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Great Basin & Range Province (Stretching (Substantial area stretched…
Great Basin & Range Province
Where?
East of Great Valley Province (which is East of SAF province)
Cause
extension
Net deficit in plate motion
Sierra Great Valley Provicnce moves N ~ 11mm/yr
Central GBP moves W~ 2.8 mm/yr
Differential extension results in intervening spaces
(West GBP?)
Normal
Faulting
Normal faults trending N-SE, N-S & N-SW
Faults define
horsts (mtns) & grabens (valleys)
Fault bound basins up to 70km wide & 300 km long
B&R Provinces
Subdivided into 3 provinces ~ structural units
∴ Heterogeneity within B & R
e.g. Central Basin Province = monolith block behaves uniformly
Stretching
Substantial area stretched by distributed normal faulting
Cause: collapse of overthickened orogen
~ 12 Ma (Sevier-Laramide)
Extension > 100 % i.e. area grew by 100% overall
Extension rate = variable but ~ tens mm/yr
2 Predominant Fault Systems
Westward & Eastward dipping normal faults
∴ surface displaced either side of faults
results in horst & graben / fault bound basins
Distinctive Morphology
Dry climate exacerbates morphology since limited erosion
Prompted theorising that high angle normal faults alone accomodate extension
Geometrical Problem
Steep normal faults have limited capacity to accomodate horizontal extension
∴ significant vertical displacement instigates limited horizontal displacement
Thus limits total extension possible
Anderson's
Law
Earth surface is free of shear stress
No surface on top i.e. other half missing
No drag exerted by atmosphere
∴ Plots on shear stress axis in Mohr space & only conveys normal stress
One of principal stresses always = perpendicular to Earth's surface
= consequence of inability to convey shear stress
High angle normal fault
Extensional settings, σ1= vertical
i.e. perpendicular to Earth surface
∴ Ideal shear fracture orientation = 30° off vertical
Hence high fault angle results
B&R Faults
GBR = underlain by high angle normal faults
which join low angle normal faults @ depth
most extension accommodated by low angle detachments