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| Titel |
The impact of splay faults on the geochemistry and fluid budget of a subduction zone |
| VerfasserIn |
Rachel M. Lauer, D. M. Saffer |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043368
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| Zusammenfassung |
| Fluid samples obtained from convergent margins indicate that pore fluids are geochemically
distinct from seawater, and characterized by reaction products derived from deeper within the
subduction zone. The geochemical anomalies indicate a hydraulic connection from the site of
these reactions to the seafloor, although it is unclear how the observed geochemical signature
is related to the underlying permeability architecture, and overall fluid budget. In erosive
margins like Costa Rica, the majority of sediments on the incoming oceanic plate are
subducted, and therefore represent significant fluid sources derived through processes of
compaction and mineral dehydration. In regions characterized by thin sediment
cover and low heat flow (eg. Costa Rica) temperatures sufficient to drive mineral
dehydration are not reached prior to subduction. In these settings, the geochemical
anomalies observed at seafloor seeps most likely represent fluids flowing from deep
within the subduction zone to the seafloor via the décollement or major subsidiary
faults that connect the plate boundary to the seafloor. Observations of geochemical
anomalies and seeps far upslope highlight the role of the latter features in both chemical
transport and overall fluid budgets. Although previous modeling studies designed to
estimate pore pressures, flow rates, and fluid budgets for convergent margins have
successfully predicted pore water geochemical signatures and flow rates at seafloor
seeps, they have not investigated the role of major splay faults. Here, we address this
problem using a 2-D numerical model of coupled fluid flow and transport in order to
quantitatively evaluate the physical and chemical hydrogeology of the forearc using the well
studied Costa Rica margin as an example. Costa Rica provides an ideal setting for the
proposed modeling study, as it has been well constrained through ocean drilling
and borehole observatories that continuously monitor pressure, temperature, and
geochemistry along a transect perpendicular to the trench. Specifically, we investigate the
effect of realistic permeability architecture on the fluid budget, distribution of fluid
pressures, and the spatial pattern of geochemical signals in the fluids expelled at the
seafloor.
Our model consists of a cross section perpendicular to the Middle American trench,
extending from 10 km seaward of the deformation front to 50 km landward. In our models,
we assign fluid sources within the subducted sediment section to represent both compaction
and clay dehydration in order to evaluate the distribution and fate of freshened (dehydration
derived) water from deep in the subduction zone to the seafloor. We assign sediment porosity
following an exponential decrease with depth, and define permeability using a relation
to porosity derived from laboratory data. Because it is not composed of accreted
sediment, we assign a uniform permeability to the overriding margin wedge. We
evaluate the effect of clay dehydration by first establishing a baseline model with
wedge and décollement permeabilities of 10-19 m2 and 10-15 m2respectively,
without including faults. In subsequent model runs, we vary the splay fault and
décollement permeability from 10-17 m2 to 10-13 m2 in order to examine the
effect of these changes on the distribution of fluid pressures, the fluid budget, and
distribution of fresh pore water through comparison with the baseline model. Comparison
of modeling results with direct measurements from seafloor seeps and borehole
observatories will clarify the link between geochemical data and the underlying physical
processes that determine fluid pressures and the distribution of fluids in the forearc.
Preliminary results show that flow rates at the seafloor are consistent with those
estimated from gravity coring (0.3-1.5 cm/yr) and the distribution of fluids (ie. fluid
budget) is sensitive to changes in the permeability of the faults and décollement,
especially in cases where fault permeability is greater than décollement permeability. |
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