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45th Lunar and Planetary Science Conference (2014)
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PLATE BOUNDARY LOCALIZATION: WHAT PROCESSES ACTIVE ON EARTH DO NO APPLY TO
OTHER PLANETARY LITHOSPHERES? L.G.J. Montesi1, 1University of Maryland, Department of Geology,
College Park MD 20742, [email protected].
Introduction: Few problems in Earth Science are
as fundamental as the origin of plate tectonics. Our
planet has adopted a global tectonic regime that appears unique in the solar system [1]. Deformation is
localized in narrow deformation zones, some of which
do no contribute to removing heat from the interior of
the planet [2].
Because of the higher strain rate inside narrow deformation zones, energy dissipation is higher in a localized shear zone than in a distributed one. Therefore,
plate boundary localization must involve a weakening
process to make deformation in a localized more favored over distributed deformation [3]. Here, I summarize localization processes proposed to be active on
Earth, their effect, and whether or not they may be
active in other terrestrial bodies.
Localization processes: Rocks deform following
brittle processes at relatively shallow pressure and
temperature and plastic mechanisms are greater depth.
Geological observations on Earth show that deformation can localize under both conditions [4], even
though plastic rheologies, being strain-rate hardening,
are fundamentally stable in the laboratory [3, 5]. Localization can be understood in either case if a state
variable enables rock strength to weaken as strain accumulates.
Brittle processes involve the formation of, and sliding along, cracks between and inside grains that form a
rock [e.g., 6]. It is therefore not surprising that brittle
failure should result in a localized shear zone. Additional processes that weaken brittle shear zones include
nucleation of weak, often hydrated minerals like serpentine, graphite, and mica [7] and the development of
a foliated cataclastic zone [8]. All these phenomena
contribute to decrease the coefficient of friction from
0.6 to 0.1. A key to achieve such weakness is that fault
gouge adopts a fabric that allows the weak phase to
form a through-going layer [9, 10].
Localization in the ductile regime is harder to understand, because of the fundamental strain-rate hardening behavior of plastic deformation process [11].
Several studies have appealed to shear heating to
weaken ductile rocks [12, 13], but there only scant
evidence of this process in the field [14,15]. It is most
efficient at relatively low temperature [5].
Ductile shear zone rocks often display marked fabric differences compared to the protolith and wall
rocks. A reduced grain size is often observed, which
leads to the many shear zones being classified as mylonites [16–19]. Reducing grain size is possible only
when dislocations are active inside the rocks [e.g. 20].
However, it leads to weakening only if grain boundary
sliding and diffusion are dominant. Therefore, grain
size reduction is a viable localization process only in
the dislocation-accommoated Grain Boundary sliding
regime (dis-BGS, [21, 22]). The mantle can localization in this regime at temperatures less than less than
800°C [18]. Ice also deform by disGBS is the superplastic regime [23].
Ductile shear zones also display a layered fabric of
anastomosing high-strain bands at many scales [4, 24].
The high-strain zone are often rich in phyllosilcates,
sometimes reaching the stage of being a phyllonite [25,
26]. Development of a layered fabric enables the
weakest phase to control the rheology of the deforming
rock. If that phase is weak or highly nonlinear, like
micas, pronounced weakening is associated with the
appearance of a layered fabric [5].
In summary, geological observations imply that
fabric development is the dominant weakening process
that enables shear zone localization. This process is
active in both brittle and ductile regimes. Montési [5]
shows that the potential for localization associated with
this mechanism far exceeds that of other processes.
Grain size evolution often accompanies shear zone
development but participates to weakening only if disGBS creep is active. This mechanism may be important in relatively low temperature mantle and in ice.
Localization in the continental lithosphere.
Gueydan et al. [27] have recently proposed a model of
strain-dependent lithospheric strength. At low strain,
the upper mantle and upper crust are the strongest regions of the lithosphere. However, fabric evolution
reduces strength in the upper crust (brittle fabric), middle crust (fabric in presence of mica) and upper mantle
(grain size reduction). The reduced strength of the lithosphere as a whole leads to increased strain rate if the
loading stress, controlled by large-scale tectonics, does
not change. The lower crust, which does not weaken,
becomes the load-bearing layer at high strain. This
model is able to reconcile different views of the lithospheric strength profile by assocating the classic “jelly
sandwich” model to low strain areas and the “crème
brûlée” model to high strain areas [28].
45th Lunar and Planetary Science Conference (2014)
Localization on Venus. Applying these concepts
to Venus requires taking into consideration the higher
temperature of the lithosphere compared to the Earth.
Most phyllosilicates are unstable just a few km below
the Venusian surface. Moreover, phyllosilicates are
often hydrated. The low water content expected in the
Venusian lithosphere makes it unlikely that these phases are present. Therefore, ductile layering is unlikely
an important player on Venus. DisGBS is limited to
~800°C in the mantle. Assuming a geothermal gradient
of 10 K/km, such temperature are present everywhere
the crust is less than 34 km, that is, everywhere except
under the highest crustal plateaus [29]. Brittle localization is less efficient than on Earth because connection
of a hydrated weak phase is unlikely on Venus.
Localization on Mars. Mars presents the same
lithological possibilities as the Earth. However, the
crust is dominantly basaltic, with only limited evidence
for felspathic rocks [30]. Many micas in terrestrial
shear zones are formed by the breakdown of Feldspar
in presence of water [27]. As this is unlikely to take
place on Mars, crustal-level localization is probably
limited to the brittle field. The Martian geotherm has
probably chaged dramatically over time. For a 30 km
thick crust (lowlands), the temperature necessary for
disGBS would have been exceeded if the geotherm
exceeded 30 K/km. These conditions would only have
have been exceeded since the early Noachian, making
localization in the shallow mantle a possibility for
most of the geological history of Mars.
Localization on Icy Satellites. Icy shells are essentially monomineralic, which limits localization to
grain size reduction in presence of disGBS. This
mechanism is thought to be dominante in icy shells
[23]. Therefore it is not surprising that icy satellite
tectonics display clear evidence for localized deformation. Indeed, it seems difficult for icy shells to retain
the rigid interiors that are necessary to define plates
[2].
Taking stock: These theoretical considerations reveal that localization in the brittle upper crust and in
the upper mantle are expected in every planet. However, the Earth is unique in the possibility of having localization in the middle crust thanks to its unusual
mineralogy. The combination of felsic minerals and
water makes it possible to form micas, which are associated with a strong potential for localization dues to
their highly non-linear rheological behavior. These
minerals are likely to be rare in the crust of Mars and
Venus because of the absence of source minerals and
water, respectively. Therefore, the Venusian and Mar-
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tian lithosphere have a structure where deep and shallow localizing levels are probably separated by a thick
layer of non-localizing materials. This makes it difficult to achieve wholesale failure of these lithosphere
which is necessary for plate tectonics [31]. The lithosphere remains strong overall, favoring a stagnant lid
regime.
The icy satellites lithosphere, like the terrestrial
oceanic crust, can be regarded as a single layer, where
localization is possible thanks to grain size reduction
and the involvement of disGBS. This behavior favors
localization. The icy satellite ice layer essentially shatters because the subjacent water layer opposes very
little resistance, unlike the oceanic asthenosphere.
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