MOTION AND EVOLUTION OF THE CHAOCHOU FAULT, SOUTHERN TAIWAN. A Thesis LAUREN E. HASSLER

Transcription

1 MOTION AND EVOLUTION OF THE CHAOCHOU FAULT, SOUTHERN TAIWAN A Thesis by LAUREN E. HASSLER Submitted t the Office f Graduate Studies f Texas A&M University in partial fulfillment f the requirements fr the degree f MASTER OF SCIENCE August 2004 Majr Subject: Gelgy

2 MOTION AND EVOLUTION OF THE CHAOCHOU FAULT, SOUTHERN TAIWAN A Thesis by LAUREN E. HASSLER Submitted t Texas A&M University in partial fulfillment f the requirements fr the degree f MASTER OF SCIENCE Apprved as t style and cntent by: David V. Wiltschk Julie Newman (Chair f Cmmittee) (Member) Vatche P. Tchakerian Richard L. Carlsn (Member) (Head f Department) August 2004 Majr Subject: Gelgy

3 iii ABSTRACT Mtin and Evlutin f the Chachu Fault, Suthern Taiwan. (August 2004) Lauren E. Hassler, B.S., Appalachian State University Chair f Advisry Cmmittee: Dr. David V. Wiltschk The Chachu Fault (CCF) is bth an imprtant lithlgic bundary and a significant tpgraphic feature in the Taiwan rgenic belt. It is the gelgic bundary between the Slate Belt t the east, and the Western Fthills t the west. Althugh the fault is knwn t be a high angle blique sinistral thrust fault in places, bth its kinematic histry and its current rle in the develpment f the rgen are prly understd. Field fabric data suggest that structural rientatins vary alng strike, particularly in the middle segment, the suspected lcatin f the intersectin f the nland Eurasian cntinent-cean bundary and the Luzn Island Arc. Fliatin/slutin cleavage is riented NE-SW and in the nrthern and suthern sectins, but ESE-WNW in the middle segment. Slip lineatins als reveal a change in fault mtin frm dipparallel in the nrth t a mre scattered pattern in the suth. This crrelates smewhat with recent GPS results, which indicate that the directin f current hrizntal surface mtin changes alng strike frm nearly perpendicular t the fault in the nrthern field area, t blique and nearly parallel t the fault in the suthern field area. The magnitude f vertical surface mtin vectrs, relative t Lanyu Island, decreases t the suth. Surface mrphlgy parameters, including muntain frnt sinusity and valley flr width/valley height rati indicate higher activity and uplift in the nrth. These bservatins crrelate well with published apatite/zircn fissin track data that indicate un-reset ages in the suth, and reset ages in the nrthern segment. Gedetic and gemrphic data indicate that the nrthern segment f the CCF and Slate Belt are currently underging rapid uplift related t blique arc-cntinent cllisin between the Eurasian cntinent and the Luzn arc. The suthern segment is significantly less active perhaps because the rgen is nt yet invlved in direct arc-cntinent cllisin.

4 iv ACKNOWLEDGEMENTS Thanks t my thesis advisr, Dr. David Wiltschk, and t my cmmittee, Dr. Julie Newman and Dr. Vatche Tchakerian, fr their guidance and assistance n this prject. Thanks t Dr. Jih-Ha Hung and his students in Taiwan fr their assistance in the field. Thanks t Jhn Spang fr assistance with the crss sectins. Thanks t Lai Lee fr help with ArcGIS and fr her friendship. Thanks als t Dr. Will Lamb fr use f the petrgraphic micrscpe. Many sincere thanks t my clleagues in Tectnphysics. My research has greatly benefited frm discussins with yu all. Thanks t Dr. Richard Allmendinger fr use f his Sterenet sftware. This prject was supprted by the NSF Summer Institute in Taiwan and grants frm AAPG and GSA.

5 v TABLE OF CONTENTS ABSTRACT ACKNOWLEDGEMENTS LIST OF FIGURES Page iii iv vii 1. INTRODUCTION Tectnic Setting f Taiwan Suthern Taiwan Gelgy The Chachu Fault Crss Sectins Line A-A Line B-B Line C-C FABRIC STUDY OF THE SOUTHERN CHAOCHOU FAULT Previus Wrk Methds Results Lithlgy GEOMORPHOLOGY OF THE SOUTHERN CHAOCHOU FAULT DISCUSSION CONCLUSIONS 43 REFERENCES 45 APPENDIX A FIELD FABRIC MEASUREMENTS 51 APPENDIX B GEOMORPHOLOGY BACKGROUND AND COMPARISON STUDIES

6 vi APPENDIX C GEOMORPHOLOGY DATA TABLES VITA.. 75

7 vii LIST OF FIGURES FIGURE Page 1 Map Shwing Gegraphic and Plate Tectnic Setting f Taiwan Index Map Shwing the Gelgy f Taiwan and the Study Area 3 3 Seismic Tmgraphic Sectin Acrss the Study Area in Suthern Taiwan 6 4 Field Area Stratigraphy 11 5 Gelgic Map and Lcatins f Crss Sectins Line f Sectin A-A 13 7 Line f Sectin B B Line f Sectin C-C Digital Terrain Map f the Study Area Shwing the Three Regins Field Fabric Data fr the Nrthern, Middle, and Suthern Regins Phtmicrgraphs Shwing Typical Rck Fabric and Shear Structures frm Slate Belt Rck Samples, Taken frm Alng the CCF Basin Size Drainage Basin Mrphmetry Plt f Stream Gradient vs. Distance frm the Muntain Frnt fr Majr Drainages Variatins in Muntain Frnt Parameters GPS Surface Mtin Data fr Suthern Taiwan Zircn/Apatite Fissin Track Ages

8 1 1. INTRODUCTION The transitin frm accretin t cllisin f an accretinary prism, frearc, and arc at a subductin zne is a fundamental feature f arc-cntinent cllisins. Structures frm each f these transitinal stages f cllisin are ften, but nt necessarily bscured by later events. Recnstructing these stages and their evlutin can pse a challenge (e.g., Rwley and Kidd, 1981; Anczkiewicz et al., 2000; Brwn et al., 2001; amng many thers). In the case f Taiwan, the Luzn vlcanic arc system is clliding with the East Eurasian margin (Fig. 1). Cllisin in Taiwan is blique and therefre is clsing frm nrth t suth in zipper-like fashin (Suppe, 1984). Because the rgen is develping t the suth, there is a space-time equivalence where structures may be traced frm the nset f cllisin (suth) t maturity (nrth) as the nature f the rgen changes prgressively frm submarine intra-cean subductin suth f N, t arccntinent cllisin between abut N and 23 N, and t arc accretin nrth f abut 23 N. The Chachu Fault (CCF) is ne structure that persists thrugh the transitin frm accretin t cllisin and appears t be a fundamental bundary frm the inceptin f the Taiwan rgen. It can be traced based n bathymetry frm ff the suthern cast f Taiwan, where the cllisin is just beginning, t the nrthernmst tip f the island (Liu et al., 1997; Fuh et al., 1997). On land the CCF is a majr prvince bundary between the sedimentary fld and thrust belt t the west, the Western Fthills, and the metamrphsed Slate Belt f the Central Range t the east (Fig. 2). The transitin frm accretin t cllisin may ccur near 23 N latitude, where a transitin is marked by changes n the upthrwn side f the Chachu Fault in a) structural rientatins such as fld axes and rck fabrics, b) slip rientatins and rates n faults, c) surface mrphlgy, d) seismicity. This thesis fllws the style f the jurnal Tectnphysics.

