What sets topographic relief in extensional footwalls

1
What sets topographic relief in extensional
footwalls?

• Alexander L. Densmore
• Nancye H. Dawers
• Sanjeev Gupta
• Roman Guidon

2
Introduction

• Amount of displacement and slip rates on fault
  surface increases away from fault tips.
• Footwall relief as a function of fault geometry
  and spacing
• Regulated by base level (theoretical lowest level
  that erosion processes reach).
• Limited by geomorphic processes rock mass
  strength.

3
Study area

• Northeastern Basin and Range province.
• Fault lengths range between 140-150 km.
• Faults composed of linked segments.
• Reached their present extent 6 Ma.

(Densmore et al, 2005)
4
Google Earth image of Lemhi Fault
5
Estimates of displacement on studied faults
(Densmore et al, 2005)
6
Catchment, outlet spacing and range half-width
(Purdie and Brook, 2006)
7
Catchment relief as a function of slip
accumulation

• Catchment relief increases away from the
  southeastern tips of the faults.
• 20 km away from the southeastern fault tips,
  catchment relief becomes relatively uniform.

(Densmore et al, 2005)
8
Catchment relief as a function of outlet spacing
and range half-width

• Catchment relief shows positive correlation
  with outlet spacing and range half-width.
• In other words, as outlet spacing and range
  half-width increase, drainage basin relief on the
  footwall increases.

9
Proposed catchment evolution model for studied
area

• First stage
• Outlet spacing and range half-width increase away
  from the southeastern tips of the studied faults.
• This stage continues until footwall relief
  reaches 1000-1500 km (which occurs 15 km from
  southeastern fault tip).

(Densmore et al, 2005)
10

• Second stage
• Catchment growth propagates towards range
  half-width.
• Once it reaches range half-width, catchments
  reorganize and begin to compete with each other.
• Result
• catchment elongation in the across-strike
  direction.
• Catchment widening in the along-strike direction.
• No change in catchment relief gtgt no change in
  footwall relief.

(Densmore et al, 2005)
11
External controls on range width and relief

• Footwall width is controlled by
• Fault dip.
• Fault spacing.

12

• Footwall relief is controlled by
• Height of base level.
• Low in the hanging wall and high in the footwall
  gtgt narrow range.
• High in the hanging wall and low in the footwall
  gtgt wide range.

13
References

• Densmore, A. L., Dawers, N. H., Gupta, S.,
  Guidon, R., 2005. What sets topographic relief
  in extensional footwalls? Geology 33, 453-456.
• Purdie, H., Brook, M., 2006. Drainage spacing
  regularity on a fault-block A case study from
  the eastern Ruahine Range. New Zealand
  Geographer 62, 97-104.