Timing

Several methods are available for determining the timing of a geologic event.  Typically, radiogenic isotopes are used to date an igneous rock.  From these dates relative times (during, before, after) of structural formations can be determined.  The Cascade Lake shear zone was determined to have been syntectonic with the formation of the Cathedral Peak pluton.  This pluton was found to have formed 85-80 Ma dated using U/Pb isotope ratios in zircons [Stern et al. 1981].

²³⁸U is a radiogenic isotope and decays to ²⁰⁶Pb.  ²³⁸U decays according to

²⁰⁶Pb = ²⁰⁶Pb_i + ²³⁸U(e^lt+1)

where i refers to the initial amount of Pb and l is the decay constant of ²³⁸U, equal to 1.55125 10-10 yr^-1.  When this technique is applied, the ratio of ²⁰⁶Pb to the stable isotope ²⁰⁴Pb is measured.  The time since the system was closed is then given by

t = (1/l)*ln[(²⁰⁶Pb/²⁰⁴Pb - (²⁰⁶Pb/²⁰⁴Pb)_i)/(²³⁸U/²⁰⁴Pb) + 1]

The same ratios can be obtained for the system of ²³⁵U decaying to ²⁰⁷Pb with a decay constant of 9.8485 10^-10 yr^-1.  These two methods should give the same age and the relation of ²⁰⁶Pb/²³⁸U to ²⁰⁷Pb/²³⁵U should lie along a concordia (figure 1).  If some amount of lead has been able to diffuse out of the crystal, this relationship will lie along a discordia where the upper intercept with the concordia represents the true age of the rock.  In zircons, very little lead will diffuse out of the crystal and the relationship of the measured ratios will lie close to the concordia.

The ages that were obtained for the syntectonic Mono Creek Granite are the same as those obtained for the Cathedral Peak pluton (85-80 Ma).  The Gem Lake shear zone is structurally continuous with the Cascade Lake shear zone.  These shear zones are therefore thought to be contemporaneous and to have been deforming during 85-80 Ma.