9 Eurasian Plate East China Sea 26 China 26 Taiwan Strait Okinawa Trugh COB Peikang Peikang High High CTFZ Taiwan Taiwan Nrth Luzn Trugh Luzn Vlcanic Arc Luzn Vlcanic Arc Phillippine Sea Plate Gagua Ridge Ryukyu Trench Trench Suth China Sea Phillippine Sea Manilla Trench 1600 Luzn Fig. 1. Map shwing gegraphic and plate tectnic setting f Taiwan. Nte the ppsing subductin systems, the Ryukyu Trend in the nrth and the Manila Trench in the west. The nland trace f the Manila Trench is nt shwn and is nt precisely knwn. The cntinent-cean bundary (COB) is shwn (after Lallemand et al., 2001). The cntinental margin is riented N60E, whereas the Luzn arc strikes N10E and mves in a N50W directin. The angle between the plate cnvergence vectr and the margin is n average 70 degrees.

10 25 N 120 E 121 E 122 E Thrust sheets Cnturs n Pekang High Axes f flds 3 25 N Lishan Fault 24 N -3km 4km 5km Castal Range 24 N 2km 2km Peikang High -3km -4km Lngitudinal Valley 23 N -2km -4km Chachu Fault B A C Pingtung Valley C' B' A' Luta Central Range Slate Belt Yuli belt Tailuk belt Backbne Range Hsuehshan Range 23 N 22 N 50 km Hengchun Peninsula Lanyu Western Fthills Castal Plain 22 N 120 E 121 E 122 E Fig. 2. Index map shwing the gelgy f Taiwan and the study area. Nte the lcatin f the Chachu Fault; particularly the bend at apprximately N 23 latitude. Nte als the lcatin f the Peikang basement high in west central Taiwan. Map mdified frm Sibuet and Hsu, 1997.

11 4 The gal f this study is t describe past, recent, and present defrmatin assciated with the CCF in Suthern Taiwan by integrating structural and petrgraphic data and landscape mrphlgy with available GPS, seismicity, and fissin track data. This study will fcus n this suthern extent f the CCF (Fig. 2). Of particular interest is the regin near the twn f Liukuei, where the fault takes a sharp bend tward the suth. In additin t a change in surface expressin f the fault, this restraining bend is assciated with changes in rck fabric rientatins, surface mrphlgy, surface velcity, and seismicity. 1.1 Tectnic Setting f Taiwan The island f Taiwan is the result f suthward prpagating blique cllisin between the Luzn Island Arc, n the Philippine Sea Plate (PSP), and the eastern margin f Eurasia (Fig. 1, Teng, 1990). The PSP is currently advancing tward Eurasia frm the sutheast, between 7 and 9 cm per year, relative t the East Asian margin (Sen et al., 1987, Yu et al., 1997, Hickman et al, 2002, Wiltschk et al., 2002, Rau and Wu, 1995, Teng, 1990). The nrth-facing Ryukyu subductin zne and assciated backarc spreading dminate tectnic prcesses in the nrth. In the suth, the Eurasian plate is subducting eastward, beneath the PSP (Teng, 1990). The change in subductin plarity ccurs belw the Hualien regin f nrtheastern Taiwan (Fig.1). A well-defined Beniff zne beneath Nrthern Taiwan crrespnds t the dwn-ging PSP slab f the Ryukyu subductin system (Rau and Wu, 1995). Smewhat less clearly defined is the Beniff zne alng the length f the island that crrespnds t the dwn-ging Eurasian slab. Teng et al. (2000) have prpsed a suthward-prpagating tear in the dwnging Eurasian slab as a mechanism fr the flip in subductin plarity beneath Taiwan. As the breakaway slab cntinues sinking int the mantle, the nrth-facing Ryukyu slab prpagates westward int the pening. The timing f the nset f slab

12 5 breakff, the rate f its prpagatin, and the current extent f are still unclear (Rau and Wu, 1995, Teng et al., 2000, Lallemand et al., 2001). A tmgraphic sectin thrugh the field area fr this study clearly shws a dense slab f Eurasian crust subducting eastward beneath the island (Fig. 3, Lallemand et al., 2001). The dwnging slab is nt everywhere cntinuus with depth. In the image shwn, a zne f lw density at the tp f the slab indicates that it may nt be attached, and that a tear may well have prpagated as far suth as ur study area (Rau and Wu, 1995). Als nte that while tmgraphy frm Nrthern Taiwan indicates a mature crustal rt beneath the Central Range (Fig. 2), a dense muntain rt is absent in the suth (Rau and Wu, 1995). The suthern prtin f the island may nt yet be invlved in direct arc-cntinent cllisin. Precllisinal extensin f the Suth China cntinental margin is well dcumented (Teng, 1990; Tillman et al., 1992; Lu, 1992; amng thers). Sibuet and Hsu, (1997), identify fur different belts f basins in the Suth China Sea that frmed during the Early Micene and subsequently changed pening directin frm N-S t NW-SE (Pautt et al., 1986 in Sibuet and Hsu, 1997). Als at this time, Luzn Island was frming as part f the Luzn Island Arc (Sibuet and Hsu, 1997). Cllisin began in the regin that is nw nrthern Taiwan arund 20 Ma, and peaked between 8 and 5 Ma (Teng, 1990). This peak in cllisin intensity crrespnds t a change in the directin f mtin f the Philippine Sea Plate frm NNW t WNW and resulted in a mre direct, rthgnal cllisin (Teng, 1990). At this time, the stratigraphy f the Castal Range (Fig. 2) shws a change frm vlcanic arc sediments t cntinental-derived sediments in the lwer Plicene (Chen and Wang, 1988 in Teng, 1990). In additin, in the Western Fthills (Fig. 2), eastward prgrading sediments derived frm Eurasia becme westward prgrading freland basin sediments as the Central Range was rapidly uplifted abve sea level (Chi and Huang, 1981 in Teng, 1990). Sediment supply increased drastically in bth regins during the Plicene (Teng, 1990).

13 6-1.5% +1.5% Taiwan A A' A 30 N 500 Eurasian Slab depth (km) A' 20 N E 130 E 1500 A. B. Fig. 3. Seismic tmgraphic sectin acrss the study area in suthern Taiwan. A lw velcity anmaly appears between the surface and the dwn-ging Suth China Sea slab indicating that the slab is nt attached. Mdified frm Lallemand, 2001.

14 7 There is n syncllisinal sedimentary recrd fr the Central Range as this area was presumably expsed during the earliest stages f cllisin and hence was erding. Radiistpe and zircn/apatite fissin track dating indicate deep burial befre cllisin, fllwed by rapid uplift arund 3 Ma, during peak cllisin (Lin, 1982, in Teng, 1990, and Liu et al., 2001). The basement rcks f the Tanana Cmplex underwent Greenschist facies metamrphism during peak cllisin and have since been exhumed frm apprximately 13 km (Ernst, 1983, Wang et al., 1982, in Teng, 1990). Seismicity varies thrughut the island. Mst fcal mechanisms frm suthern Taiwan are cnsistent with a NW-SE cmpressive stress frm PSP cllisin (Yeh et al., 1991, Rau and Wu, 1998). Fcal mechanisms in the Lngitudinal Valley indicate sinistral slip mtin, which crrespnds with plate mtin (Yeh et al., 1991). At least ne right lateral mechanism is fund in suthern Taiwan, in the Pingtung Valley regin, perhaps indicating extrusin t the suth (Ka and Jian, 2001). Strike-slip faulting ccurs in suthern Taiwan beneath the Central and Fthills Ranges (Ka and Jian, 1991), suth f what has been called the Chuchi Transfer Fault Zne (Lacmbe et al., 2001). This bundary represents a majr change in the character f seismicity in Taiwan. Althugh the nature f this bundary is nt knwn, it may represent the suthern extent f direct arc-cntinent cllisin in Taiwan. 1.2 Suthern Taiwan Gelgy The Taiwan rgenic belt is divided int fur gelgic prvinces (Fig. 2; H, 1986). Frm east t west they are the Castal Range, the Central Range (subdivided int the Tanzania Cmplex and Slate Belt), the Western Fthills Range, and the Castal Plain. The Castal Range is made up f accreted Luzn Vlcanic Arc material verlain by syncllisinal sediments f Late Micene t Pleistcene age (Teng, 1990). The Castal Range is bunded n west by the Lngitudinal Valley Fault (LVF) system, which is thught t be the surface expressin f the Eurasian/PSP bundary (Teng, 1990). The Central Range, bunded n the east by the Lngitudinal Valley Fault, and n

