Mag netic Sur vey of Tai wan and Its Pre lim i nary In ter pre ta tions

netic anom a lies lie over a ma jor part of the moun tain ranges, ex cept in the south ern seg ment of the Cen tral Range. Ac cord ing to the am pli tude of mag netic anom a lies and their cov er age, this im plies that a broad ex tru sive ultramafite body could ex ist be neath the south ern segment of the Central Range.


IN TRO DUC TION
Mag netic anom a lies are the re sult of con trasts in magnetization due to sus cep ti bil ity con trasts in crustal rocks of dif fer ent mag netic prop er ties. Com pre hen sive mag netic anomaly maps are es sen tial for a com plete un der stand ing of regional tec ton ics and geo log i cal char ac ter is tics.
Mag netic anom aly maps of Tai wan have been constructed pre vi ously by the Chi nese Pe tro leum Cor po ra tion for oil ex plo ra tion (Hsieh and Hu 1972;Chang and Hu 1981). In gen eral, their data, how ever, are re stricted to poten tial oil-re serve ar eas, mostly over the west ern plains region. In ad di tion, Hsu et al. (1998) com piled both land and ma rine mag netic data to il lus trate sev eral new in sights into the Tai wan-Luzon re gion. Wang et al. (2002) used this magnetic ev i dence to de scribe the for ma tion of Tai wan as a sol itary wave along the Eur asian con ti nen tal plate mar gin. How-ever, none of these sur veys pro vided much data on the moun tain ous re gions of Tai wan. Con se quently, un til a recent com pre hen sive mag netic anom aly sur vey, Tai wan's mag netic data was in com plete, es pe cially in the in ac cessible moun tain ous re gions.
The com pre hen sive mag netic sur vey of the whole is land was con ducted from July 2003 to Feb ru ary 2004 via a joint ef fort be tween the In sti tute of Geo phys ics, Na tional Cen tral Uni ver sity and the In sti tute of Earth Sci ences, Ac a de mia Sinica. Much thought ful plan ning re gard ing the uti li za tion of in stru ments, man power, and sur vey routes was re quired to re duce the ef fects of geo mag netic sec u lar vari a tions. All mag netic points were sur veyed with an av er age spac ing of 2 km along high ways, by ways, and moun tain ous trails (Fig. 1). This pa per de scribes the mag netic sur vey and construc tion of the mag netic anom aly map based on these new mag netic data. In ad di tion, pre lim i nary in ter pre ta tions of the tec tonic sig nif i cance of these maps are also dis cussed. Terr. Atmos. Ocean. Sci., Vol. 20, No. 2, 309-314, April 2009doi: 10.3319/TAO.2008

GE OL OGY AND TEC TONIC SET TING OF TAI WAN
Tai wan is the re sult of com pli cated in ter ac tion be tween two con verg ing plates, the Eur asian plate to the north west and the Phil ip pine Sea plate to the south east. The complications arise due to a change in subduction po lar ity in the vicinity of Tai wan and the fact that the North Luzon arc, on the Phil ip pine Sea plate, in ter sected the Eur asia con ti nen tal mar gin (Fig. 2) fol low ing the subduction of the oce anic and tran si tional litho sphere of the Eur asian plate. The moun tains in Tai wan are very young, geo log i cally speak ing, formed as a re sult of the col li sion be tween an is land arc sys tem and the Asian con ti nen tal mar gin (Wu 1978;Ho 1982;Tsai 1986).
The geo log i cal struc tures of Tai wan trend mainly in a NNE-SSW di rec tion as shown in the sim pli fied geo log i cal map of Fig. 2. These trends are par al lel to the main top ographic trends. The sur face ge ol ogy of Tai wan is dom i nated by Ter tiary rocks, ex cept on the east side of the Cen tral Range, where pre-Ter tiary meta mor phic com plexes are exposed.
East of the meta mor phic com plex is the Coastal Range, where Mio cene and youn ger rocks rep re sent ing a for mer is -land arc are found. These rocks com posed of andesitic volca nic units are thrust up along a se ries of en ech e lon faults, and these thrust blocks were suc ces sively accreted to the island, be gin ning around 4 -6 mybp (Lee et al. 1991). The Lon gi tu di nal Val ley (LV) sep a rates the Coastal Range from the Cen tral Range to the west. The LV is con sid ered to be the su ture that jux ta poses older con ti nen tal rocks and young island arc ma te ri als.
West of the pre-Ter tiary meta mor phic com plex are the main Cen tral Range, the Foot hills, and the Coastal Plain. In north ern Tai wan, the Cen tral Range is com posed of two ranges, the Back bone Range in the east and the Hsuehshan Range in the west, but the south ern Cen tral Range is a single range. The Back bone Range is com posed mainly of slates, but in the Hsu eh shan Range al ter nat ing sand stone and shale lay ers dom i nate. Most of the older Ter tiary rocks were de rived from the Chi nese main land, but since the Late Plio cene, sed i ments in the west ern Tai wan Ba sin have come from the Cen tral Range (Chou 1973), sig ni fy ing the rise of the Cen tral Range at that time. The Coastal Plain of west ern Tai wan is com posed of Qua ter nary al lu vial de pos its, and the Neo gene strata un der neath are gently folded and thin to the west. The Peikang high, where the pre-Ter tiary base ment is at the shal low est in west ern Tai wan, is a nat u ral di vide between north and south Tai wan. The other prom i nent basement is called the Kuanying high, which un der lies the north - 310 Yen et al.  west ern off shore area (Sun and Hsu 1991). An ad di tional fea ture is the Tatun andesitic vol cano group lo cated at the north ern tip of the is land. It is be lieved to be as so ci ated with the subduction of the Phil ip pine Sea plate (Chen 1975).

