Patterns of mineral transformations in clay gouge, with examples from low-angle normal fault rocks in the western USA

Rapid dip-slip (11.7&#177;3.5 mm/yr) on the active Mai'iu low-angle normal fault in SE Papua New Guinea enabled the preservation of early formed microstructures in mid to shallow crustal rocks. The corrugated, convex-upward shaped fault scarp dips as low as 16&#176;&#8211;20&#176; near its trace close to sea level and forms a continuous landscape surface traceable for at least 28 km in the NNE slip-direction. Structurally, offset on the Mai'iu fault has formed a metamorphic core complex and has exhumed a metabasaltic footwall during 30&#8211;45 km of dip slip on a rolling-hinge style detachment fault. The exhumed crustal section records the spatiotemporal evolution of fault rock deformation mechanisms and the differential stresses that drive slip on this active low-angle normal fault.The Mai'iu fault exposes a <3 m-thick fault core consisting of gouges and cataclasites. These deformed units overprint a structurally underlying carapace of metabasaltic mylonites that are locally >60 m-thick. Detailed microstructural, textural and geochemical data combined with chlorite-based geothermometry of these fault rocks reveal a variety of deformation processes operating within the Mai'iu fault zone. A strong crystallographic preferred orientation of non-plastically deformed actinolite in a pre-existing, fine-grained (6&#8211;33 &#181;m) mafic assemblage indicates that mylonitic deformation was controlled by diffusion-accommodated grain-boundary sliding together with syn-tectonic chlorite precipitation at >270&#8211;370&#176;C. At shallower crustal levels on the fault (T&#8776;150&#8211;270&#176;C), fluid-assisted mass transfer and metasomatic reactions created a foliated cataclasite fabric during inferred periods of aseismic creep. Pseudotachylites and ultracataclasites mutually cross-cut both the foliations and one another, recording repeated episodes of seismic slip. In these fault rocks, paleopiezometry based on calcite twinning yields peak differential stresses of ~140&#8211;185 MPa at inferred depths of 8&#8211;12 km. These differential stresses were high enough to drive continued slip on a ~35&#176; dipping segment of the Mai'iu fault, and to cause new brittle yielding of strong mafic rocks in the exhuming footwall of that fault. In the uppermost crust (<8 km; T<150&#176;C), where the Mai'iu fault dips shallowly and is most severely misoriented for slip, actively deforming fault rocks are clay-rich gouges containing abundant saponite, a frictionally weak mineral (&#181;<0.28).In summary, these results combined with fault dislocation models of GPS velocities from campaign stations in this region suggest a combination of brittle frictional and viscous flow processes within the Mai'iu fault zone. Gouges of the Mai'iu fault have been strongly altered by fluids and are frictionally weak near the surface, where the fault is most strongly misoriented. At greater depths (8&#8211;12 km) the fault is stronger and slips both by aseismic creep and episodic earthquakes (a mixture of fast and slow slip) in response to locally high differential stresses.

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Mixed-Mode Seismic Slip and Aseismic Creep on a Highly Active Low-Angle Normal Fault System in Papua New Guinea

10.5194/egusphere-egu2020-230 ◽

2020 ◽

Author(s):

James Biemiller ◽

Laura Wallace ◽

Luc Lavier

Keyword(s):

