Ge 122 Field Trip to Taiwan
By Division of Geological and Planetary Sciences
Great adventures often begin with but a paragraph's worth of enticement...
Ge 122 Geologic Hazards Mapping
12 units (1-8-3); summer term. Prerequisites: Ge 120 or equivalent, or consent of instructor. Two weeks of intensive field-based description and evaluation of the deposits and landforms related to a geologic hazard. Field location will vary from year to year, but will focus on a particular locale, either within the U.S. or abroad, where a seismic, volcanic, slope-stability, or other hazard can be documented and evaluated. In September 2002, we will travel to Taiwan once again. Our focus will be on the active structures of fore-arc/volcanic-arc collision, about 60 km north of our field area in 2001. Instructor: Sieh.
For eleven days in mid-September, 2001, six Caltech students joined with six of their Taiwanese counterparts from National Taiwan University to assess the hazards of earthquake rupture, flooding and landsliding in southeastern Taiwan (Figure 1). Their work centered on the southern end of the Longitudinal Valley, the locus of active collision of a volcanic arc and a fore-arc basin, on the western edge of the Philippine Sea plate (Figure 2).
Geodetic levelling and GPS measurements show that the Longitudinal Valley is closing at a rate of more than 20 mm/yr, as the young volcanic arc rocks of the Coastal range on the east collide with the forearc sedimentary rocks of the Central range on the west (Figure 3). Mapping revealed that the active fault dipping beneath the Coast range splits into two main structures in the valley - a thrust fault at the base of the Peinan hills, and a left-lateral strike-slip fault in the river valley east of the hills (Figure 4). Synclines, anticlines and secondary faults between these faults are also active and have implications for the nature of deformation on the principal thrust fault (Figure 5).
Mapping revealed that a large landslide rests upon an ancient riverbed 70 meters above the modern stream bed, east of the Peinan river (Figure 6). This slide alerted students to the possibility of flooding upstream of future landslides. Judging from its thickness, emplacement of the slide might well have impounded a giant lake upstream, more than 100 m deep and 20 km long
Profiles of the fluvial terraces that cut across the "Foot" show the magnitude of fault scarps and style of folding (Figure 7).
Our thanks go to Professor Yue-Gao Chen and his students at NTU for hosting us on this excursion. And also, we appreciate the financial support of the Gordon and Betty Moore Foundation, which made the trip possible.
Images
Figure 1. Caltech students and Taiwanese counterparts on a bridge just downsteam from the scarp and waterfall produced by fault rupture in 1999. Front row: J.D. Godchaux (grad student), Eric Cowgill (post-doc).
Figure 2. The Taiwan orogeny is occurring in two belts. On the west, sediments of the fore-arc basin are riding up over the continental shelf of the Chinese mainland, most recently during the M 7.6 Chi-chi earthquake. The red line is the 1999 fault rupture. On the east, the volcanic arc is colliding with the sediments of the fore-arc. The red circles are the field areas of 2001 and 2002.
Figure 3. Neotectonic map of the Longitudinal Valley shows active left-lateral and reverse faults. Creep meters show that the Longitudinal Valley fault is creeping at over 20 mm/yr. Geodetic profile W-E shows this as well and indicates that the Central Range is tilting toward the valley at about 1 mm/km/yr. We studied the "Foot" in 2001 and will study the area around the Shiouguluan River in 2002.
Figure 5. An oblique aerial view of the "Foot" shows the thrust fault and associated syncline and anticline.
Figure 7. Profiles of the modern and ancient river beds of the Luyeh river. See Figure 4 for locations.Last updated: Tue Oct 09 21:11:33 -0700 2007
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