GEOLOGICAL CONSTRAINTS ON
ACTIVE DEFORMATION
presented by S. Marshak (UIUC)
Key References:
*Burbank, D.W., & Anderson, R.S., 2001, Tectonic Geomorphology.
*Keller, E.A., & Pinter, N., 2002, Active Tectonics (2e).
*Yeats, R.S., Sieh, K., & Allen, C.R., 1997, The Geology of Earthquakes
Definitions
-Active deformation: Ongoing tectonism generating
geologic
structure (folds; faults; warping; fabrics)
-Land-surface consequences:
•uplift and subsidence
- Geomorphic consequences:
* depositional landscapes
* erosional landscapes
- Epeirogeny vs. orogeny
Rates of Active Deformation
-Geologic strain rates: 10-14/s
-This translates into:
- horizontal rates of 1 - 15 cm/y
- vertical rates of 1 - 15 mm/y
-Implications:
- 10 cm/y = 1 km/10,000y = 1,000 km/10,000,000y
- 1 cm/y = 1 km/100,000y = 100 km/10,000,000y
- 1 mm/y = 1 km/1,000,000y = 10 km/10,000,000y
-Rate-controlling factor
-flow rate of the asthenosphere
The "Battle"
- Active deformation builds features.
- manifestation of internal processes
- driven by internal heat of Earth.
- Features are torn down by mass movement and erosion.
-manifestation of external processes
- driven by gravity and solar energy.
- Everything is tempered by isostasy.
Concept of Geomorphic Markers
-Identifiable geomorphic features or surfaces that provide a reference
frame against which to gauge differential or absolute motion.
-Basic characteristics:
- readily recognizable landform
- known undeformed geometry
- high preservation potential
- known age
Marine or Laucustrine Terraces
-Definition: Planar surface formed by interaction of ocean (or lake)
and the coast.
-Terraces result from a combination of sea-level change and
tectonic movement.
* Pleistocene glaciation causes 120 - 150 m.
* 18,000 years ago, sea level was 150 m lower.
Constructional Terraces:
* Gilbert-type delta
* Reef
- requires 18°C clear water
- coral can grow up at rate of 10 cm/y
- need stable sea-level for terrace to build.
Destructional Terraces:
* wave-cut bench
* abrasion ramp
Examples:
* coastal California
* Papua-New Guinea
River terraces
- flat region on either side of the river.
Lower Sonkul River, Kyrgyzstan
photograph: Marli Miller
- over length of river, it slopes downstream.
- two types:
- aggredational: formed by sediment fill, then downcutting
- degradational: formed b erosion of substrate
-if it cuts into bedrock, it's a "strath terrace"
Kazakhstan - Kyrgyztan / 42kauka Fotograf: Michael Richter
-Can understand river-terrace formation in terms of stream power.
- Stream Power = rate of expenditure of potential energy per unit
length of the stream.
-Stream power is proportional to gradient and discharge. A change
in power produces a terrace.
-Threshold of critical power: Power is just sufficient to transport
sediment from upstream, so height of bed is constant;
this is an "equilibrium condition."
-Stream power is controlled by:
- change in gradient
- change in bed roughness
- change in sediment load or sediment size
Other Geomorphic Markers
- Alluvial fans
- Formed by change in
competence
when a stream emerges from a
narrow canyon and spreads over
a broad plain.
- Lava flows
- Landslides
- Erosional surface
- pediments
- "peneplains"
- Glacial Deposits
- moraines
- Linear markers
Pediment
- river-channel trace Gobi Altai Mountain
Photo by M. Walther
- ridge crests
FEMA
Alluvial fan
Death Valley (CA)
Other Landscape Features Indicative of Neotectonism
- fault scarps (vs. fault-line scarps)
- triangular facets
- mountain-front sinuosity
- sinuosity is less for tectonically active areas
- pressure ridges; sag ponds; tension gashes
Triangular facets, Wasatch Front,
Utah
San Andreas fault delineated by small pressure ridge in this
view to the south across Cholame Valley. Minor,
discontinuous surface fractures with right-lateral offset of
about 5 mm were observed here. Photo b y W. Bryant.
p.276-277a
Original artwork by Gary Hincks
Fig. 10.06g
Photo courtesy of Paul “Kip” OtisDiehl, USMC, 29 Palms, CA
• pressure ridge along the San Andreas
Fig. 10.06a-f
W. W. Norton
Fig. 10.35bc
W. W. Norton
Fig. 10.35a
W. W. Norton
More Landscape features:
-folded land surfaces (topography uplifted by folds)
* rollover folds vs. thrust fault-propagation
* San Andreas (Wheeler Ridge)
* Zagros Mts.
-faulted land surfaces
- mountain-front migration
Wheeler Ridge
Zagros Mountains, Iran
Drainage-System Development:
-overall degree of development (network density)
- longitudinal-profile development (and nick points)
- drainage-basin asymmetry
- stream-length index SL = (ΔH/ΔL)L for a reach.
- depends on gradient and rock type
- identify recent tectonism by identifying anomalously high
index values.
- stream diversion; water-gaps; wind-gaps.
Correlation of geomorphic markers
• problem is that some geomorphic systems respond faster
than others, in a regional context.
• fluvial system responds faster than a glacial system.
