Inferring the rheology of the crust from the uplift observed above the Altiplano-Puna Magma Body

Abstract

SUMMARYGeophysical imaging techniques together with numerical models have shown that the surface uplift measured above the Altiplano-Puna Magma Body (APMB) can be explained by the presence and propagation of a diapir from the top of the APMB itself. In this work, we model deformation that characterizes the crustal region above and around APMB through the use of a viscoplastic rheology. That is, we assume that at large scale the crust that surround the magmatic mushy diapir behaves as a yield-stress fluid described by the Herschel–Bulkley (HB) model, whereby motion develops only when the local deviatoric stress is greater than a critical value, the yield stress. In this scenario, laboratory and numerical results show that there are two main critical conditions needed for the growth and subsequent rise of a diapir: (1) the ratio between the yield stress and viscous stresses, namely the Bingham number Bi, has to be less than 1, that is Bi ≤ 1 and (2) the ratio between buoyancy stresses and the yield stress, namely the inverse Yield number Yinv, has to be larger than a critical value $Y_{{\rm inv}_C}$. Using these two conditions allows us to estimate the bulk rheological properties of the heterogeneous crust above APMB as a function of the diapir’s size and density contrast with the crust. For the development of a 10–100 km wide diapir, 100–400 kg m–3 lighter than the surrounding crust, the crust yield stress should range between 0.5 and 15 MPa. Then, the regional uplift velocity measured at the surface implies a strain rate greater than ∼10−15–10−16 s−1 and a crust maximum bulk effective viscosity of ηc=1021 Pa.s.