15 8 the west by the Chachu Fault, is divided int tw prvinces, the Tanana Cmplex and the Slate Belt. The Tanana Cmplex represents the metamrphsed eastern Eurasian cntinental basement (Teng, 1990). The metamrphic grade in the Tanana Cmplex increases frm Prehnite-Pumpellyite facies in the west t Greenschist and Blueschist facies in the east. The Slate Belt, subdivided int the Backbne and Hsuehshan ranges, represents the tectnized Cenzic sedimentary east Asian basement cver, and is cmpsed f Palecene t Micene slates, argillites, and quartzites (H, 1986). The Backbne Range t the east cnsists f Ecene t Micene pelitic sediments, which are thught t be mre distal t the cntinental margin than thse f the Hsuehshan Range (Tillman et al., 1992, Lee et al., 1997). The Hsuehshan Range grades frm Prehnite-Pumpellyite facies in the west t Greenschist facies in the east. The Hsuehshan Range, which is cmpsed f predminantly Ecene and Oligcene cntinental margin sediments and bund t the east by the Lishan Fault, may be a large, left-laterally sheared pp-up structure, pssibly and inverted Negene basin frmed during a perid f extensin in the Suth China Sea (Biq, 1989; Clark et al., 1992; Tillman and Byrne, 1995). The range is a structural as well as gegraphic high, being bund t the east and west by yunger units in the Western Fthills and Slate Belt respectively (Clarke et al., 1992). Ging east acrss the Lishan Fault, t the Backbne Range, the metamrphic grade changes back t Prehnite-Pumpellyite facies (Lee et al., 1997). The Western Fthills cnsist f flded and thrust faulted precllisinal cntinental shelf sediments and westward prgrading, syncllisinal clastics that cllected in a rapidly subsiding freland basin (Teng, 1990). Sedimentary units decrease in thickness frm greater than 8000 m in the east t 500 m in the west. Units typically carsen and thicken t the suth where uplift began later than in the nrth (H, 1986). The Western Fthills are separated frm the Slate Belt by the Chachu Fault, the fcus f this study.

16 9 1.3 The Chachu Fault The Chachu Fault is a majr mid-rgen fault between the Western Fthills and Central Range. The main trace f the fault is knwn by several names, e.g. Chuchi, Suantung, Tulungwan, and Hungchun amng them. In the suth it is knwn as the Chachu Fault (Yen and Tien, 1986). In the suth, the fault separates the Slate Belt frm the Pingtung Valley, which has been interpreted as a dwn-drpped Negene basin, filled with alluvial and fluvial material shed frm the Western Fthills and Central Ranges t the nrth (Fig. 2, H, 1986). Initial gravity surveys in suthern Taiwan identified the CCF as a high-angle reverse fault, dipping t the east at arund 75 t 80 degrees (Chiang, 1971). Biq (1989) interprets the fault as an blique wrench fault with cmpnents f bth sinistral and reverse-slip mtin. This assessment is based n the ccurrence f left-lateral splay faults that branch ff frm the main CCF in the nrth and the ccurrence f peaks f relatively high elevatin n the eastern side f the fault with assciated gravity anmalies that indicate crustal thickening. Recent GPS data indicate that the bliquity f mtin changes alng trace f the fault (Wiltschk et al., 2002). Bth vertical and hrizntal mtin assciated with the fault changes alng its strike and lw cnvergence n either side f the fault indicates that it is high angle (Wiltschk et al., 2002). 1.4 Crss Sectins In rder t illustrate the upper crustal structure acrss the CCF, we present three crss sectins frm suthwestern Taiwan. These sectins crss the study area in the nrthern, middle, and suthern regins and were chsen t illustrate the develpment f the rgen frm suth t nrth (Fig. 2). Gelgic maps f the regin prvide surface gelgy frm which we infer sub-surface structure (see figure captins fr details). Each sectin extends t a depth f apprximately 12 t 15 km and ranges 40 t 85 km in length and assumes displacement frm the ESE. These crss sectins cmplement thse

17 10 f Hickman et al. (2002), but ur interpretatin yields less shrtening. We extend the lines int the Slate Belt bth t illustrate ur interpretatin f the CCF gemetry at depth. Stratigraphy is summarized in Figure 4. Fr a mdified gelgic map and sectin lcatins, see Figure 5. Variatins in stratigraphic thickness f units represented in these sectins may ccur where extensinal basement faults have created additinal accmmdatin space prir t cllisin. Hwever, well data are t sparse t dcument these changes. Therefre, units are drawn with unifrm thicknesses thrughut the three crss sectins. We have placed a basal detachment at apprximately 13 km belw sea level t the sutheast rising t 10 km in the nrthwest in all three lines. N seismic r well data cnfirms the lcatin f this detachment. Hwever, this is a minimum depth t accmmdate stratigraphy and structures expsed at the surface Line A-A Line f sectin A-A (Fig. 6) crsses the nrthern extensin f the Nuishan anticline, which is cut by the Lunhu fault and a small splay. The Lunhu fault places the Micene Tangenshan sandstne n tp f the Plicene Pelia shale and accmmdates apprximately 3 km f displacement. Because the surface trace indicates it is lw angle, the Kuhsiali fault is interpreted as a backthrust t the Lunhu fault. We add anther small, blind splay fault in the ftwall f the Lunhu fault t accunt fr west-dipping beds bserved at the surface. We draw this splay flattening at the base f the Chutuchi frmatin and ramping up again t frm a small fault prpagatin anticline at the westernmst extent f the Western Fthills. Stratigraphic ffset indicates displacement n the Chutuchi fault is apprximately 2 km. The Chutuchi fault is shwn t cut up-sectin frm the basal detachment and utilizes east-dipping bedding in the hanging wall anticline east f the Lunhu fault. This interpretatin implies ut f sequence faulting f the Chutuchi fault. T accmmdate stratigraphy seen at the surface, we interpret a large sectin f basement carried by the Pingchi fault. As drawn, the Pingchi fault accmmdates 5 km

18 11 Qt Age Nrth Alluvium Suth Alluvium Terrace Depsits Terrace Depsits Lingku Cnglmerate Lingku Cnglmerate H Pleist. Typical Thickness Variable Variable Variable Lithlgy Peilia Sh. Negene Late Micene Plicene Chutuchi Fm Maupu Sh. Ailiachu Fm. Yenshuikeng Sh. Tangenshan Ss. Changchikeng Fm. Hunghuatzu Fm. Gutingkeng Fm. Nanshihlun Ss. Kaitzulia Sh. Wushan Fm m 500 m m 1200 m Sanmin Sh. >800 m Chachu Fault Middle Micene U. Changshan Mutan Fm. Shihtzuta Slate Belt Ss. >5000 m L. Changshan Chachu Fm. Alluvium Terrace Depsits Cnglmerate Shale Muddy Sandstne Calcareus sandy Shale Fine-grained Sandstne Cal and Shale Lw-grade Slate Fig. 4. Field area stratigraphy. Mdified frm H, 1986, and CPC Map 6, Tainan, Stratigraphy is cmpiled frm CPC gelgic maps f suthwest Taiwan, Kahshiung- Pingtung and Tainan Sheets (1992, 1989), CGS gelgic maps 51 (Chiasien) and 67 (Fanglia) (2000, 1993), and H, Detailed lithlgic descriptins are published in H, 1986.