• A climate change or specific earthquake may provide
motivation for forming the surface. Thus, if you know the
age of the climate change, can determine age of the
surface or feature. (i.e., need to know paleoclimatic
context.)
Relative age vs. numerical age
• ultimate goal: have a basis for determining
rates up uplift and erosion
Clast seismic-velocity
• fresh rock has a sharp clink, weathered
rock has a dull thump
• due to the number of microcracks and the
degradation of bonds between grains
• pound on many boulders to get a statistical
sense
• can do the same thing by measuring the
seismic velocity through a boulder
• this is only for relative dating
Weathering rinds and hydration rinds
• measure the thickness of weathering rind
• best to use most easily weathered clasts,
like basalts
• in glassy materials, can use the hydration
rind
Soil development
• soil evolves (depends on climate, duration,
and slope
• soil thickness is proportional to the age of
the surface, slope, rainfall, etc.
• other factors indicative of age
-horizon development
-degree of ped development
-degree of mineralization reflect
• can get a chronosequence by studying development on
comparable material
Fig. 7.12a
W. W. Norton
Lichenometry
• diameter of a specific type of lichen is a
proxy for the time a rock has been exposed
• calibrate for a given locality by studying
lichen growth on objects of known age
• e.g., tombstones and monuments; tree rings
• can resolve discrete episodes of rockfall
material associated with seismic activity
Tree rings (dendrochronology)
• growth is rapid in wet season, slow in dry or
winter
• get alternating low-density and high-density wood
• tree-ring time series are a proxy for climatic time
series
• get a master time series from a set of trees with
ring-width overlap
• Trauma to tree is recorded in the rings
• submerged forests
Radiocarbon dating
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works for surfaces less than about 30 to 40 Ka (max about 50 Ka)
14C and 13C are formed in the atmosphere by interaction with cosmic
rays
plant absorbs these isotopes in atmospheric proportions
after death, the radioisotopes decay
accelerator mass spectrometry (AMS) counts all the atoms, so sample
size is reduced
but AMS still doesn't allow dating much beyond 50 Ka
pitfall is variation in cosmic-ray flux. So there is calibration using
tree rings.
use of bristlecone pines takes calibration record back to 10 Ka
Uranium/thorium dating
• uranium series consists of several parent isotopes and
intermediate products
• U substitutes for Ca in calcite. So it allows dating of
carbonates
• but shells can leak daughters, so they are not reliable
clocks
• coral works well, so you can date marine terraces
• can also date carbonate coatings on sediments in soil
• high-precision dating of coral can have uncertainty of
± 10 years
Amino acid racemization
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in life have left-handed amino acids
in death, they flip to right-handed form
the ratio of left to right is a clock
time frame is 100 Ka to 1 Ma, depending
on climate
Luminescence dating
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basis is trapping and release of energy
decay by nearby radioisotopes traps electrons between two valence levels
addition of addition energy, allows the electrons to return to lower level
energy is released in the form of light (luminescence)
additional energy is in the form of
- heat (thermoluminescence)
- light (optically stimulated luminescence)
the number of electrons that can jump depends on how long the sample was
exposed to radiation without being reset
age = paleodose / dose rate
works well for loess or dunes (these were zeroed before deposition) allows
dating from 1 Ky to 1 My
Dating Seismic Recurrence
(Paleoseismology)
Offset markers
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offset stream channels
offset lava flows
displaced shoreline features
growth interruption in corals
Successive landslide deposits
The Waingake Landslide Dam formed in
August 2002 when a high intensity rainstorm
reactivated part of a pre-existing landslide.
http://www.geonet.org.nz/landslidecat.html
Trenching
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Overlap unconformity
draping and overlap of fault scarp
colluvial wedge or landslide debris
fissure filled with alluvium
sand blow and injection dike under draping strata
folding and bed rupture due to liquefaction
offset soil horizons
Fig. 10.31
W. W. Norton
Fig. 10.35d
W. W. Norton
trenching examples
Dr. Tom Rockwell at San Diego State
has dug trenches across the
Rose Canyon Fault.
Minjar fault scarp and wedge of colluvial material overridden by thrust slip
along the central section of the Main Pamir Thrust.
Tsunami features
• seismic vs. subaerial landslide vs.
submarine landslide types
• examples from Norway, Newfoundland, and Hawaii
• geomorphic evidence for tsunami
- sand and gravel layers well inland and at high levels
- tossed boulders
- coastal erosion feature
above storm wave height
Scotland
Boulders
(Australia)
Layer of marine sand deposited during the
1952 Great Kamchatka tsunami (shown by arrow).
L.I. Bazanova
Penecontemporaneous deposition
• soft-sediment
folding
• turbidites
Meander restructuring
• river sinuosity
• position of the meander belt
Neotectonic studies active faulting
• active growth faults, studied by seismic
reflection
• incorporation of dated layers in structures
- conglomerates (e.g., Pyrenees; Salt Range)
- dated ashes (e.g., Bishop tuff)
- strontium isotopes in marine sediments
•
http://www.geo.cornell.edu/geology/faculty/RWA/movies/growthFBF.html
Extensional growth fault
Current Issues
• are there "steady-state" orogens (e.g., Olympics)
• erosion rate laws
• Relationships between climate and uplift
- A climate change or deformation may
provide motivation for forming a surface.