Download 03 Structural Control mod 4b

Structural Control
of Landforms
Mostly Chapter 12
Plus a review of folds and faults
Photo from Drury: Two distinct units. One dendritic
drainage pattern is sparsely vegetated. Parallel
contours suggest it is horizontal. Other formation
banded, with straight wooded ridges, controlled by
steep dips. The boundary truncates the ridges.
Horizontal unit lies unconformably on the steeply
dipping strata (angular unconformity).
The wide spacing of drainage in the younger unit
suggests that it is a massive, coarse clastic
rock. The older unit comprises shales and
limestones. From Steve Drury, Image Interpretation
in Geology, adopted for this course
Some photos in this PowerPoint made available online,
courtesy of Steve Dutch, click here
SAND, HOSES, Slickensided Rock, Pencil, Rubber
bandGum, Foam sediments, Cardboard fault models, 2
Plastic boxes, Food Coloring ,Paper, wood, Ice
From our lab workbook Image
Interpretation in Geology by Steve Drury
Erodability
• Relative Erodability
– Layered rocks = wide range
• Sedimentary
• Volcanic
– Massive rocks = narrow range
• Metamorphic
• Intrusive igneous
– Erodability is not absolute but
• typically shale > limestone > sandstone ~ gneiss
Canadian Shield.
Pale granite and
darker metavolcanic
rocks, the granite
having resisted
glaciation best.
Drury IIG
Erodability
• "… shale, limestone, marble and some
types of [mica] schist are less resistant
"valley-makers" in humid climates" …
• "whereas [quartz] sandstone, quartzite,
[quartz] conglomerate and various igneous
rocks [ granite has ~20% quartz H= 7] are
resistant "ridge-makers" ….
• Easterbrook (1969) Principles of Geomorphology
[words in brackets added]
Lithology/Climate
Erodability: shale > limestone > sandstone ~ gneiss
In humid areas, weathering and erosion are faster, slopes are more eroded,
gentler after the same duration of exposure to weathering
Horizontally
layered rocks
– outcrops
parallel
topographic
contours.
In arid terrains (a) the
intermittent violent
erosion develops
steep-sided gullies
and valleys. Note
differential erosion
In humid climate the
topography is more muted.
Monadnocks resistant rock ridges Colorado
Undisturbed Sediments
showing differential erodability
Dry Climate, intermittent strong storms
Review: Stream Vees
Vees are pointing in direction of dip
Tablelands
In horizontal beds, rock outcrops
would follow contours
In horizontal beds, rock outcrops
would follow contours
Butte
chimney
Tablelands: note horizontal
layers, differential erosion
mesa
Inselberg
Pediment (gentle slope < 5%,
erosional concave up surface
w thin veneer of gravel etc.)
Dry Climate, intermittent strong storms
• Plateau>mesa>butte>chimney
• Ratio surface area of top to height
Alluvial Fan
Desert Landforms
near Mountains
Rain-shadow desert in
the lee of mountains
(often exposed bare rock
with gravel veneer)
Mountains eventually erode away to Inselbergs
Compression, Tension,
and Shearing Stress
Convergent
Divergent
Transform
Convergent Plate Boundaries
and Folding
Subduction causes Arc: Under
Ocean Lithosphere Japan,
Aleutians, Cent. Am.; under
continent Andes, Cascades
Continent-Continent
collision forms
Fold and Thrust Mountains:
Alps, Himalayans,
Appalachians
Strike and Dip
Map Symbols: Strike shown as long line, dip
as short line. Note the angle of dip shown: 45o
Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
Tilted Strata
• Monoclinal folds, or one
side (limb) of a fold
• Name = f(dip angle)
– Cuesta (moderate dip)
– Hogback (steep dip)
– Flatiron remnant of
dissected Hogback w
triangular face
Dip Slope vs. Scarp slope
Cuesta
Hogback
Hogback dip slope greater 30°
- 40° with near symmetric
slope on each face
Ridges
• Dip of Cuesta < Hogback
Copyright http://www.alperry.com/coal/grand_hogback.html
© J. Michaelhttp://www.aureo.org/conference/boulderconference.html
Daniels 2002
Folds are typical of convergence
Folded Rock Before Erosion
Folded Rocks, Hwy 23
Newfoundland, New Jersey
Note highest point
Source: Breck P. Kent
Adjacent Anticline and Syncline
Folded Rock After Erosion
Eroded Anticline, older rocks in center. Syncline is opposite.
Topography may be opposite of Structure
Anticline Before/After Erosion
Notice center rock oldest
Topography may be opposite of Structure
Syncline Before/After Erosion
Notice center rock youngest
Various Folds
Various Folds (cont'd)
Various Folds (cont'd)
Various Folds (cont'd)
Axis
Axial plane near axis should be close to horizontal
Plunging Folds and Nose Rules
Demo: Plastic box, water, paper folds
Up
End
Down
End
Nose of anticline points direction of plunge, syncline nose in opposite direction
Plunging Folds
Nose
Nose
Nose
Joints: Fractures – with no movement
vs. Faults with relative movement
Sandstone, note no streams here, too many cracks
Source: Martin G. Miller/Visuals Unlimited
Demo: Cardboard Models
Dip-Slip
Faults
Continental Rift into Ocean Basin - Tension => Divergence
Rift Valleys and
Oceans are the
same thing
Normal Faults
Normal Faults at Divergent
Margins - Iceland
A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR
Overhanging
Block
Fault Line scarp
(High-angle
Normal Fault)
Convergent Margins
Shallow Reverse Fault = Thrust Fault
Lewis Thrust Fault (cont'd)
Same layer
Lewis Thrust Fault (cont'd)
Source: Breck P. Kent
PreCambrian Limestone over
Cretaceous Shales
Geologists are frequently called upon to find the ore body
This
guy is
rich
What phase of magma fractionation would result in the
placement of this ore body?