19 12 Tainan A B B' A' 1 Lunhu 2 Kuhsiali 3 Chutuchi 4 Pingchi 5 Chishan Plicene Pleistcene Liukuei Faults 6 Kachung 7 Chachu 8 Lishan 9 Liukuei 10 Lungchuan 11 Gutingkeng 5 12 Fungshan Micene Kasiung 12 Pre- Micene C C' Kilmeters Fig. 5. Gelgic map and lcatins f crss sectins. Mdified frm H, 1986, and CPC Map 6, Tainan, Stratigraphy is cmpiled frm CPC gelgic maps f suthwest Taiwan, Kahshiung-Pingtung and Tainan Sheets (1992, 1989), CGS gelgic maps 51 (Chiasien) and 67 (Fanglia) (2000, 1993), and H, Detailed lithlgic descriptins are published

20 West A East A' Lunhu F. Kuhsiali F. Chutuchi F. Pingchi F. Chishan F. Lachung F. Chachu F. Lishan F. Fig. 6. Line f Sectin A-A. Crss sectins acrss the Western Fthills and Slate Belt in Suthern Taiwan. Structural and stratagraphic data frm CPC gelgic maps 6 (Tainan) and 7 (Kahshiung-Pingtung), and CGS gelgic maps 67 (Fangli) and 51 (Chiahsien), stratigraphic descriptins in H, 1989, nearby wells, and ne seismic sectin acrss the eastern Pingtung Valley by Chang, N vertical exaggeratin. 13

21 14 f displacement. We draw a small blind thrust fault frm the Pingchi fault int the ftwall t prduce the west-dipping beds bserved at the surface. Several small faults with minr amunts f displacement cut the Pingchi hanging wall blck. These faults are interpreted as splay faults t the Pingchi. The Chishan fault may utilize bedding in the Pingchi hanging wall anticline. This interpretatin is develped further in the crss sectin fr line B-B. The Kachung fault is shwn here as a small splay t the CCF and is perhaps a precursr t the CCF. The CCF is interpreted here as a steeply-dipping reverse fault that cuts up-sectin frm the basal detachment after ramping up frm a deeper prtin f the basement. This gemetry allws fr apprximately 10 km f shrtening in the Western Fthills and exhumes pre-micene basement, which is expsed in the Eastern Central Range. The Lishan fault merges with the CCF suth f line B-B and is shwn here as a backthrust t the CCF. The regin between the CCF and the Lishan fault is the suthern extensin f the Hsuehshan Range. Slate Belt rcks adjacent t the CCF are early- t mid-micene in age (CGS maps 67 and 51) and are brught up frm at least 13 km in rder t prduce the reset Zircn fissin track ages bserved by Liu, 2001 and Fuller, The Slate Belt is nt mapped in detail in the regin thrugh which line A-A passes. Therefre, units are nt differentiated in the crss sectin Line B-B Line B-B (Fig. 7) crsses the Lunhu and Kuhsiali faults 12 km suth f line A-A. In the B-B crss sectin, bth faults are interpreted t dip westward. The Kuhsiali fault, n lnger a backthrust as in line A-A, frms a steep reverse fault. The Lunhu fault places Pleistcene units abve Plicene with apprximately 2 km f displacement, less than in the nrthern, A-A sectin. The small, blind splay fault is still present in the ftwall, but we interpret it t have stepped up-sectin t the Late Plicene Chutuchi frmatin.

22 West B East B' Lunhu F. Kuhsiali F. Chutuchi F. Pingchi F. Chishan F. Liukuei F. Chachu F. Lishan F. Fig. 7. Line f Sectin B-B. Crss sectins acrss the Western Fthills and Slate Belt in Suthern Taiwan. Structural and stratagraphic data frm CPC gelgic maps 6 (Tainan) and 7 (Kahshiung-Pingtung), and CGS gelgic maps 67 (Fangli) and 51 (Chiahsien), stratigraphic descriptins in H, 1989, nearby wells, and ne seismic sectin acrss the eastern Pingtung Valley by Chang, N vertical exaggeratin. 15

23 16 The Chutuchi and Pingchi faults merge apprximately 1 km suth f the line f sectin. Therefre, the Chutuchi fault is, in this interpretatin, a minr splay fault t the Pingchi. By ur interpretatin, the Pingchi fault steepens up-sectin due t subsequent frmatin f and displacement n the Kuhsiali fault. Displacement accmmdated by the Pingchi fault is apprximately 5 km. The Chishan fault accmmdates apprximately 10 km f displacement. We interpret this fault t be an ut f sequence thrust that utilizes east-dipping units in the hanging wall blck f the Pingchi fault t cut up sectin. This line f sectin cuts acrss the Western Fthills and CCF in the lcatin f the restraining bend at Liukuei. Out f sequence thrust faulting may result frm defrmatin assciated with this restraining bend. We interpret smaller faults in the hanging wall blck f the Chishan fault as abandned thrust faults that have since been passively steepened by uplift and blck rtatin f units abve the Chishan fault. The Liukuei fault diverges frm the CCF 10 km nrth f line B-B. T the suth, the fault trends int the Pingtung Valley, where it is buried by Quaternary alluvium. The Liukuei fault is interpreted as a precursr t the CCF that exhumed Slate Belt rcks befre abandnment and burial by Pleistcene gravels. The CCF and Lishan faults are drawn with similar gemetries as in sectin A-A. The basement ramp belw the basal detachment is drawn further east t accmmdate the apprximate 17 km f shrtening in the Western Fthills frm this interpretatin Line C-C There are few surface data alng line C-C (Fig. 8). It was drawn t incrprate a seismic sectin frm Chang, 1971, n the Pingtung Valley and western Slate Belt margin. The line extends t 3500 m and indicates that Plicene strata are absent belw the eastern Pingtung Valley and that the uppermst units, the Lingku cnglmerate and the Changchikung Frmatin, thin ut t the east. Belw the depth f the seismic line, we draw stratigraphy cntinuing nrmally and with the same thicknesses as in the tw nrthern crss sectins. Absence f the Pli-Pleistcene units may indicate cpius

24 Wanlung F. Chachu F. West C East C' Fig. 8. Line f Sectin C-C. Crss sectins acrss the Western Fthills and Slate Belt in Suthern Taiwan. Structural and stratagraphic data frm CPC gelgic maps 6 (Tainan) and 7 (Kahshiung-Pingtung), and CGS gelgic maps 67 (Fangli) and 51 (Chiahsien), stratigraphic descriptins in H, 1989, nearby wells, and ne seismic sectin acrss the eastern Pingtung Valley by Chang, N vertical exaggeratin. 17

25 18 alluvium was being shed frm muntains t the nrth and east. Alternatively, during that time the valley may have been expsed and has since been dwn-drpped. Because the Taiwan rgen is presumably develping suthward, we expect structures alng the suthernmst line f sectin t have less displacement. We draw a nascent, blind thrust fault frm the basal decllement, acrss the Pingtung Valley, where it frms an anticline at the suthernmst extent f the Western Fthills. We draw anther blind thrust t accunt fr thinning strata t the east. The Wanlung thrust, imaged in seismic survey (Chang, 1971) is interpreted t be a shallwer-dipping precursr t the Chachu fault carrying Slate Belt rcks. It dips 45 west while the Chachu fault dips apprximately 75 west. Bth faults are buried by Quaternary sediments.

