Our next technical event is a lunch on Monday 19th November, by Dr Kristina Tietze, visiting from GFZ, Potsdam, Germany. Kristina’s talk is titled, ‘Imaging fluid imprints along active and fossil margins with 3D magnetotelluric inversion – examples from the San Andreas fault and the Gawler craton.’ Abstract and bio below. This event is co-hosted by the Geological Survey of South Australia, Department for Energy and Mining. Lunch will be provided.
Date & Time: Monday 19th November, 12-1 pm
Location: Rm 4.06A/B, Level 4, 11 Waymouth St (ANZ building), Adelaide.
RSVP: To Philip Heath at Philip.Heath@sa.gov.au by midday, Friday 16th November for catering purposes (and please let Phil know if you have any dietary requirements).
Abstract
Presence of fluids alters stress distribution and lowers mechanical strength of rocks. Moreover, fluids transport minerals e.g. from the mantle into the crust and play an important role in the formation of mineral deposits. Thus, imaging the distribution of fluids is crucial to unravel current and past tectonic regimes as well as to understand the prevalence of mineral resources.
Mapping the electrical resistivity structure is particularly useful in this respect as fluid phases and their remnants usually reduce electrical resistivity of rock formations. Magnetotelluric (MT) measurements are capable of recovering the electrical resistivity structure from surface to mantle depths and may be one of the best means to image fluid networks over wide regions.
At the San Andreas fault (SAF) near Parkfield, 2D and 3D MT surveying with more than 250 sites revealed “along-strike” changes on fluid systems at lower crustal to upper mantle depths where the fault changes its mechanical behaviour from creeping to being locked and where the source region of non-volcanic tremors has been located. Narrow, subvertical low-resistive features were interpreted as a migration pathways for fluids from the mantle into the SAF system which lead to fault weakening and associated creep. Where the fault is locked, a crustal fluid channel is absent. Fluids are suggested to be trapped beneath an impermeable layer at mantle depths and co-located NVT events may be caused by episodic fluid release.
In this context, a new MT inversion approach using the MT phase tensor was implemented which improves 3D resistivity models where data is distorted by near-surface inhomogeneities below the resolution scale of the MT experiment. For the SAF data set, application of the new scheme resulted in more reliable images of the 12 km deep sedimentary basin of the Central Valley. Currently, a comparative study of the new and conventional inversion schemes is run for a set of ~280 AusLAMP sites across South Australia where MT is used to uncover former tectonic processes along fossil margins and understand their relation to mineral deposits.
Biography
Kristina joined the German Research Centre for Geosciences (GFZ Potsdam, Germany) in 2007 after completing her Diploma studies in Geophysics at the Johann Wolfgang Goethe University, Frankfurt. As member of the Geo-Electromagnetics working group she focused on application and development of 3D modelling and inversion approaches for magnetotelluric (MT) data. In 2012, she received her PhD from Freie Universität Berlin and GFZ Potsdam for studies of the electrical conductivity structure along the San Andreas fault system with 3D MT inversion. From 2012-2017 she has been heading a project on developing controlled-source electromagnetic methods for monitoring of fluid flow in reservoirs at GFZ Potsdam.
Currently, she is a visiting researcher at the Geological Survey of South Australia and the University of Adelaide working on 3D inversion and modelling of MT data from the AusLAMP project.
