GreatBreak: Grand GreatBreak: Grand Challenges in GeodynamicsChallenges in Geodynamics

Characteristics of a Desirable Characteristics of a Desirable Geodynamic ModelGeodynamic Model

• Ties together observational constraints on current state of the lithosphere/upper mantle with physical/chemical processes as currently understood

• Process-oriented• Multi-disciplinary• Useful

– a hypothesis testing tool (is my interpretation consistent with the physical/chemical processes as we know them?)

– an exploratory tool that helps identify which of many are the dominant controlling processes.

– a predictive tool that can be used to guide future studies– an analysis tool that helps constrain physical properties

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t= 80 my

Hot lithosphere => all weakt=260 my

Type 1: Diffuse extension in West Antarctica

t= 60 my

Type 3: Extension eventually focuses somewherewithin West Antarctica

t=160 my High heat loss=> relatively strong

t= 60 my

Region of low heat loss=> relatively weak

t=90 my

Type 2: Extension eventually focuses at theEast/West Antarctica boundary

-12.5 12.50heat flowto left

heat flowto right

Heat Flow(mW/m^2)

Driving MechanismDriving Mechanism

• Potential Energy plays a role, but what else?– Relation to Plate Boundary Processes– Basal Shear– Slab Windows, Mantle Drips, and Delamination

• Are Driving Mechanisms viewed differently at different depths, places, and scales?

=> We need: Better kinematic history at all scales; detailed mantle images at ca. 50-100 km scales; density structure; T(x,y,z) and heat flow data.

Rheology of the LithosphereRheology of the Lithosphere

• Is it net strength, or the “jelly sandwich” model? => Thin Sheet vs. Fully Dynamic models.

• Is deformation style more strongly related to boundary conditions, or dynamically evolving rheological structure?

• What role does tectonic inheritance play?• Does the P.E. concept apply to the lithosphere as a

whole, or do we need a “mobile” layer (e.g., weak lower crust) to facilitate collapse?

=> We need: More laboratory constitutive data; more lab and field thermal data; a direct comparison of the various modeling approaches.

Coupling Deformation at Different Coupling Deformation at Different DepthsDepths

• Decollement tectonics controls surface faulting, but what controls the decollement?

• The flat moho problem, lower crustal flow, and underplating

• Coupling mantle and crust. Is the uppermost mantle strong or weak?

=> We need: data of deep deformation rates & patterns; better information on age & composition of lower crust; more rheological and thermal data.

List of ChallengesList of Challenges• Driving Mechanism for Extension• Rheology of the Lithosphere• Coupling Deep and Shallow Deformation• Spatial and Temporal Changes in Patterns of

Deformation and Magmatism• Thermal Processes & Mode of Isostatic

Compensation• Role of Fluids• Understanding Faulting• Exporting our Knowledge

Exporting our KnowledgeExporting our Knowledge

• How do we incorporate all these knowledge pieces into a useful geodynamic model?

• Great Basin as a prototypical diffuse zone of continental extension– What does it tell us about other parts of the Basin and Range?– The West Antarctic Rift System?– Why aren’t diffuse zones of continental extension more

common? And why isn’t the Great Basin an narrow rift?

=> What we need: A cross-disciplinary involvement in model development; a community approach to model-building; a geodynamic modeling tool that can be used by non-geodynamicists; education on how to undertake outreach & infrastructure support.

Spatial and Temporal Changes in Spatial and Temporal Changes in Deformation & MagmatismDeformation & Magmatism

• The relation between deformation and magmatism is still uncertain

• Tectonic inheritance vs. dynamic controls– Sevier plateau & local preexisting heterogeneities– Dynamic temperature changes & effect on strength

• Changes in melt source vs. time – Lithosphere vs. asthenosphere– Contributions from the crust

=> We need: easily queried data base of timing, location, rates, and geochemistry of faults & magmatism

Role of FluidsRole of Fluids

• Rheological effects– Generally weaken rocks in both brittle & ductile

regime– May cement fractures

• Source• Relation to thermal regime=> We need: mapping of fluid flow indicators,

correlation to structural elements, knowledge of volumetric flow rates; knowledge of fluid types (hydrous fluids vs. melt).

Thermal Processes & Isostatic Thermal Processes & Isostatic CompensationCompensation

• Patterns of heat flow– Vertical and horizontal– Role of fluid flow

• Causes and effects– Where’s the heat coming from?– How does T(x,y,z,t) relate to changes in deformation pattern?– Turning on and off melt production

• Asthenosphere thermal state (then and now)• What’s holding up the lithosphere?=> We need: more physical properties & heat flow data;

indicators of thermal state in the past; more detailed images of the upper mantle.

FaultsFaults

• Nucleation & growth of Fault Systems• Relationship between Fault Systems and

regional & local stress state (and how it varies with depth)

• Understanding low angle normal faults=> We need: More detailed kinematic

indicators of fault slip history & fault system geometries; More rheological data; Better numerical algorithms