic compounds are ubiquitous in the Earths surface, sediments and many rocks, and preserve records of geological, geochemical and biological history; they are also critical natural resources and major environmental pollutants. The naturally occurring stable isotopes of volatile elements (D, 13C, 15N, 17,18O, 33,34,36S) provide one way of studying the origin, evolution and migration of geological organic compounds. The study of bulk stable isotope compositions (i.e. averaged across all possible molecular isotopic forms) is well established and widely practised, but frequently results in non-unique interpretations. Increasingly, researchers are reading the organic isotopic record with greater depth and specificity by characterizing stable isotope ‘structures’ – the proportions of site-specific and multiply substituted isotopologues that contribute to the total rare-isotope inventory of each compound. Most of the technologies for measuring stable isotope structures of organic molecules have been only recently developed and to date have been applied only in an exploratory way. Nevertheless, recent advances have demonstrated that molecular isotopic structures provide distinctive records of biosynthetic origins, conditions and mechanisms of chemical transformation during burial, and forensic fingerprints of exceptional specificity. This paper provides a review of this young field, which is organized to follow the evolution of molecular isotopic structure from biosynthesis, through diagenesis, catagenesis and metamorphism.