Lunar and Planetary Science XLVIII (2017)
2111.pdf
STRIKE-SLIP TECTONISM AND SHEAR FAILURE ON GANYMEDE. M.E. Cameron1, B. R. Smith-Konter1,
1
2
3
4 1
L. Burkhard , D.A. Patthoff , R.T. Pappalardo , and G. C. Collins , University of Hawaii at Manoa, Department of
2
Geology and Geophysics, [email protected], [email protected], [email protected], Planetary Science Insti3
tute, [email protected], Jet Propulsion Laboratory California Institute
of Technology, Rob4
[email protected], Wheaton College, Physics and Astronomy Department, [email protected].
Introduction: The surface of Ganymede displays
several candidate regions of strike-slip tectonism, with
shear failure presumably driven by a combination of
global and local stress sources. As Ganymede orbits
Jupiter every 171.6 hours, variations in gravitational
tidal forces, due in part to the satellite’s eccentric orbit,
(e = 0.013) act to deform the moon’s surface [1], with
diurnal stresses on the order of a few kPa. Greater eccentricity in the past [2] could have resulted in greater
diurnal stresses. Nonsynchronous rotation stresses
(NSR) may arise if a tidally flexed satellite has an outer icy shell that is decoupled from its interior [3], likely
by a global liquid layer. As the outer shell rotates, the
surface migrates eastward relative to the tidal bulge
and can result in an additional source of stress within
the icy shell. We assume an NSR rate for Ganymede of
~105 years [4], and steady-state rotation of a viscoelastic ice shell of viscosity ~1022 Pa s (as in [5]), yielding
stresses on the order of MPa. To better understand the
role of tidal stress sources and implications for strikeslip tectonism on Ganymede, we investigate the relationship between shear and normal stresses at nine
target regions we previously mapped: Anshar Sulcus,
Arbela Sulcus, Byblus Sulcus, Dardanus Sulcus, Nippur/Philus Sulci, Nun Sulci, Tiamat Sulcus, Transitional Terrain, and Uruk Sulcus.
We use the numerical code SatStress [5] to calculate both predicted diurnal and NSR tidal stresses [6]
as plausible mechanisms for strike-slip tectonism at
Ganymede’s surface. SatStress tensor components are
resolved into shear stress (τs) and normal stress (σn)
based on discrete fault segment positions of varying
orientation. We use the Coulomb stress equation [7] to
determine the failure potential of a fault segment as a
function of mean anomaly (orbital position) m, with
shear failure occurring when the resolved shear stress
is greater than the frictional stress. We investigate the
mechanics of shear failure along major fault zones of
each target region and consider a range of plausible
friction coefficients [8] (µf = 0.2 – 0.6) and brittle fault
depths, to evaluate how failure predictions vary as a
function of depth, ice friction, geographic location, and
fault geometry.
Summary of Results: Global tidal stress models
limited to only present-day diurnal stresses do not
permit Coulomb shear failure along any of the major
fault zones of the nine regions investigated here. However, a combination of both diurnal and NSR stress
mechanisms readily generate shear and normal stress
magnitudes in all nine regions that could give rise to
Coulomb shear failure today. For the assumed presentday fault geometry and location on the surface (i.e.,
measured planform geometry and assumed vertical
dip) of the major fault zones of each region, these results suggest shear failure is possible down to depths of
~1-2 km for high friction (µf = 0.6) cases and >2 km
for low friction (µf = 0.2) cases at all nine target regions. For example, Figure 1 illustrates the digitization
of two major fault zones within the Nun Sulci, their
corresponding strike-slip indicators (i.e., pronounced
offset along the south branch, right-stepping en echelon structures along the north branch [9]), and modeled
normal, shear, and Coulomb stresses for these structures, given their current fault orientation and prescribed µf.
In addition to assessing each fault zone’s ability to
accommodate shear failure, we also compare each fault
zone’s predicted sense of shear to the inferred shear
directions from structural mapping efforts [10]. Using
high-resolution Galileo solid-state imager (SSI) data,
we have mapped in detail major strike-slip indicators
(en echelon structures [11,12], strike-slip duplexes
[13], strained craters [14], and offset of pre-existing
structures) within all nine target regions. Multiple examples of strike-slip indicators, of both right and leftlateral shear, are documented in various combinations
at each site, with ubiquitous examples of en echelon
structures and at least one example of other indicators.
We use rose diagrams and diagrammatic strain ellipses
to examine trends and assess consistency between
mapped structures, and we also infer main stages of
tectonic deformation at each region. To perform a firstorder comparison of mapped and modeled shear sense,
we limit our analysis to mapped inferences of shear
along major fault zone structures within each target
region and compare these to the predicted shear sense
for each modeled fault zone. We find compatible senses of shear among six of the nine regions; however, we
note that these results are sensitive to both fault strike
(affecting our models) and our inferred morphology of
strike-slip indicators (from our mapping). Because
confidence in shear sense is greatest for easilyidentified strike-slip offsets (which also provide the
strongest inference of brittle failure), we organize our
results below into three groups: (1) fault zones with
notable offset and compatible shear sense, (2) fault
zones with other strike-slip indicators and compatible
shear sense, and (3) fault zones with other strike-slip
indicators but incompatible shear sense.