26 19 2. FABRIC STUDY OF THE SOUTHERN CHAOCHOU FAULT REGION 2.1 Previus Wrk Our purpse in examining rck fabric is t dcument bth past mtin and defrmatin related t the CCF and hw it changes alng strike. The suthern extent f fabric characterizatin in the Slate Belt is the suthern Crss Island highway, which runs E-W acrss the nrthern prtin f the study area. Based n restred vein gemetries and syndepsitinal nrmal faults frm the Slate Belt f central and suthern Taiwan, Tillman et al. (1992) identify three distinct extensinal regimes, which predate cllisinal defrmatin. The directin f extensin prgressively rtates frm NNEdirected t N-directed in the Ecene and Oligcene, and frm N t NW-directed during the in the early Micene. NW extensin cntinued int the middle and late Micene (Tillman et al., 1992). Sme tectnic features f the Taiwan muntain belt may be reactivated extensinal structures (Biq, 1989, Deffntaines et al., 1994). Intensity and cmplexity f defrmatin increase eastward acrss the range. Alng the western bundary, strngly asymmetric flds with shrt and steep t verturned NW limbs and lng planar SE limbs are prminent (Clark et al., 1992). These flds shw dwn-dip stretching lineatins cnsistent with flexural slip flding (Tillman and Byrne, 1995). Fld axes trend NNE t NE. The flds are islclinal t inclined with axial surfaces that dip t the sutheast (Tillman and Byrne, 1995). Pressure shadw fibers in these flds are curved and indicate tp t the west thrusting and nn-caxial simple shear defrmatin (Tillman and Byrne, 1995). Secnd and third phases f flding are distinguished in the eastern prtins f the Backbne Range (Clark et al., 1992). An S 1 slutin cleavage, the character f which varies with lithlgy, is a prminent structural feature f the Slate Belt rcks (Tillman and Byrne, 1995). The S 1 cleavage dips mderately t steeply t the sutheast and is axial planar t flds (Tillman and Byrne, 1995). S 1 is typically parallel t bedding and intersects slickenlines n the flexural fld surfaces. This indicates that the develpment f the slutin cleavage

27 20 ccurred sme time after flding and that the bserved flds were flattened during develpment f S 1 (Tillman and Byrne, 1995). Dwn-dip stretching lineatins f the S 1 fabric are cnsistent with plane-strain defrmatin (Tillman and Byrne, 1995). Additinally, flds and S 1 cleavage are cut by late late-stage strike-slip faults, cnjugate sets that indicate NW-SE shrtening (Tillman and Byrne, 1995). Based n structural relatinships, fabric rientatins, and zircn fissin track annealing ages, Tillman and Byrne, (1996), identify subbelts within the Hsuehshan Range, which indicate that sme thrusting within the Slate Belt may have ccurred ut f sequence frm the traditinal mdel f prgressive, imbrecating thrusts within an rgenic belt. 2.2 Methds Field fabric data were cllected at 187 lcatins alng the mapped trace f the CCF. Mst measurements were taken within 1-2 km f the mapped fault trace. Measurements f fliatin, bedding, fld axes, slip surfaces and slip lineatins are gruped tgether int three regins. The nrthern regin is nrth f the fault bend at Liukuei. The suthern regin is suth f the bend. The middle regin is in the fault bend (Fig. 9). The middle regin is als the lcatin where the Liukuei and Lishan Faults diverge frm the CCF. 2.3 Results In the nrthern regin bedding and fliatin strike NE, nearly parallel t the CCF and dip mderately t the SE; bedding generally dips mre steeply t the SE (Fig. 10). It is difficult t distinguish between riginal bedding and fliatin in the nrthern regin. Orientatins f bedding and fliatin measurements are almst identical and I have decided t cmbine the bedding and fliatin data sets. The equal area plt f cmbined bedding and fliatin data shws tw cncentratins. The strngest dips mderately t

28 ' Chachu Fault Lishan Fault 5 Liukuei Liukuei Fault 66 Chachu Fault 24 Pingtung Valley 22 30' 80 Fanglia Philippine Sea Kilmeters Fig. 9. Digital terrain map f the study area shwing the three regins. Prminent gegraphic and tectnic features are labeled. Sample lcatins and numbers fr XRD analysis are shwn as circles.

29 22 N30E Fliatin and Bedding Fld Axes Slip Surfaces Slip Lineatins a. b. c. d. N=116 N=40 N=46 N=38 e. f. g. h. N=84 N=23 N=13 N=8 CCF i. Far Field Mtin j. k. l. N=145 N=23 N=53 N=42 Relative Plate Mtin N55W, 7-9cm/yr Fig. 10. Field fabric data fr the nrthern, middle, and suthern regins. The schematic t the side shws the lcatins f the regins relative t the CCF. An arrw indicates the relative mtin f the Philippine Sea Plate. Plts are lwer hemisphere equal area steregrams. Cnturing is via mdified Kamb methd with an interval f 2 Sigma. Significance is set at 3. The number f datapints is shwn belw each steregram.

30 23 the SE, and the weaker ne t the NW. Bth strike NE-SW, parallel t the fault. The tw cncentratins in fliatin rientatins prbably crrespnd t the same S 1 and S 2 fabrics described by Tillman and Byrne in the same regin, the weaker, nrth-dipping fliatin being the S 2 crenulatin cleavage and the strnger sutheast-dipping S 1 fabric, a slutin cleavage. Outcrp-scale flds in the nrthern regin are asymmetric, with fld axes plunging gently t the SW. NW-dipping fld limbs are generally shrt and near vertical, whereas SE-dipping limbs are generally lnger and mderately dipping This tp t the NW mtin is cnsistent with bservatins frm previus studies (Tillman and Byrne, 1995, 1996; Clark, 1992). Slip surfaces are rughly parallel t the strng fliatin directin. Slip lineatins (slickenlines and slickenfibers) are cncentrated almst directly dwn-dip f the average slip surface plane. Dwn-dip slip lineatins and fliatinparallel slip surfaces are cnsistent with bservatins frm previus studies in r near the nrthern regin that indicate flding in the Slate Belt was accmmdated by flexural slip alng cmpetent bedding planes and by flexural flw in weaker layers (Tillman and Byrne, 1995). The character f fabrics in the middle regin is significantly different frm that f the areas t the nrth and suth. Cmbined bedding and fliatin measurements shw a single strng preferred rientatin, striking WNW, parallel t the CCF, and dipping gently t the SSW (Fig. 10). It is easier t distinguish between bedding and fliatin in this sectin than t the nrth. Hwever, bedding and fliatin still appear t have nearly identical rientatins. Fld axes, which represent bth flds in bedding and flds in fliatin, are riented apprximately N-S, similar t thse in the Nrth, but in the Middle regin are perpendicular t the strike f fliatin. Slip surface and slip lineatin data are scarce fr this sectin but shw a weak cncentratin parallel t fliatin. Slip lineatins were als few, but shw a weak dwn-dip cncentratin relative t average slip surface rientatin. Tw fliatin directins in the suthern regin are similar t thse in the nrth (Fig. 10). As in the nrthern regin, the strnger fliatin dips tward the SE and the

