1. Ganymede’s Tectonics Robert T. Pappalardo Jet Propulsion Laboratory, California Institute of Technology Robert T. Pappalardo Jet Propulsion Laboratory, California Institute of Technology Copyright 2012 California Institute of Technology. Government sponsorship acknowledged.

2. Ganymede’s Tectonics: Outline Dark terrain furrows Light grooved terrain: –Extensional tectonics –Multiple wavelengths => unstable extension –Tectonic resurfacing and high strain –Cryovolcanism? –Lithospheric spreading? Strike-slip tectonics Synthesis model Summary and outstanding questions Harpagia Sulcus

3. Ganymede’s Dark Terrain Segregated bright and dark materials. Thin lag over cleaner, brighter substrate. Shaped by impacts and tectonics. Some furrows probably ancient basin rings. [after McKinnon & Melosh, 1980] [Prockter et al., 2000] [Oberst et al., 1999]

4. Dark Terrain: Furrow Flexure Topography shows flexural signature Very warm lithosphere implied during flexure: ~ 60 – 80 mW m -2 ; ~ 15 – 20 K km -1 ; BDT ~ 2 – 3 km [Nimmo & Pappalardo, 2004]

5. Grooved Terrain: Voyager-Based Model [Parmentier et al., 1982] Erech Sulcus (Voyager 2) Light “ grooved” terrai n consists of subparallel ridges and grooves Sharp boundaries common Topographic wavelengths ~8 km Cryovolcanism followed by extensional fracturing or faulting

6. Grooved Terrain: Galileo Observations Fine-scale tilt-block normal faulting, superimposed on larger pinches-and-swells. Horst-and-graben style, with small-scale faults. [Pappalardo et al., 1998] tilt block style horst & graben style stereo-based topography ~ 2–3 km ~ 8 km ~ 1 km

7. Grooved Terrain: Fourier Analysis of Topographic Wavelength Fourier analysis of brightness profiles confirms multiple deformation wavelengths [Patel et al., 1999] Byblus Sulcus

8. Grooved Terrain: Unstable Extension [Dombard & McKinnon, 2001] Unstable extension promoted by: high heat ( ≥ 20 K km -1 ; ≥ 80 mW m -2 ; BDT ≤ 3 km), modest strain rate (~10 -12 – 10 -15 s -1 ), low surface temp. T s, and small ice grain size d. = q d =

9. Tectonic Resurfacing Model Pervasive tilt-block normal faulting of dark terrain, with bright icy material revealed.

10. Extensional Strain Deformed craters indicate high extensional strain: –10s % extension may be typical of grooved terrain. [Pappalardo & Collins, 2005]

11. Light Terrain: Cryovolcanic Resurfacing?  Scalloped depressions suggest icy calderas and cryovolcanism.  Even smoothest regions are tectonized  Perhaps tectonism typically erases signs of cryovolcanism. G28 “calderas” Harpagia Sulcus

12. Europa-Like Lithospheric Separation and Spreading on Ganymede? Possible reconstruction of Arbela Sulcus suggests ~25 km opening and ~65 km left-lateral offset [Head et al., 2002]. Arbela Sulcus

13. Ganymede Strike-Slip Tectonics 50 km Dardanus Sulcus lateral offsets Strike-slip faulting may be fundamental to Ganymede’s tectonics, including the transition from dark to light material A large stress source (e.g., nonsynchronous rotation) is required to overcome frictional resistance to fault slip. fault duplexes Uruk Sulcus en echelon strictures Marius Regio [Pappalardo et al., 2012]

14. Present and past strike-slip creeping or lock-slip are feasible from NSR. diurnal + NSR stress (MPa) compression tension right-lateral left-lateral perijove right-lateral fault creeping present (e = 0.0013) Dardanus Sulcus: Present and Past Strike-Slip Coulomb failure:     s   f  n +  gz  shear stress normal stress frictional stress perijove orbital position diurnal + NSR stress (MPa) right-lateral slip locked past (e = 0.05) ice shell density  = 1000 kg/m 3 gravity g = 1.43 m/s 2 fault friction  f = 0.3 fault depth d = 2 km layer thickness H = 2 km observation depth z = 1.6 km lithostatic stress  gz = 2.3 MPa [Pappalardo et al., 2012]

15. Ganymede Tectonics: Synthesis Model I Some grooved terrain can form by extending and tectonically erasing the pre-existing terrain. Strike-slip faulting is commonly associated. Dark terrain is thin lag above icy substrate. Furrows may focus subsequent tectonism. 2. tectonic resurfacing1. furrow formation [Pappalardo et al., 2004]

16. Ganymede Tectonics: Synthesis Model II Cross-cutting groove lanes can isolate dark polygons and erase older structures. Cryovolcanic features are commonly erased. Cryovolcanic resurfacing can probably smooth and brighten grooved terrain. Rare lithospheric spreading my occur. 3. cryovolcanic resurfacing4. cross-cutting groove lanes [Pappalardo et al., 2004]

17. Global Groove Trends Global mapping of groove trends suggests orientations (when assumed extensional) are consistent with nonsynchronous rotation, true polar wander (TPW), or differentiation + TPW [Collins, 2009, pers. comm.]. youngest intermediate oldest

18. Ganymede’s Tectonics: Summary Furrow tectonics and grooved terrain both imply high heat flow during formation Light grooved terrain: Unstable extension with pervasive small-scale normal faulting –Strains can be locally high (10s%). Strike-slip tectonics commonly associated Little evidence for contraction (so far) Cryovolcanism is elusive –Evidence may be erased by tectonics Minor strike-slip faulting is common –En echelon, duplexes, lateral offsets. Local lithospheric spreading is plausible Global groove trends suggest association with satellite differentiation

19. Outstanding Questions What are the relative roles of tectonic and cryovolcanic resurfacing? What strain is typical of grooved terrain? What are the driving mechanisms for extension and strike-slip faulting? Are there contractional structures? What are the global and regional processes responsible groove trends? How does Ganymede’s tectonics compare to other icy satellites? Is Ganymede tectonically active today?

20. Backup Slides

21. Extensional Tectonics: Analog Models Morphological characteristics of Ganymede structures are well- matched to extensional structures in stretched sandbox models. Extended “craters” still identifiable at >30% extension [Wyrick, 2012]! [Sims et al., LPSC, 2006] Ganymede dark terrain Analog sandbox model 4 cm

22. Saturn’s Satellites: Tectonic Resurfacing Dione Rhea Enceladus Cassini imaging suggests that “wispy” and smooth terrains on the saturnian satellites are areas of tectonic resurfacing.