GreatBreak: Grand Challenges in Geodynamics

Characteristics of a Desirable Geodynamic 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

km km 500 km 550 km 600 km 0 m.y. 10 m.y. 20 m.y. 30 m.y. 40 m.y km

Mantle Yield Stress Stress (Mpa) m.y. Moho Temperature Temperature (Celsius) Distance From Origin (km)

Driving 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 km scales; density structure; T(x,y,z) and heat flow data.

Rheology 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 Depths 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 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

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 Deformation & 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 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 Compensation 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.

Faults 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