31 24 weaker ne t the NW. Bth strike NE-SW, but in the suthern regin, due t the fault bend, are blique rather than parallel t the CCF. Slip surfaces are generally parallel t bedding. Hwever, slip lineatins d nt indicate a preferred rientatin f slip, but instead are scattered within the average plane f slip. Many slip surfaces are themselves flded such that the scatter in slip lineatins may indicate either flexural slip r pst-slip flding. Fld axes, like in the middle regin, are perpendicular t fliatin. Hwever, in the suthern regin they trend NW-SE and plunge tward the SE, unlike fld axes in the middle and Nrthern regins (Fig. 10). Fld axes in the suthern regin d nt seem t reflect the expected reginal shrtening directin, NW-SE, nr d they cincide with the expected fld axes directin if the tw different fliatin rientatins are assumed t represent large scale flding. Fr this reasn, the tw directins f fliatin are nt thught t be fld limbs. 2.4 Lithlgy Thin sectins frm rck samples taken alng the fault generally shw greater strain tward the nrth. Rcks in the mst nrtheast crner f the nrthern regin, alng the Suthern Crss-Island highway, cmprise mstly pelitic rck subjected t lw greenschist facies metamrphism. Veins are cncentrated in sandier layers and are typically cmpsed f fibrus quartz and r calcite. Veins are ften sheared alng grain bundaries; particularly where they crss beds f finer grained material (Fig. 11, A and B). Veins crsscut bedding and predate frmatin f fliatin. Thin sectins reveal dmainal spaced cleavage characterized by truncated grains (mstly quartz, but ccasinal micas), and rtated grains (mstly quartz and pyrite frambids) between undulus layers f fine, paque material. Fibrus quartz fringes are cmmn n large grains and display mderate shear rtatin (Fig. 11, E and F). Shear structures indicate tp t the nrthwest shear and are cnsistent with previusly described utcrp-scale

32 25 BDG // FOL SE NW SW NE BDG // FOL BDG // FOL NE SW SW NW SE BDG ~90 t FOL SE NW SE NW SE NW BDG ~23 t FOL BDG // FOL Fig. 11. Phtmicrgraphs shwing typical rck fabric and shear structures frm Slate Belt rck samples, taken frm alng the CCF. A-F are frm the nrthernmst prtin f the nrtherny regin, alng the Suthern Crss-Island Highway. A and B shw CCW sense f shear; C-F shw CW sense f shear. G-J are frm the middle regin ans shw CW sense f shear. K and L are frm the suthern regin. Tp is up in each picture. Gegraphic rientatins are shwn.

33 26 structures. In sme instances, shear structures indicate a nrmal sense f shear (Fig. 11, C and D). These samples are thus interpreted t be fund in verturned fld limbs. Rck lithlgies in the middle regin cmprise similar lithlgies t thse in the nrth, but are generally finer grained. Thin sectins frm samples in the middle regin shw similar slaty cleavage t thse f the nrth. Hwever, shear structures are much less develped and therefre are difficult t interpret (Fig. 11, G, H, I, and J). Rcks in the suth are carser grained and bedding was easier t distinguish frm fliatin than is the case with nrthern and middle units. Slutin cleavage is nt as strng, thugh it is still present (Fig 11, K and L). Shear structures were nt bserved in the suth. Absence f shear structures indicates that there has been less simple shearing in this regin. Micrscale structures indicate a qualitative decrease in strain intensity frm nrth t suth. In rck samples frm all study regins, grains large enugh t identify in thin sectin are predminantly quartz and muscvite, with trace grains f micrcline, calcite, and pyrite. X-ray diffractin (XRD) patterns n seven samples (see Fig. 9 fr sample lcatins) frm alng the CCF indicate that the dminant mineralgies in Slate Belt units are quartz and muscvite mica. Diffractin patterns als indicate that the transitin frm smectite t illite appears t be cmplete thrughut the study area. H, (1986) indicates that the western Slate Belt units are prehnite-pumpellyite facies.

34 27 3. GEOMORPHOLOGY OF THE SOUTHERN CHAOCHOU FAULT A key indicatr f current mtin f the CCF is its tectnic gemrphlgy. We emplyed a 40 m Digital Elevatin Mdel (DEM), frm which we identified twentythree majr drainages that empty ut either nt the Pingtung Plain r int the Lanung River. Stream drainages and drainage divides were manually digitized using the hillshade surface DEM in cnjunctin with the index cntur layer (cntur interval 200 m). Majr drainage basins were further subdivided int 3 rd, 4 th, and 5 th rder basins accrding t Strahler methd (Strahler, 1952). Large, high rder basins can indicate either lng-term stability r weak underlying bedrck. The tw largest, mst mature basins are fund just suth f the bend at Liukuei (Fig. 12). The ccurrence f these tw large basins suth f the fault bend culd indicate that this regin has undergne less uplift in recent times. Hwever, we als see that there are n high, 5 th -rder basins nrth f the fault bend. The size and shape f each basin is cnsidered t indicate its maturity r the resistance f the underlying lithlgy. The larger and mre equant the basin and the lnger its trunk stream, the mre time it has had fr bth lateral and headward ersin (Burbank and Andersn, 2001). Active muntain frnts will have shrt, narrw drainage basins that are clsely spaced, while inactive frnts will have large, rund basins that are als necessarily further apart (Burbank and Andersn, 2001). Spacing between drainages was examined by measuring the distance f the drainage muth where it crsses the muntain frnt, frm the suthern bundary f the field area (apprximately N20 22 ). Basin spacing is relatively clse in the nrthern and middle regins and wider in the suthern regin. Clser spacing indicates higher activity t the nrth (Fig. 13). Basin elngatin rati was measured fr each f the majr drainages alng the frnt by cmparing the lng axis, which is in mst cases rughly perpendicular t the frnt, fllwing a majr stream, t the diameter f a circle having the same area as the given basin (Cannn, 1976).

35 22 30' 28 A ' ' Kilmeters N Basin Size B. Area (km^2) Nrth Middle Suth Frnt Range Basins 5th Order Basins 4th Order Basins 3rd Order Basins Distance (km) Fig. 12. Basin size. A) Lcatin and size f majr range-frnt drainages alng the suthern CCF. B) Drainages are further subdivided and classified via Strahler methd (Strahler, 1952) int 5th, 4th, and 3rd rder basins.

36 29 Distance between basins (km) Basin Spacing Nrth Middle Suth A. Distance (km) Elngatin Rati Nrth Middle Suth B. R e Distance (km) Asymmetry Basin Asymmetry Nrth Middle Suth Distance (km) C. Fig. 13. Drainage basin mrphmetry. Variatins are shwn fr basin spacing, elngatin, and asymmetry alng the CCF. See text fr equatins.

37 30 R e = diameter f circle with the same area as the basin distance between the tw mst distant pints in basin (1) Basin shape will affect the spacing f drainages alng the frnt. Majr basins tend t be mre elngate in the nrthern and middle regins, and mre rund in the suthern regin. Basin elngatin crrelates with basin spacing because mre elngate streams allw fr clser spacing (Fig. 13). Basin asymmetry is determined by fllwing the main trunk stream up until it branches int t lwer-rder streams. Frm there the midline becmes the drainage divide between the tw sub basins. The drainage basin asymmetry factr (A s ) was determined by calculating the area f the basin n the right side f the midline (A r, facing dwn-stream). That area was then calculated as a percentage f the ttal basin area (A t ) as defined in Hare and Gardener (1985): Ê A s =100* Ar ˆ Á (2) Ë At Thse basins fr which the A s is less than 50 percent are tilted relatively t the left, and fr thse which the A s is greater than 50 percent are tilted relatively t the right. Asymmetry must be cmpared amng basins whse trunk streams are all similarly riented (Keller and Pinter, 2002). Streams in the middle regin may be tilted slightly tward the suth (Fig. 13). Strng, sutheast-dipping fliatin may als accunt fr the appearance f suthward tilting. Stream gradient pltted alng the length f the majr trunk streams fr each basin reveals that the verall stream gradient is higher in the nrthern regin (Fig. 14). This is expected as basins in the nrth are generally smaller and at a higher elevatin. Tw streams ccur in the nrthern regins in which the gradient increases very rapidly in the middle f their lengths. The ccurrence f these increases in stream gradient culd indicate that the frntal CCF recently underwent active uplift, the affects f which are prpagating back alng the streams as knickpints. Knickpints are generated where

38 Stream Gradient A 0.15 B Gradient C D Distance frm muntain frnt (km) Fig. 14. Plt f stream gradient vs. distance frm the muntain frnt fr majr drainages. Heavy black lines = nrthern regin streams, heavy white lines = middle regin streams, thin black lines = suthern regin streams. Tw nrthern streams (A and B) shw bvius knickpints in their middle reaches. The tw largest drainages (C and D) have a steeper gradient near their muths than in their middle reaches.