Papua New Guinea ◽

Normal Fault ◽

New Guinea ◽

Fault System ◽

Normal Faults ◽

Fault Mechanics ◽

Fault Rocks ◽

Near Surface ◽

Geological Evidence ◽

Seismic Slip

Whether low-angle normal faults (LANFs; dip < 30&#176;) slip in large earthquakes or creep aseismically is a longstanding problem in fault mechanics. Although abundant in the geologic record, active examples of these enigmatic &#8216;misoriented&#8217; structures are rare and extension rates across them are typically less than a few mm/yr. As such, geodetic and seismological observations of LANFs are sparse and can be difficult to interpret in terms of earthquake cycles. With a long-term slip rate of ~1 cm/yr, the Mai&#8217;iu fault in Papua New Guinea may be the world&#8217;s most active LANF and thus offers an outstanding natural laboratory to evaluate seismic vs. aseismic behavior of LANFs. Here, we use new results from a campaign GPS network to determine the degree of locking vs. aseismic creep on the Mai&#8217;iu fault and evaluate these results in the context of geological evidence for mixed seismic and aseismic slip in exhumed Mai&#8217;iu fault rocks.We derive velocities from GPS measurements with 3-4 km station spacing above the shallowest portions of the fault, which dips 21-25&#176; at the surface. Dislocation modeling of these velocities is consistent with 6-8 mm/yr of horizontal extension, corresponding to ~1 cm/yr dip-slip rates on a 27-35&#176;-dipping fault. Strain rates and vertical derivatives of horizontal stress rates derived from these velocities confirm localized extension across the fault. We compare and evaluate two interseismic locking models that fit the data best: one in which the fault deforms by shallow near-surface creep updip of a deeper zone of increased interseismic coupling which soles into a steadily creeping shear zone at depth, and one in which the fault creeps steadily downdip of a shallowly locked patch. These results combined with field and microstructural evidence from the exhumed fault rocks suggest that the fault slips by a mixture of brittle frictional (seismic slip, fracturing, and cataclastic creep) and viscous (stress-driven dissolution-precipitation creep, or pressure solution) processes. Using depth-constrained mechanical properties and stress conditions inferred from exhumed fault rocks, we model the time-dependent competition between frictional slip and viscous creep to assess where and how elastic strain accumulates along the Mai&#8217;iu fault, and whether the fault is capable of hosting or nucleating earthquakes.

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Spatiotemporal history of fault–fluid interaction in the Hurricane fault, western USA

Solid Earth ◽

10.5194/se-11-1969-2020 ◽

2020 ◽

Vol 11 (6) ◽

pp. 1969-1985

Author(s):

Jace M. Koger ◽

Dennis L. Newell

Keyword(s):

Damage Zone ◽

Carbon Sources ◽

Normal Fault ◽

Western Usa ◽

Calcite Vein ◽

Regional Flow ◽

Fault Strength ◽

History Of ◽

Fault Damage Zone ◽

Fluid Interaction

Abstract. The Hurricane fault is a ∼250 km long, west-dipping, segmented normal fault zone located along the transition between the Colorado Plateau and the Basin and Range tectonic provinces in the western USA. Extensive evidence of fault–fluid interaction includes calcite mineralization and veining. Calcite vein carbon (δ13CVPDB) and oxygen (δ18OVPDB) stable isotope ratios range from −4.5 ‰ to 3.8 ‰ and from −22.1 ‰ to −1.1 ‰, respectively. Fluid inclusion microthermometry constrains paleofluid temperatures and salinities from 45 to 160 ∘C and from 1.4 wt % to 11.0 wt % as NaCl, respectively. These data suggest mixing between two primary fluid sources, including infiltrating meteoric water (70±10 ∘C, ∼1.5 wt % NaCl, δ18OVSMOW ∼-10 ‰) and sedimentary brine (100±25 ∘C, ∼11 wt % NaCl, δ18OVSMOW ∼ 5 ‰). Interpreted carbon sources include crustal- or magmatic-derived CO2, carbonate bedrock, and hydrocarbons. Uranium–thorium (U–Th) dates from five calcite vein samples indicate punctuated fluid flow and fracture healing at 539±10.8 (1σ), 287.9±5.8, 86.2±1.7, and 86.0±0.2 ka in the upper 500 m of the crust. Collectively, data predominantly from the footwall damage zone imply that the Hurricane fault imparts a strong influence on the regional flow of crustal fluids and that the formation of veins in the shallow parts of the fault damage zone has important implications for the evolution of fault strength and permeability.

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