Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps

TY - JOUR

T1 - Strain rates at high temporal resolution from curved inclusion trails in garnet, Passo del Sole, Central Swiss Alps

AU - Berg, C. A.

AU - Carlson, W. D.

AU - Connelly, James

PY - 2013

Y1 - 2013

N2 - Quantitative strain rates at outcrop scale are very difficult to obtain, but they may be estimated from crystals with curved inclusion trails by calculating rotation rates from growth rates and corresponding deflections of the internal foliation. Garnet in a quartzose pelite at Passo del Sole in the central Swiss Alps is extraordinarily valuable for calculation of strain rates during Alpine orogenesis, because the unusual zoning patterns clearly define the kinetics of its nucleation and growth. Complex concentric zoning patterns can be correlated from one crystal to another in a hand sample, based on compositional and microstructural similarities; the ubiquity of these features demonstrates that all garnet crystals nucleated at nearly the same time. Compositional bands whose radial widths are proportional to crystal size provide evidence for growth governed by the kinetics of intergranular diffusion of locally sourced nutrients. Together, these constraints increase the reliability of estimates of rates of garnet growth, and the strain-rate calculations that depend on them. To obtain growth rates, P-T conditions during garnet crystallization were modelled in a series of pseudosections, and compositional evolution was connected to rates of garnet growth by means of an independently determined heating rate. These growth rates, combined with measured amounts of curvature of inclusion trails, indicate that the time-averaged strain rate at Passo del Sole during Alpine metamorphism was on the order of 10-14s-1. Strain rates calculated using rotational v. non-rotational models are similar in magnitude. The constraints on crystallization kinetics also allow direct calculation of strain rates during individual stages of garnet growth, revealing short-term increases to values on the order of 10-13s-1. These higher strain rates are correlated with the growth of concentric high-Ca or high-Mn zones in garnet, which implies that strain softening associated with the transient passage of fluids is responsible for acceleration of deformation during these intervals.

AB - Quantitative strain rates at outcrop scale are very difficult to obtain, but they may be estimated from crystals with curved inclusion trails by calculating rotation rates from growth rates and corresponding deflections of the internal foliation. Garnet in a quartzose pelite at Passo del Sole in the central Swiss Alps is extraordinarily valuable for calculation of strain rates during Alpine orogenesis, because the unusual zoning patterns clearly define the kinetics of its nucleation and growth. Complex concentric zoning patterns can be correlated from one crystal to another in a hand sample, based on compositional and microstructural similarities; the ubiquity of these features demonstrates that all garnet crystals nucleated at nearly the same time. Compositional bands whose radial widths are proportional to crystal size provide evidence for growth governed by the kinetics of intergranular diffusion of locally sourced nutrients. Together, these constraints increase the reliability of estimates of rates of garnet growth, and the strain-rate calculations that depend on them. To obtain growth rates, P-T conditions during garnet crystallization were modelled in a series of pseudosections, and compositional evolution was connected to rates of garnet growth by means of an independently determined heating rate. These growth rates, combined with measured amounts of curvature of inclusion trails, indicate that the time-averaged strain rate at Passo del Sole during Alpine metamorphism was on the order of 10-14s-1. Strain rates calculated using rotational v. non-rotational models are similar in magnitude. The constraints on crystallization kinetics also allow direct calculation of strain rates during individual stages of garnet growth, revealing short-term increases to values on the order of 10-13s-1. These higher strain rates are correlated with the growth of concentric high-Ca or high-Mn zones in garnet, which implies that strain softening associated with the transient passage of fluids is responsible for acceleration of deformation during these intervals.

KW - Alpine metamorphism

KW - Garnet

KW - Lepontine Dome

KW - Strain rate

KW - Strain softening

U2 - 10.1111/jmg.12017

DO - 10.1111/jmg.12017

M3 - Journal article

AN - SCOPUS:84874686476

SN - 0263-4929

VL - 31

SP - 243

EP - 262

JO - Journal of Metamorphic Geology

JF - Journal of Metamorphic Geology

IS - 3

ER -