Regional Tectonics
Geos 425/525
Fall 2014
Tectonic rates
LECTURE 3
70 mins
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Plate tectonic rates
• We already know that plate tectonic rates are
in the 1-20 cm/year range, or 10- 200 km/yr.
• For example, in the case of fast
subduction/collision (say 10 cm/yr), it only
takes about 1 million years before a material
point at the trench is at about 100 km depth,if
subduction angle is 45 degrees. Wow.
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• Rates of uplift. The changing vertical position of a point
on the land surface as a function of time = surface uplift.
• Surface uplift = bedrock uplift + deposition - compaction denudation.
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Uplift rates
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Think denudation as being erosion for now, although it is a little more
complicated (see later). Bedrock uplift rate is the rate at which bedrock is
being carried upward by tectonic processes. Deposition and compaction apply
only to sedimentary basins and are ignored when dealing with bedrock. There
could be significantly different response of an orogenic belt locally vs.
regionally. Regional uplift is governed by isostatic response to loading or
unloading, whereas individual points (e.g. Mt Whitney) do not. Example: the
Santa Lucia Mountains, CA.
Uplift rates can be isostatic and tectonic. Tectonic is what is not attributable to
isostatic response, but to shortening. The total uplift rate is a sum of the
tectonic and isostatic components.
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Rates of denudation
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Denudation is removal of material from a point on the Earth’s surface.
Denudation leads to exposure of different crustal levels.
There are two types of denudation: erosional and tectonic. Erosion is
mechanical and chemical weathering of rocks and removal by geomorphic
agents.
Rates of erosion are typically 0.0x mm/yr (see table for summary for major
rivers). At the absolute maximum, rates of erosion have been recorded to be
0.3 mm/yr.
Tectonic denudation is due to crustal thinning and leads to unroofing via fault
slicing. They can be an order of magnitude higher or more, up to cm/yr for
relatively short periods of time. For example, the Catalina Mts, north of
Tucson, have been denuded at a good 4 mm/yr rate at around 20 Ma.
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Denudation rates can be determined using river
sediment yield, structural and stratigraphic controls etc.
By far the most common tool is thermochronology
–some major thermochronometers are shown in Fig below.
Denudation is about one order of magnitude less than plate motions (on average),
but can be much higher for short times in orogenic belts – if tectonic. All high mountain
belts have strong evidence for extensional collapse. Mt Everest, for example, has a major
detachment fault slicing through it.
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Example: apatite U-Th-He
Slope tells the denudation rate, some 2 km over 20 my, or 1mm/yr
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Rates of continental extension
• How fast did the Basin and Range form? Does it add the missing
displacement along the San Andreas, and thus is it representing an
integral part of the plate boundary? How fast does a forearc collapse
after tectonic erosion?
• Extension rates can be several mm/yr, up to plate rates. One way to
determine extension rates is using thermochronology. Continental
extension is a major “within plate” phenomenon, that is directly tied to
plate kinematics. Basin and Range extension accommodates part of the
obliquity in the PAC-NA slip vector at various times during the late
Cenozoic.
• Normal faults move in short pulses of high rates. Consequently, basin
development and range uplift has to mimic those rates.
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Rates of thrust belt propagation
• Fold and thrust belts propagate towards the foreland at
rates of several mm to cm/yr.
• For example, the NA Cordilleran FTB has migrated about
1000 km within ~100 Ma (De Celles, 2004).
• Same numbers apply to major collisional belts like the
Himalayas or accretionary margins (Alps, Cascadia, etc).
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Rates of metamorphic processes
• Dynamo-thermal metamorphism is fast. The theory of kinetics has
shown that metamorphic minerals (such as garnet) grow at rates of mm
to cm per year, suggesting that once a rock has been exposed to
metamorphic conditions it re-equilibrates almost at lab time scales.
• Prograde reactions tend to be erased because of that. Windows into the
prograde history are rare.
• Retrograde paths are kinetically more sluggish. Typical path of a
metamorphic rock is shown in fig below.
• PT and time determinations for several orogens indicate that this loop
can be closed at rates of several mm to even cm/yr. That is a rock can
be taken down to 60 km and back up within a couple of million years
(and experience metamorphism).
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Fig. Shows a typical PT time loop for metamorphic rocks formed during
continental collision (Data from Medaris et al., 2003, Lithos) are from the Carpathians,
and reflect Variscan metamorphism.
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Rates of ductile deformation
• They are potentially much faster than plate rates, implying
that if there is a pressure gradient and conditions for crustal
flow are met in the mid-crust, the flow will accommodate
the pressure gradient over short time periods. A material
point can be traveling in the mid crust at speeds of several
meters per year.
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Ductile marble, Coast Mountains, BC.
Ductile deformation in granite, now gneiss, Catalina Mts., AZ.
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Rates of basin development
• Basins are great indicators of surrounding
tectonic environments – the rate at which a
basin accumulates sediment mimics the
dynamics of the source region. Range: from
0.001 mm/yr to several mm per Ma.
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Miocene syntectonic sediments formed in the Tucson basin during detachment
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faulting
Rates of intracontinental deformation
• Within plate deformation, is much slower than plate kinematics.
Vertical motions are related to (1) isostatic glacial rebound, (2) steady
state decay of old topography, and (3) eustatic loading around passive
margins. All are around 0.0x mm/yr. Glacial rebound is used to
constrain the viscosity of the upper mantle.
• Lateral rates were difficult to constrain before GPS days. We do know
now that a plate like NA is far from being rigid and that several major
brittle faults moving at rates much (tens of times) slower than plate
boundaries break up a rigid continent in several smaller blocks.
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PACIFICPLATE & SAN ANDREAS
TRANSFORM FAULT
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