Tracking changes in crustal thickness during orogenic evolution with Sr/Y: An example from the North American Cordillera

Abstract

Global compilations indicate that the geochemistry of arc magmatism is sensitive to Moho depth. Magmatic products are prevalent throughout the history of Cordilleran orogenesis and can be employed to constrain the timing of changes in crustal thickness as well as the magnitude of those changes. We investigate temporal variations in crustal thickness in the United States Cordillera using Sr/Y from intermediate continental arc magmas. Our results suggest that crustal thickening began during the Late Jurassic to Early Cretaceous and culminated with 55-65-km-thick crust at 85-95 Ma. Crustal thicknesses remained elevated until the mid-Eocene to Oligocene, after which time crustal thicknesses decreased to 30-40 km in the Miocene. The results are consistent with independent geologic constraints and suggest that Sr/Y is a viable method for reconstructing crustal thickness through time in convergent orogenic systems. 1 GSA Data Repository item 2015308, Figure DR1 (unfiltered Great Basin rock analyses with proposed data filters); Table DR1 (Great Basin geochemical data); Table DR2 (accepted Great Basin data subsets by area); Table DR3 (discarded Great Basin data subsets by area); Table DR4 (global geochemical data for Quaternary rock analyses); and Table DR5 (compiled global geochemical data by arc),

Geochronology and geochemistry of early Paleozoic igneous rocks from the Zhangguangcai Range, northeastern China: Constraints on tectonic evolution of the eastern Central Asian Orogenic Belt

“…505-496 Ma monzogranites over time in the northern SZB ( Fig. 11A; Wang et al, 2016a) is consistent with crustal thickening (Chapman et al, 2015) associated with the Cambrian continent-continent collision. The decrease in Sr/Y recorded by the ca.…”

Raising the Gangdese Mountains in southern Tibet

“…Feldspar fractionation increases Rb/Sr ratio in felsic melts because Sr is compatible, whereas Rb is incompatible. Consequently, the elevated Rb/Sr ratio of samples can be used as an indicator of highly fractionated magmas [ Chapman et al ., ]. Thus, to eliminate the effect of differentiation on primary magma composition, geochemical data used in this study were restricted to rocks in the range of 55 to 68 wt % SiO 2 (volatile free), with MgO values 1–6 wt % and Rb/Sr = 0.05–0.2.…”

“…Crustal thickness in the Andean convergent margins correlates with geochemical indices of arc lavas [ Plank and Langmuir , ; Haschke et al ., ]. This correlation was corroborated and formulated by examining geophysically determined crustal thickness (Moho depth) and recently emplaced continental calc‐alkaline rocks in continental arcs [ Mantle and Collins , ; Chiaradia , ; Chapman et al ., ; Profeta et al ., ]. However, whether this correlation was present in syncollisional to postcollisional settings has not yet been tested [ Profeta et al ., ], making it equivocal to track crustal thickness using geochemical indices of samples from such settings.…”

“…The empirical fit defined by the La/Yb ratios of global intermediate rocks with crustal thickness [ Profeta et al ., ] is used to track the crustal thickness of the Gangdese arc in southern Tibet. This is because (1) the basaltic rocks that are available for the approach of Mantle and Collins [] are rare in the arc and (2) some intermediate rocks show very high Sr/Y ratios (>100) that are greater than expected for differentiation within even the thickest crust based on global compilations [ Chapman et al ., ]. (La/Yb)n (n denotes normalized to chondritic values of McDonough and Sun []) and calculated crustal thickness, following the method of Profeta et al .…”

“…Comparison between the compositions of recently emplaced intermediate continental calc‐alkaline magmatic rocks and geophysically determined crustal thickness, as well as studies from global correlations and specific orogens, show that geochemical indices can be used to track quantitatively temporal variations of crustal thickness in continental arcs [ Chiaradia , ; Chapman et al ., ; Profeta et al ., ]. An extended application of this approach in this paper to the Gangdese arc in southern Tibet (Figure a), and assuming Airy isostatic compensation, enables us to quantify the crustal thickness of the arc and reconstruct the spatial and temporal uplift history of the Gangdese Mountains.…”