39 32 baselevel lwers faster than the stream can erde, and can prpagate upstream great distances frm their surce, especially in rivers that are incising bedrck (e.g. Ritter et al., 2002). Hwever, that surce may nt necessarily be lcated at the muth f the stream (Ritter et al., 2002). The knickpints bserved in this study may als indicate that the streams crss ver an active subsidiary t the CCF. The gradient prfile fr the tw largest drainages, ne f which empties int a large entrenched alluvial fan, is cncave upward and indicates the stream is nt at equilibrium with uplift and/r climate cnditins. Uplift r flding at the muths f these drainages, r a rapid increase in sediment supply culd prduce a cncave up prfile. Increased uplift at the muth, des nt favr alluvial fan entrenchment, which is bserved in this area (discussed belw). Sediment supply naturally increases as a functin f upstream basin area. As these are by far the largest basins in the study area, their size may accunt fr the abundance f sediment. Other catastrphic events, such as heavy rainfall that triggers mudslides r stream piracy at the headwaters culd accunt fr increased sediment fluxes at the muths f these drainages. Alternatively, rapid uplift and steepening in the interir f the range wuld increase stream pwer and therefre sediment supply. It is mre likely that increased sediment supply is respnsible fr the bserved cncave up gradient prfiles. Hwever, further study is needed t determine the factrs cntrlling this increase. The large entrenched alluvial fan fund at the muth f nrthernmst f the tw largest drainage basins may indicate either a lessening f tectnic uplift, r a Hlcene climate change frm dry t mre humid cnditins. Entrenched alluvial fans are characteristic f a mderately active muntain frnt (e.g. Bull and McFadden, 1977). Stream incisin ften implies a change in lcal r reginal base level. The muntain belt may experience uplift and thus fster rapid dwn cutting f a stream by steepening the channel and increasing stream pwer. Cntrary t these bservatins, entrenched alluvial fans imply a lessening f uplift thrugh time, particularly uplift at the muntain frnt. As uplift slws t belw the rate f dwncutting, depsitin migrates frm the apex f the alluvial fan utward and entrenchment ccurs as the stream equilibrates (Bull, 1977).

40 33 In additin t tectnic cntrls, alluvial fan entrenchment may als be initiated by sea level fluctuatins, climate change, r changing sediment supply. Marine regressin, such as that bserved since 8-6 ka, lwers base level and favrs incisin. Climate change frm humid t dry cnditins decreases vegetatin, thereby increasing weathering n slpes. This leads t increased sediment supply and aggradatin f alluvial depsits. Climate change frm dry t humid cnditins has the ppsite effect. Mre vegetatin decreases surface weathering and sediment supply while increasing stream discharge, thereby prmting ersin f alluvial depsits. Recent studies f pllen spres frm suthern Taiwan indicate dryer cnditins may have ccurred existed befre 13 and 10 ka, and have subsequently becme mre humid (Sung et al., 1997). The valley flr width t valley height rati (Bull and McFadden, 1977), V f = V fw (E ld - E sc ) + (E rd - E sc ) 2 (3) cmpares the width f a stream valley t its height. Small ratis indicate narrw valleys with high walls, whereas high ratis indicate brad stream valleys whse width is significantly greater than the valley height. These measurements are made alng frnt parallel transects f the stream valleys between 2 and 5 km east f the muntain frnt using the prfile tl in ArcGIS t create the valley prfile. Alng this transect, the elevatins f the stream (E sc ), the left and right drainage divides (E ld, E rd ), and the valley flr width (V fw ) were measured. Smaller drainages have less stream pwer with which t dwncut. As a result, smaller streams will tend t have a lwer Vf value (meaning a strnger V-shape rather than a brad flat valley) than will larger streams perating in the same tectnic regime. The nrthern and middle lengths f the muntain frnt are characterized by narrw, steep valleys, and indicate rapid recent uplift alng the frnt (Fig. 15 A). Larger ratis in the suth indicate brad valleys that have had time t incise the muntain frnt as well as erding laterally.

41 34 Muntain Frnt Sinusity (S mf ) is used as a prxy fr muntain frnt activity. The muntain frnt was described by manually digitizing the slpe break, set where slpe increased/varied by ne standard deviatin acrss slpe f the Lanung River valley and Pingtung Valley and int the Slate Belt. T determine muntain frnt sinusity (S mf), the actual ttal length (L t ) f the frnt and the straight-line length (L s r distance made gd, D MG ) were calculated (Dan Rathert, persnal cmmunicatin). Sinusity: S mf = L mf L s = L t D MG (4) was then calculated with the ttal length (r length f muntain frnt, L mf ) and the D MG (Bull and McFadden, 1977). Using this equatin, the lnger the ttal length f the muntain frnt is relative t the straight-line length, the greater the sinusity. A high sinusity is thught t be related t, 1) a less active frnt fr which stream ersin and embayment ccurs faster than uplift alng the fault, r 2) t the presence f a weaker lithlgy in which dwncutting and ersin ccurs mre quickly than in a mre resistance bedrck (Bull and McFadden, 1977; amng many thers). Sinusity is expected t be higher where basins are clsely spaced. Narrw, elngate basins dente rapid uplift and subsequent dwncutting f the stream, and allw fr clser spaced drainages and increased sinusity. This may give the appearance f slwer uplift rates alng the frnt. The muntain frnt was split int segments f equal length t evaluate changes in sinusity alng the muntain frnt. Length intervals f apprximately five, ten, and twenty kilmeters were used t assess the effect f line segment length n sinusity (Fig. 15 B). The different line lengths yield similar trends. Hwever, the shrter segment lengths are mre sensitive t the lcal effects f stream drainages alng the muntain frnt. Muntain Frnt Sinusity (S mf ) alng the CCF is much lwer nrth f the fault bend at Liukuei and is lwest just suth f the bend. Sinusity values crrelate well with

42 35 valley flr width/height ratis f stream valley prfiles and indicate less activity alng the frnt t the suth (Fig. 15). Taken tgether, the gemrphlgical measures indicate that the nrthern and middle regins have been mre recently active than the suthern regin. Basins in the nrth tend t be smaller and f a lwer rder. The mrphlgy f nrthern and middle regin basins is mre elngate and they are clser spaced. Stream gradients are higher in the nrthern and middle regins. In additin t stream and basin parameters, muntain frnt mrphlgy als suggests mre recent activity in the nrth and middle regins. Values f bth V f and S mf are lwest in the nrth, particularly at the fault bend near Liukuei.

