Geology Reference
In-Depth Information
Box 9.2
Geological versus geodetic rates of deformation.
The proliferation of GPS data on relative velocities across orogens and plate boundaries has
provided a far more complete view of the modern patterns of crustal deformation. Although it
is tempting to use these decadal rates as representative of rates applicable to hundreds of
thousands of years, this equivalence has relatively rarely been tested. Along some well-studied
strike-slip fault zones, such as the southern San Andreas Fault, where numerous paleoseismic
studies define rates of slip on multiple faults at time scales of many thousands of years, geodetic
and geological rates are quite closely matched.
Across contractional orogens, however, this match of rates is less clear. For example, in the
Himalaya, geomorphic studies (Lavé and Avouac, 2000) clearly show that, at Holocene time
scales,
∼
20 mm/yr of slip occurs at the Main Frontal Thrust (see Fig. 7.25). Yet, the geodetic
shortening across the same zone is only a few millimeters per year. This large mismatch is
interpreted to result from the presence of a locked megathrust, such that elastic strain is
accumulating in the Greater Himalaya and is episodically released in large earthquakes that
translate slip to the frontal thrust (Avouac, 2003). The Tien Shan, which span from China across
Kyrgyzstan to Kazakhstan, are not underlain by a single megathrust, but instead comprise a
series of fault-bounded ranges that are separated by narrow basins (see figure A). Recent
earthquakes in the Tien Shan suggest that most fault planes dip quite steeply (
∼
45
°
) and extend
to depths of
∼
20 km (Ghose
et al.
, 1997, 1998), which is near the brittle-ductile transition.
Hence, these faults are unlikely to be linked by a relatively shallow crustal detachment.
North
South
A
GPS site
Fault trace
Thrust Fault
Aksay
At-Bashi
4
Naryn
Tarim
Basin
Kochkor
Chu
3
Kazakh
Platform
2
Tien Shan Sites
AZOK
1
0
Kazakhstan
Kyrgyz Republic
China
B
20
GPS velocity (relative
to AZOK)
Cumulative late Quater-
nary shortening rate
Local shortening
(95% uncertainty)
18
0.3
+1.6
0.9
+0.3
-0.3
16
4.2
+0.6
-0.2
14
-0.7
2.2
+1.6
-0.2
12
3.2
+2.0
10
-0.8
North
South
8
2.2
+1.6
-
0
.2
6
4
Geologic versus
Geodetic Rates
2
A
Z
OK
0
0
100
200
300
400
500
Distance (km)
A. Topographic profile of the Tien Shan with major thrust faults, GPS sites, and studied fault indicated. B.
Comparison of GPS north velocities with late Quaternary shortening rates. Modified after Thompson
et al.
(2002).
In an effort to define fault-slip rates across the Tien Shan, Thompson
et al.
(2002) analyzed
numerous fold scarps, fault scarps, trenches across faults, and deformed terraces on major faults
