ganymede’s tectonics robert t. pappalardo jet propulsion laboratory, california institute of...
TRANSCRIPT
Ganymede’s Tectonics
Robert T. PappalardoJet Propulsion Laboratory,
California Institute of Technology
Copyright 2012 California Institute of Technology. Government sponsorship acknowledged.
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
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]
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]
Grooved Terrain: Voyager-Based Model
[Parmentier et al., 1982]
Erech Sulcus (Voyager 2)• Light “grooved” terrain consists of subparallel ridges and grooves
• Sharp boundaries common• Topographic wavelengths ~8 km• Cryovolcanism followed by
extensional fracturing or faulting
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
Grooved Terrain: Fourier Analysis of Topographic Wavelength
Fourier analysis of brightness profiles confirms multiple deformation wavelengths
[Patel et al., 1999]
Byblus Sulcus
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. Ts , and small ice grain size d.
l =
qd =
Tectonic Resurfacing Model
Pervasive tilt-block normal faulting of dark terrain, with bright icy material revealed.
Extensional Strain• Deformed craters indicate high extensional strain:
– 10s % extension may be typical of grooved terrain.
[Pappalardo & Collins, 2005]
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
Europa-Like Lithospheric Separationand Spreading on Ganymede?
Possible reconstruction of Arbela Sulcus suggests ~25 km opening and ~65 km left-lateral offset [Head et al., 2002].
Arbela Sulcus
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]
Present and past strike-slip creeping or lock-slip are feasible from NSR.
diur
nal +
NS
R s
tres
s(M
Pa)
compression
tension
right-lateral
left-lateral
perijove
right-lateral fault creeping
pres
ent
(e =
0.0
013)
Dardanus Sulcus: Present and Past Strike-Slip
Coulomb failure: | ts | mf (n + rgz)
shearstress
normalstress
frictional stress
perijove orbital position
diur
nal +
NS
R s
tres
s(M
Pa)
right-lateral slipright-lateral slip locked
past
(e
= 0
.05)
ice shell density = 1000 kg/m3
gravity g = 1.43 m/s2
fault frictionf = 0.3
fault depth d = 2 km layer thickness H = 2 km observation depth z = 1.6 km lithostatic stress rgz = 2.3 MPa
[Pappalardo et al., 2012]
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]
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 resurfacing 4. cross-cutting groove lanes
[Pappalardo et al., 2004]
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.].
youngestintermediate
oldest
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
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?
Backup Slides
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
Saturn’s Satellites: Tectonic Resurfacing
Dione RheaEnceladus
Cassini imaging suggests that “wispy” and smooth terrains on the saturnian satellites are areas of tectonic resurfacing.