Which formed first, the ore body or the fault?
What common mineral is mostly likely in the ore body?
Younger
Reverse
Normal
This poor guy is out of luck
Miners pay geologists to
find their lost orebody
One friend earned
enough to buy a house
Horizontal Movement Along
Strike-Slip Fault
Landscape Shifting, Wallace Creek
San Andreas Fault
Normal Fault Quake - Nevada
Reverse Fault Quake - Japan
Divergent
HW Down
HW Up
Convergent
Transform
Strike Slip Fault Quake - California
http://pangea.stanford.edu/~laurent/english/research/Slickensides.gif
Fracture Zones and Slickensides
Part 2 Structural Control
of Streams mostly Ch. 12
• Consequent streams follow slope of the land
over which they originally formed.
• Subsequent streams are streams whose course
has been determined by erosion along weak
strata.
• Resequent streams are streams whose course
follows the original relief, but at a lower level
than the original slope
• Obsequent streams are streams flowing in the
opposite direction of the consequent drainage.
consequent (c follow slope)
obsequent (o opposite main slope)
subsequent (s along weak)
Insequent (random dendritic)
resequent streams (original slope but
lower level)
Insequent Streams= Initial Consequent
• Almost random drainage often forming dendritic
patterns.
• Typically tributaries - developed by headward
erosion on a horizontally stratified rocks, or a
substrate with ~ constant composition.
• NOT controlled by the original slope of the
surface, its structure or the type of rock.
Headward Erosion
Drainage Patterns with and without structural control
None
Joints
Volcano, exposed pluton, diapir
fold limbs
Dendritic Patterns
• Underlying bedrock has no structural control
over where the water flows.
• Characteristic acute angles
• No repeating pattern.
Trellis Patterns
• Form where underlying bedrock has
repeating weaker and stronger types of
rock.
• Streams cut down deeper into the weaker
bedrock
• Nearly parallel streams
• Branch at higher angles.
Rectangular patterns
• Branching of tributaries at nearly right
angles
• Form in jointed igneous rocks or horizontal
sedimentary beds with well-developed
jointing or intersecting faults.
Parallel Erosion
• Form on unidirectional regional slope or parallel
landform features. Small areas.
Radial Erosion
•
•
•
•
Flow of water outward from a high point
Down a volcano cone
or an intrusive dome, or
down an alluvial fan.
Annular patterns
• form on domes of alternating weak and
hard bedrocks.
• The pattern formed is similar to that of a
bull's-eye when viewed from above
• weaker bedrocks are eroded and the
harder are left in place.
Centripetal patterns
• Form where water flows into a central location,
such as a round bowl-shaped watershed, or a
karst limestone terrain where disappearing
streams flow down into a sinkhole and then
underground.
Structural Control of Drainage
• Contorted
Folded Rocks
Stream Capture
Headward Erosion
Stream Capture vs. Structural Control
Subsequent Susquehanna
does not reach Beaverdam
Creek flowing through
water gap
Susquehanna captures
headwaters of Beaverdam
Creek, diverting upper
Beaverdam trunk to
Susquehanna channel.
Godfrey Ridge
Headward erosion from Water
Gap area cut through Godfrey
Ridge and captured Brodhead
Creek which was flowing east
behind Godfrey Ridge
Stream Capture
Terraces 1
1. Old river meanders
across floodplain
2. Base level drops
(how?), or region
uplifts. Area now
much higher above
sea level than before.
Potential energy
increases, water flows
faster, better erosion,
stream straightens and
cut down to base
level, less floodplain
width and cut lower.
3.Terrace forms from
previous floodplain.
Further incision cuts
another terrace
Next time Terraces 2 and 3:
Isostatic Rebound and high
water shorelines as glaciers
melt
Potential rgh to Kinetic Energy 1/2mV2
A flight of river terraces
• Antecedent Streams and Superimposed Streams
• Meanders in steep, narrow valleys
– Caused by a drop in base level or uplift of region
Delaware Water Gap
Incised (entrenched) meanders
• River is older than uplift
"In this panorama in southwestern Colorado, a stream flows from the right
across an uplift (anticline) in the rocks. As soon as the stream enters the uplift,
its canyon becomes deep. Note the entrenched [incised] meanders, a couple of
which were cut through and abandoned when the canyon was about half its
present depth. As soon as the river exits the uplift, the canyon once again
becomes shallow. Clearly, the river was there first and the rocks arched upward
across its course." Steve Dutch
Some photos in this PowerPoint made available online, courtesy of Steve Dutch, click here
Pediments
and Alluvial Fans
Alluvial fans typically develop at the
exits of intermittent streams
draining arid mountainous regions.
And on Mars …
Link courtesy Melissa Hansen
An example of a v-shaped stream, with fairly constant
slope and cross section
Conservation of Energy with frictional losses
An example for the homework calc.
• A stream channel has been uplifted to 300
meters above base level. It’s cross sectional
area, slope, and water depth is close to
constant. The stream is full of large boulders. At
300 meters it flows out of an alpine lake, where it
has an average velocity of 0.01 meters/sec, that
is, it has mostly potential energy. At base level it
has a velocity of 15 meters per second (so all
kinetic energy, plus frictional losses on the way
down. Estimate the percent energy lost to
friction.