43 Valley Flr Width-Height Rati Nrth Middle Suth A V f Distance (km) Muntain Frnt Sinusity Sinusity (ttal lenght/straight line length) Nrth Middle Suth 20 km segments 10 km segments 5 km segments B Distance (km) Fig. 15. Variatins in muntain frnt parameters. Vf (A.) and Smf (B.) are shwn vs. distance. Lwer values fr bth parameters indicate higher activity in the nrthern and middle regins.

44 37 4. DISCUSSION Our data indicate that activity alng the CCF increases t the nrth and is cnsistent with a suthward-develping rgen. The nrthern and middle regins are active and have been fr sme time, while the suthern regin appears t be less active and at a yunger stage f develpment relative t the nrth. The nrthern and middle regins have the highest current uplift rates based n GPS data. Gemrphic parameters indicate that the nrthern and middle regins have undergne recent uplift as indicated by elngate, clsely spaced basins, steep stream gradients, high S mf, and lw V f values (Figs. 13, 14, and 15). Yunger apatite and zircn fissin track ages indicate that rcks in the nrthern and middle regins are exhumed frm greater depth than thse in the suth and that the rgen is mre mature in the nrth. Quartz fringes in the nrth shw greater rtatin than thse in the middle regin (Fig. 11). Fabric rientatins in the nrthern regin suggest NW-SE-directed shrtening that are clinear with the far-field stress directin inferred frm the NW mtin f the Philippine Sea Plate (Fig. 10). Inferred shrtening rientatins fr brittle and ductile fabrics d nt cincide in the middle and suthern regins. Fabric rientatins change abruptly in the middle regin, perhaps due t fault blck rtatin. Tw faults diverge frm the CCF in this regin and may be respnsible fr late-stage rerientatin f preexisting fabric. Fabric rientatins in the suthern regin are different frm thse in the nrthern and middle regins, thugh fliatin rientatins are similar t thse in the nrth. The suthern regin is tectnically yunger than the nrth and represents a shallwer prtin f the accretinary wedge. Because brittle and ductile fabrics d nt indicate the same directin f cmpressive stress, as is the case in the middle regin, this may indicate either that the stress state changes with depth r that rcks in the western Slate Belt are rtated as they are brught up frm depth. The actual transitin between the different fabric rientatin seen thrughut the different study regins ccurs

45 38 abruptly between the nrthern and middle regins, at apprximately 23.1 N latitude, and is mre gradatinal between the middle and suthern regins. GPS surface mtin data shw variatin in current surface defrmatin alng the CCF. Present mtin alng the CCF is nt well cnstrained because GPS data are sparse within the Slate Belt and Central Range. Hwever, it is clear that there is sme deflectin f surface mtin vectrs acrss the fault. When surface mtins are calculated relative t the Philippine Sea Plate, vectrs rtate suthward traversing the island frm west t east (Fig. 16, Wiltschk et al., 2002). Nrth f the fault bend, mtin vectrs trend tward the sutheast. Suth f the bend they trend due almst due suth. The magnitude f vertical mtin als decreases acrss the fault. When surface mtins are calculated relative t sites alng the CCF, mtin vectrs east f the fault trend tward the NW, whereas east f the fault velcity vectrs trend mstly due west. Vertical velcity data frm GPS indicate that uplift rates vary alng the fault. Greatest uplift rates ccur in the nrthern regin n bth sides f the CCF. Uplift is negligible t the suth. In additin t current defrmatin, lng term uplift frm apatite/zircn fissin track ages als indicates mre active uplift t the nrth. Liu et al. (2001) bserved that zircn and apatite fissin track ages generally increase frm east t west, traversing the island, and indicate that greatest uplift ccurs at the cllisinal bundary and decreases with increasing distance. Liu et al (2001) bserved that the ccurrence f reset units decreases t the suth where the rgen is presumed t be yunger. Fuller (2003) fund that yung reset ages becme similar t the nrth, indicating that the rgen has achieved steady state apprximately 80 t 130 km frm the suthern tip f the island. Fuller (2003) ntes a repeat in the prgressin frm reset t unrest ages acrss the Hsuehshan Range. This range may expse rcks frm mre shallw prtins f the defrmatin wedge. Reset ages indicate higher uplift rates in central Taiwan, well west f the plate bundary. The Peikang High basement anmaly may influence uplift rates in this regin by acting as an indentr (Liu et al., 2001).

46 39 A B 8 cm/yr 5 cm/yr Vertical Surface Velcity (m/yr) Vertical Surface Velcity (m/yr) * * * * Kilmeters Kilmeters Fig. 16. GPS surface mtin data fr suthern Taiwan. Cnturs represent vertical velcity. Black arrws represent hrizntal mtin velcity vectrs. A) Surface defrmatin rates relative t the Philippine Sea Plate. B) Defrmatin rates relative t GPS sites alng the CCF (marked with * s).

47 40 In the nrthern regin, fissin track ages frm samples adjacent t the fault are un-reset and indicate that the rck is uplifted frm shallwer depth than thse directly east and suth (Fig. 17). Hw far these lder, un-reset ages extend eastward int the Slate Belt is nt knwn. The suthern-mst tip f the Hsuehshan Range extends dwn int the nrthern regin and may be respnsible fr the regin f unreset units. Fissin track ages adjacent t the fault in the regin f the bend are yung and suggest uplift and exhumatin frm a deeper prtin f the rgen and perhaps mre rapid uplift alng this segment. This culd be due t the influence f the Peikang High indentr r the presence f the restraining bend, r pssibly bth as the Peikang High may be respnsible fr the develpment f the restraining bend (Biq, 1989). Mving suthward, fissin track ages change frm yunger, reset ages t intermediate ages tward the suth (Fig. 17). Fissin track ages in the suthern regin indicate unreset strata. The yunger ages in the suthernmst Hengchun Peninsula are interpreted t have been reset upn deep burial within the accretinary wedge rather than exhumed, basement material (Fuller, 2003). Fissin track ages tell a slightly different stry than the GPS surface velcity data. While the nrthern regin appears t be mst active tday, fissin track data indicate that the middle regin, where the lwest track ages are fund directly adjacent t the fault, has seen the mst uplift. Bth current and lng-term data suggest that the suthern CCF has experienced slwer uplift than regins t the nrth. Seismic data further supprt a change in the amunt and style f active defrmatin in suthern Taiwan. Beneath suthern Taiwan, the Chuchi Transfer Fault Zne (CTFZ) cuts acrss the island at high angle t the reginal structural trend f the island (Fig. 1, Lacmbe et al., 2001, Lallemand et al., 2001). The CTFZ, recgnized primarily by seismicity, is thught t be a precllisinal basement transfrm fault that has since been reactivated, perhaps by cllisin with the Peikang High (Lacmbe et al., 2001). Suth f the CTFZ, the number f seismic events falls ff sharply (Lacmbe et al., 2001, Teng et al., 2000).

48 ' Ma Ma 4 Ma 8 Ma 12 Ma 20 Ma 36 Ma Liukuei Ma 8 Ma 4 Ma Ma 4 Ma 22 30' Fissn Track age cnturs (Ma) 4 Ma Kilmeters 8 Ma Sample Lcatins Reset Partially Reset Un-reset Fig. 17. Zircn/Apatite fissin track ages. Ages are cntured and sample lcatins are indicated with clred circles. As in previus studies, samples are designated as ttally reset, unrest, and partially reset when they are significantly yunger than the stratigraphic age f the hst rck (<6 Ma, in Taiwan), significantly lder than the hst (>90), r intermediate between thse tw end members ( Ma), respectively (Liu et al., 2001, Fuller, 2003, Willet et al., 2003).Nte the yungest samples are fund near Liukuei. Ages increase t the suth. Data frm Fuller, 2003.