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.