STRIKE-SLIP TECTONISM AND SHEAR FAILURE ON GANYMEDE. M.E. Cameron1, B. R. Smith-Konter1, L. Burkhard1, D.A. Patthoff 2, R.T. Pappalardo3, and G. C. Collins4, 1University of Hawaii at Manoa, Department of Geology and Geophysics, mecamero@hawaii.edu, brkonter@hawaii.edu, liliane@hawaii.edu, 2Planetary Science Insti-tute, donald.a.patthoff@jpl.nasa.gov, 3Jet Propulsion Laboratory California Institute of Technology, Rob-ert.Pappalardo@jpl.nasa.gov, 4Wheaton College, Physics and Astronomy Department, gcollins@wheatonma.edu.

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 ec-centricity in the past [2] could have resulted in greater diurnal stresses. Nonsynchronous rotation stresses (NSR) may arise if a tidally flexed satellite has an out-er 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 viscoelas-tic 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 strike-slip tectonism on Ganymede, we investigate the rela-tionship between shear and normal stresses at nine target regions we previously mapped: Anshar Sulcus, Arbela Sulcus, Byblus Sulcus, Dardanus Sulcus, Nip-pur/Philus Sulci, Nun Sulci, Tiamat Sulcus, Transi-tional Terrain, and Uruk Sulcus.

We use the numerical code SatStress [5] to calcu-late 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. How-ever, 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 present-day 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 re-sults 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 re-gions. 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 eche-lon structures along the north branch [9]), and modeled normal, shear, and Coulomb stresses for these struc-tures, given their current fault orientation and pre-scribed 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 ex-amples of strike-slip indicators, of both right and left-lateral 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 first-order 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 sens-es 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 easily-identified 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.

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Figure 1: Nun Sulci [45°N, 45°W, west positive longitude conven-tion] (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 map-ping and modeling approaches are Byblus Sulcus, Nippur/Philus Sulci, and the Transitional Terrain re-gion (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 du-plexes, and strained craters found in each region sug-gest prevalent left-lateral shear. Assuming that a domi-nant 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 (right-lateral), 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 dif-ferent orientation or location than it is presently locat-

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 tecton-ically 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). Further-more, when considering a ~50 - 90° backrotation of the tidal bulge corresponding to NSR stresses, models are able to predict a matching sense of shear.

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 in-ferred 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 image-ry and mapping efforts. Modeling that does not match the present-day inferred sense of shear may be con-sistent with a migrating ice shell as due to NSR, per-haps allowing for reorientation of modeled strike. Lo-cal 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. References: [1] Greenberg R. et al. (1998), Icarus, 135, 64-78. [2] Showman A.P. and Malhotra R. (1997) Icarus, 127, 93-111. [3] Ojakangas G.W. and Stevenson D.J (1989) Icarus, 81, 220-241. [4] Hoppa G. et al. (1999) Icarus, 141, 287-298. [5] Wahr J. et al. (2009) Icarus, 200, 188-206. [6] Greenberg R. et al. (1998) Icarus, 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] Pappa-lardo 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, 827-838. Additional Information: This research is supported by NASA Outer Planets Research Program (NNX14AE15G). Government sponsorship acknowledged.

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