Fault zones with notable offset: Significant offset,
as inferred in Galileo imagery, has been suggested at
three of our target regions [9, 10]: Nun Sulci (50 km,
left-lateral), Dardanus Sulcus (45 km, right-lateral),
and Tiamat Sulcus (40 km, right-lateral). Likewise,
modeled shear stresses along theses offsets are in
strong agreement with their respective inferred shear
senses: the Nun Sulci (Figure 1) are dominated today
by left-lateral shear stress, and Dardanus Sulcus and
Tiamat Sulcus are dominated by right-lateral shear
stress.
Lunar and Planetary Science XLVIII (2017)
2111.pdf
Nun Sulcus
(a) Imagery
a
right stepping
en echelon
structures
North
Branch
Upuant
Mont
Nefertum
South
Branch
0
50
100
Kilometers
(b) Map View: Diurnal + NSR Coulomb Failure
compressional
Normal Stress
right-lateral
Shear Stress
0
MPa
tensional
left-lateral
free to slip
free to slip
locked MPa
(c) Depth View
W
E
distance (km) from origin
E
300
distance (km) from origin
0
South Branch
μf = 0.2
0
W
0
400
400
locked MPa
free to slip
South Branch
μf = 0.6
depth (km)
depth (km)
E
300
distance (km) from origin
free to slip
North Branch
μf = 0.6
depth (km)
depth (km)
W
0
locked MPa
0
North Branch
μf = 0.2
0
MPa
Coulomb Stress
μf = 0.6
Coulomb Stress
μf = 0.2
0
ed. For example, a small backrotation of the modeled
strike of Arbela, Anshar, and Uruk Sulcus by ~10-30°
counterclockwise results in subtle but reversed sense of
shear that is compatible with mapped shear indicators.
It is also important to note that for each of these tectonically complex regions, the major fault zone structure
adopted for our shear calculations may not have been
formed by strike-slip tectonism, but instead by tensile
stresses (as suggested by several en echelon features
associated with several of these structures). Furthermore, when considering a ~50 - 90° backrotation of the
tidal bulge corresponding to NSR stresses, models are
able to predict a matching sense of shear.
W
0
E
locked MPa
distance (km) from origin
Figure 1: Nun Sulci [45°N, 45°W, west positive longitude convention] (a) Galileo imagery; (b) map view of modeled diurnal +
NSR normal, shear, and Coulomb stresses at depth z = 1 km, for
µf =0.2 and 0.6 for each example structure at mean anomaly m =
0; and (c) predicted Coulomb stresses presented as a function of
depth. Gray segments represent shallow regions of high tensile
stress, not subject to Coulomb failure.
Fault zones with other strike-slip indicators (and
compatible shear sense): Three additional fault zones
that present compatible shear sense between the mapping and modeling approaches are Byblus Sulcus,
Nippur/Philus Sulci, and the Transitional Terrain region (Figure 2). While these regions do not display any
clear examples of offset of pre-existing structures, the
inferred sense of en echelon structures, strike-slip duplexes, and strained craters found in each region suggest prevalent left-lateral shear. Assuming that a dominant shear sense should be evident in the major fault
zones of each of these regions, we calculate shear
stresses along each major zone and find that all should
be dominated today, by left-lateral shear stress, the
same manner as we observe.
Fault zones with other strike-slip indicators (but
incompatible shear sense): The inferred sense of slip at
Arbela Sulcus (left-lateral), Anshar Sulcus (rightlateral), and Uruk Sulcus (right-lateral) do not strongly
agree with present-day shear sense predicted by our
tidal stress modeling. A possible explanation for this
may be due to the migration and reorientation of the
ice shell, where a feature might have formed in a different orientation or location than it is presently locat-
Figure 2: Transitional Terrain [centered on 32°N, 173°W, west
positive longitude convention]. (a) Galileo imagery and (inset)
digitized hypothetical major fault structures.
Conclusions: We find that present-day diurnal and
NSR tidal stressing mechanisms, in combination, are
sufficient to induce shear failure along all of the inferred regions of strike-slip tectonism on Ganymede
that we have studied. In addition, our models generally
predict the same sense of shear as inferred from imagery and mapping efforts. Modeling that does not match
the present-day inferred sense of shear may be consistent with a migrating ice shell as due to NSR, perhaps allowing for reorientation of modeled strike. Local conditions and pre-existing faults may affect sense
of shear predictions. In future work, additional secular
stress mechanisms, such as true polar wander, will also
be considered as a possible alternative stress model.
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135, 64-78. [7] Byerlee J. D. (1978) Pure Applied Geophysics, 116,
615-. [8] Fortt A. L. and Schulson E. M. (2009) Acta Materialia, 57,
4382-4390. [9] Seifert F. et al. (2015) LPSC XLVI, Abstract #2985.
[10] Cameron M.E. et al. (2016) LPSC XLVII, Abstract #2630. [11]
Collins G.C. et al. (1998) LPSC XXIX, Abstract #1755. [12] Pappalardo R.T. et al. (1997) LPSC XXVIII, Abstract #1231. [13] DeRemer
L.C. and Pappalardo R.T. (2003) LPSC XXXIV, Abstract #2033. [14]
Pappalardo R.T. and Collins G.C. (2005) J. Struct. Geol., 27, 827838.
Additional Information: This research is supported by NASA
Outer Planets Research Program (NNX14AE15G). Government
sponsorship acknowledged.