Mylonite, cataclasite, and gouge: reconstruction of mechanical heterogeneity along a continental low-angle normal fault (pdf)
(Journal of Structural Geology) Mylonite, cataclasite, and gouge: reconstruction of mechanical heterogeneity along a low-angle normal fault: Death Valley, USA - ScienceDirect
Graphical abstract for in review paper on fault mechanics of the Boundary Canyon detachment, Death Valley area.
The study area
Is NE of Death Valley, within the Funeral and Grapevine Mountains.
The detachment fault core was studied in detail at seven distinct sites along the surface trace.
Exposures reveal a heterogeneous suite of fault rocks that record slip at different depths and temperatures during fault evolution.
Using my thermo-kinematic model of the fault exhumation, the different fault rocks are placed into paleo-mechanical context.
Field photos
showing some of the mylonites exposed along the fault surface trace.
Fault cores
were measured and sampled at different locations along the detachment
The mineralogy of gouge zones and wall rocks was determined using XRD. These results help understand the possible friction along the fault where gouge-controlled friction, but also help us estimate the relative contribution of footwall vs hanging wall protoliths to the gouge zones.
Brittle-ductile mylonites
in outcrop have NW-trending lineations and S-C fabrics consistent with regional BCD kinematics
They are mostly calc-mylonites, but include quartz-muscovite mylonites as well.
Oriented sections were cut and polished for microstructural analysis under petrographic microscope and electron microprobe.
Microstructure
of brittle-ductile mylonite layers.
Calcite twinning, dilatant jog filled with cataclasite.
Measurements of dynamically recrystallized grains for piezometry.
Cataclasites
provide evidence of cosiesmic slip in the deep brittle crust.
These rocks have textures consistent with granular flow and were emplaced as bands subparallel to pre-existing foliations (A-C) and as injection veins (D).
They have a distinct hematite coating that may represent rapid heat flux and decarbonation of the calc-mylonite and schistose protoliths.
Reactivation
of inherited mylonitic fabrics is the mechanism for generation of the cataclasites.
This is observed from the outcrop to microscopic scales.
Reactivation and embrittlement was focused near strength contrasts both within the mylonites (e.g., quartz porphyroclasts) and within the section (e.g., breccia-mylonite contacts)
Cataclasite zones
have complex embayments, aligned tabular wall-rock clasts, bent micas in S-C’-preferred orientation
Elemental maps
of cataclasite generation surfaces show high-K zone localized shear
fragmental and comminuted qtz grains & secondary calcite
Mutually overprinting cataclasite injection veins and ultramylonite (B). Interpreted to represent cyclic coseismic slip (brown-red stuff) followed by plastic flow (mylonite) near the brittle ductile transitions zone where earthquakes nucleated along this low-angle normal fault.
pyrite is preferentially converted to hematite in brittle-deformed domains of the footwall mylonites.
Strain fringes
of pyrite and hematite are unidirectional around this injection vein. They point upward, suggesting formation during fluid injection and upward-directed active deformation around the injection.
Illite-rich gouge
was observed and sampled along most of the surface trace.
This gouge is mostly foliated, but some gouge zones are structureless and pulverized, containing clasts of foliated gouge within them. This supports coseismic slip, possibly via dynamic rupture propogation.
Mechanical transitions
along the reconstructed fault. Mineral phases within and microstructure of the fault rocks are used to infer the frictional properties and temperatures of formation, based on comparison to published laboratory experiments.
Combination with thermo-kinematic models allows depth intervals of different mechanical behavior to be mapped on the existing fault.
Simple 2D static stress models were developed to estimate stress at various depths along the reconstructed fault.
The earthquake nucleation zone
shallowed and thinned with fault slip due to elevated geotherm during footwall exhumation.
The thin earthquake nucleation zone may help explain the rarity of large-magnitude earthquakes on seismically active analogues to the Boundary Canyon detachment.
Collaborators on this project:
