MI2021 - 12 Will McCarthy: Combined AMS and geochronology on late Caledonian transcurrent tectonism
Channel: MagNetZ
Date: 2021-02-18

Lost in Translation: A combined AMS and geochronology study to provide new temporal constraints on late Caledonian transcurrent tectonism

Will McCarthy, University of St Andrews, UK

To unravel the deformation history of an orogeny, strain markers that record distinct phases of deformation throughout its duration need to be identified. However, the availability of strain markers that can be accurately and precisely dated are highly limited. Importantly, the intrusion of granitoid magma, which can be easily dated, is common during orogenesis and the utility of magnetic anisotropy methods (e.g. AMS, AARM, AIRM, opAMS), can provide a record of syn- and/or post-emplacement deformation.This study investigates the idea that incremental strain is effectively recorded as magma is progressively injected into the crust to form plutons during orogenesis, i.e. plutons emplaced at different times will record ambient far field strain as they crystallize and a record of the evolving orogenic stress field will be preserved. The utility of temporally constrained strain markers greatly enhances our understanding of the timing and stress kinematics of orogenic events. For example, the transcurrent regime during the late Caledonian Orogeny ~430 Ma to ~400 Ma is conveniently divided into an earlier transpressive and a later transtensive kinematic regime, however, the precise timing of this stress field change is debated. Construction of the 425 Ma to 380 Ma Galway Granite Complex in western Ireland transcends this time interval, it thus provides an ideal geological setting to test our hypothesis.We present data from over 500 AMS block samples and four new U-Pb zircon age determinations from four plutons of the Galway Granite Complex. These data show that the K3 axis of AMS ellipsoids collected from intrusions aged 425 Ma - 410 Ma are horizontal, these results are consistent with a regional compressive tectonic regime. Data collected from intrusions aged between 410 Ma to 402 Ma return subhorizontal K3 axis, this points towards a regional extensional regime and possible post orogenic regional transtension. We conclude that many mesozone syntectonic intrusions can be used as effective strain markers when high-quality age determinations and AMS data are available and that such an approach could be used to enhance the resolution of kinematic tectonic models across ancient orogeny.