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| Titel |
Rheological structure of the lithosphere in plate boundary strike-slip fault zones |
| VerfasserIn |
Vasileios Chatzaras, Basil Tikoff, Seth C. Kruckenberg, Julie Newman, Sarah J. Titus, Anthony C. Withers, Martyn R. Drury |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130858
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| Publikation (Nr.) |
 EGU/EGU2016-11177.pdf |
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| Zusammenfassung |
| How well constrained is the rheological structure of the lithosphere in plate boundary
strike-slip fault systems? Further, how do lithospheric layers, with rheologically distinct
behaviors, interact within the strike-slip fault zones? To address these questions, we
present rheological observations from the mantle sections of two lithospheric-scale,
strike-slip fault zones. Xenoliths from ∼40 km depth (970–1100 ˚ C) beneath the San
Andreas fault system (SAF) provide critical constraints on the mechanical stratification
of the lithosphere in this continental transform fault. Samples from the Bogota
Peninsula shear zone (BPSZ, New Caledonia), which is an exhumed oceanic transform
fault, provide insights on lateral variations in mantle strength and viscosity across
the fault zone at a depth corresponding to deformation temperatures of ∼900 ˚
C.
Olivine recrystallized grain size piezometry suggests that the shear stress in the SAF
upper mantle is 5–9 MPa and in the BPSZ is 4–10 MPa. Thus, the mantle strength in both
fault zones is comparable to the crustal strength (∼10 MPa) of seismogenic strike-slip faults
in the SAF system. Across the BPSZ, shear stress increases from 4 MPa in the surrounding
rocks to 10 MPa in the mylonites, which comprise the core of the shear zone. Further, the
BPSZ is characterized by at least one order of magnitude difference in the viscosity between
the mylonites (1018 Pa⋅s) and the surrounding rocks (1019 Pa⋅s). Mantle viscosity
in both the BPSZ mylonites and the SAF (7.0⋅1018–3.1⋅1020 Pa⋅s) is relatively
low.
To explain our observations from these two strike-slip fault zones, we propose the
“lithospheric feedback” model in which the upper crust and lithospheric mantle act together
as an integrated system. Mantle flow controls displacement and the upper crust controls the
stress magnitude in the system. Our stress data combined with data that are now available for
the middle and lower crustal sections of other transcurrent fault systems support the
prediction for constant shear strength (∼10 MPa) throughout the lithosphere; the stress
magnitude is controlled by the shear strength of the upper crustal faults. Fault rupture in the
upper crust induces displacement rate loading of the upper mantle, which in turn, causes
strain localization in the mantle shear zone beneath the strike-slip fault. Such forced
localization leads to higher stresses and strain rates in the shear zone compared
to the surrounding rocks. Low mantle viscosity within the shear zone is critical
for facilitating mantle flow, which induces widespread crustal deformation and
displacement loading. The lithospheric feedback model suggests that strike-slip
fault zones are not mechanically stratified in terms of shear stress, and that it is the
time-dependent interaction of the different lithospheric layers—rather than their relative
strengths—that governs the rheological behavior of the plate boundary, strike-slip fault zones. |
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