David Foster - University of FloridaPaul Mueller - University of FloridaDavid Mogk - Montana State University

David Foster - University of FloridaPaul Mueller - University of FloridaDavid Mogk - Montana State University

Regional overview and workshop themeRegional overview and workshop theme

• Theme: >3.5 Ga of lithospheric evolution: segregation from the mantle, stabilization and modification.

• This area contains the largest record of major lithospheric events to be analyzed by EarthScope.

• The Archean lithosphere of Wyoming is still being modified thermally and chemically by the Yellowstone system

Goals:• Identify major geological

problems that EarthScope can help address

• Identify what additional groundwork needs to be done to maximize EarthScope results

• Formulate hypotheses and pose tests of the major questions

• Prioritize major objectives and possible additional projects to maximize gains while the instruments are active in the area

• Plan education and outreach activities

• Inform and engage the public• Integrate Northern Rockies

research in to broader EarthScope science goals

SW Laurentian Basement• Complex mosaic of Archean and

Proterozoic lithospheric blocks • Poorly exposed outside of Laramide

basement uplifts• What are the province boundaries?• Do the provinces extend into the

mantle lithosphere?• How was the basement and lithosphere

modified by younger events? • What is the nature of the edge of

Laurentia– Idaho shear zone?

• Has the ancient structure played a roll in Phanerozoic tectonomagmatic events?

– Does it continue to do so today?

• What constitutes a craton in terms of geochemical and geophysical properties

Margins of Wyoming craton

• Nature of the Great Falls tectonic zone and relationship to THO

• Selway accreted terrane• Medicine Hat block - how far

does it extend?• Priest River block - is it a

separate late Archean-Paleoproterozoic block?

• How do terranes defined by magnetic data equate to crustal and lithospheric structure?

• Does the basement surface map equate to what is at depth in the lower crust and mantle lithosphere?

– probably not

Sims et al. (2004)Sims et al. (2004)

Deep Crustal & Lithospheric Investigations• LITHOPROBE DEEPPROBE

seismic data & U-Pb ages of xenoliths suggest that a thick Paleoproterozoic mafic underplate underlies the northern Wyoming province

• Underplate could be a major source for tonalite-grandiorite magmas. Many of these plutons age give Nd TDM = 1.7.1.9 Ga (e.g. Tobacco Root)

• is some of it Archean?

• Implications for PC orogeny and cratonization

• Less data than southern Rocky Mountains

Clowes et al., 2002

Belt basin and Neoproterozoic rift• What controlled rift geometry?• What drove rifting?• How was the crust modified?• Metallogenesis and magmatism

– Western edge of Belt basin? Is most of it here or is most of it missing?

• What is the Lewis and Clark line & other tectonostratigraphic boundaries?

• Implications of recently recognized widespread Neoproterozoic rocks west of Belt basin in Idaho

• Was the margin truncated• What age is the reentrant?

– Neoprotoeroic or Mesozoic or both

• Rich and complex history of continental modification

• Cretaceous-Eocene magmatic provinces

• Cordilleran fold-thrust belt• Laramide basement uplifts• Eocene metamorphic core

complexes• Cenozoic-Recent magmatism• Mineralized zones• Mesozoic accreted terranes• Idaho shear zone• Lewis and Clark fault zone• Hot Spot• Neotectonics • Seismic activity• Shallow subduction• Slab window

Cordilleran Orogen

SW Montana magmatic province

• Orthogonal to orogen axis

• Intruded well into foreland at ~75 Ma

• Relationships between thrusts and plutons

• Basement control, shallow subduction, or truncated margin?

Lithospheric response to tectonics through time

• In southern Rocky Mts major variations in strength and fertility occur between the Phanerozoic and Proterozoic lithosphere

• In northern Rocky Mts there are major differences in strength and partial melt productivity between the Proterozoic and Archean as well.

• How far northeast did the effects of Laramide shallow subduction extend?

• Asthospheric upwelling at 54-50 Ma (older than in the south)

Humphreys et al., 2003Humphreys et al., 2003

Lewis and Clark fault zone

• Basin bounding fault along axis of Belt basin

• A lateral ramp during Cretaceous-Paleocene thrusting

• A dextral accommodation zone during Eocene extension

• Basement control?

Eocene extension and magmatism

• Reworking lower crust

• Collapse of orogenic wedge

• Lithospheric response to a slap window?

• Relationship between magmatism and extension

– e.g., Bitterroot - older, deep Eocene plutons in footwall, younger shallow plutons and volcanic rocks in hanging wall

Integration

Relationships between Precambrian provinces and Phanerozoic tectono-magmatic provinces

The Yellowstone system is currently modifying the Archean lithosphere of Wyoming. What is the nature of the hot spot? Is there lithospheric control on the neotectonics and magmatism?

Humphreys et al., 2000Humphreys et al., 2000

Summary

• Understanding the processes by which the Rocky Mountains lithosphere formed and evolves requires the integration of many disciplines (structural geology, geochemistry, petrology, geophysics, geochronology, geomorphology, etc.).

• EarthScope resources can play a pivotal role in elucidating the complex history of the northern Rocky Mountains, and thereby add significantly to our understanding of both crustal genesis and continental evolution, by providing images of crust and mantle structures to combine with the age and composition information of material added to the continental crust.

Example Questions for Northern Rockies Workshop Discussion

• How did the ancient lithosphere and structures control crustal evolution?

• What is the structure of the oldest craton in southern Laurentia?• How are cratons stabilized and once “cratonized” how are they

reactivated? • Is there a typical crustal/lithospheric structure associated with major

mineralization?• How strong is crust in orogenic zones and why does it extend?• The nature of magmatic underplating and repeated mafic magma

intrusion in the craton?• Is the Yellowstone Hot Spot a deep mantle plume?• How diffuse is the plate boundary of western North American?• What is driving modern extension and seismicity?• Why are the Northern Rocky Mountains still at high elevation?• How has the Cenozoic-Recent magmatism in the Snake River Plain

modified the lithosphere?• What is the large-scale structure of the Mesoproterozoic Belt basin?• Can we answer all important questions with passive seismic?• What other methods are needed?• What is the best way to involve educators and public in the science of

EarthScope?