MAG NETIC SUR VEY AND RE DUC TIONS
A new geo mag netic net work of twelve sta tions equipped with con tin u ous re cord ing sys tems (GSM90F with 0.1 nT sen si tiv ity and 1 min. sam pling rate) was es tab lished by the In sti tute of Geo phys ics, Na tional Cen tral Uni ver sity ). All the mag netic in stru ments were checked si multa neously at the Lunping (LP) sta tion, an old ref er ence station of the is land-wide geo mag netic net work pre vi ously deployed. Six geo mag netic per ma nent sta tions ( Fig. 1) from the new geo mag netic net work were cho sen to mon i tor di urnal vari a tions in the earth's mag netic field. Of these sta tions, the Tsengwen (TW) sta tion is lo cated in west ern Tai wan while the other five sta tions: Neicheng (NC), Yuli (YL), Taitung (TT), Hengchun (HC), and Pingtung (PT) are distributed over east ern and south ern Tai wan.
Ten por ta ble pro ton pre ces sion mag ne tom e ters (including Geometrics mod els G-826/G-856 with sen si tiv ity 1/0.1 nT, re spec tively) were used in the is land-wide magnetic sur veys. These in stru ments were cal i brated at the TW sta tion prior to de ploy ment to cor rect any sys tem atic vari ation in each in stru ment.
The Cen tral Range of Tai wan, reach ing nearly 4000 m in el e va tion at its high est point, is an area where few magnetic mea sure ments were taken prior to 2002 (Hsu et al. 1998;Wang et al. 2002). To achieve an im proved over all understanding of the tec ton ics of Tai wan, es pe cially in the more in ac ces si ble moun tain ous re gions, an is land-wide mag netic sur vey was ini ti ated in July 2003 and com pleted in Feb ru ary 2004. In to tal, 6063 mag netic points were surveyed (Fig. 1). Ef forts were made to pro vide as uni form coverage in the moun tain ranges as phys i cally pos si ble.
Each mag netic point was cho sen af ter care ful in spec tion of the sur round ing area. Ba si cally, points were se lected far away from pop u lated ar eas, all vis i ble iron ob jects and power lines to avoid un wanted or man made "noises" and interference. To en sure mea sure ment re li abil ity, five readings of to tal mag netic in ten sity were mea sured over a few min utes at each mag netic point. If the stan dard de vi a tion among read ings was less than 10 nT, which is con sid ered the di ur nal vari a tions of Tai wan (Yeh et al. 1981;Chen et al. 2004), the ob served read ings were av er aged as the to tal mag netic in ten sity of the given point. The lo ca tion and el eva tion of the mag netic points and the ob served time were deter mined by a por ta ble GPS ac cu rate enough for this largescale sur vey.
Di ur nal vari a tion is due to the part of the earth's magnetic field orig i nat ing in the ion o sphere. At any point on the earth's sur face the ex ter nal field var ies with a range of ampli tude of about 10 -30 nT dur ing the day (Chap man and Bartels 1940) as the earth ro tates be neath dif fer ent parts of the ion o sphere. The mag ni tude of the di ur nal vari a tion depends on the lat i tude at which it is ob served. For the Tai wan re gion, the mag ni tude of di ur nal vari a tion is 10 -50 nT ). Di ur nal vari a tion may be cor rected by install ing a con stantly re cord ing mag ne tom e ter at a fixed base sta tion within the sur vey area gen er ally. Re cords from the geo mag netic per ma nent sta tion (Fig. 1) near est the measur ing point can be used to cor rect the di ur nal vari a tions.
In re gional stud ies, cor rec tions for lat i tude and lon gitude are in her ent in the ref er ence field that is sub tracted. The In ter na tional Geo mag netic Ref er ence Field (IGRF), for the field of in ter nal or i gin and its sec u lar vari a tion (Regan and Cain 1975;Barton 1997), was pro mul gated by the In ter national As so ci a tion of Geo mag ne tism and Aeronomy (IAGA) Com mis sion 2. A global model of the ref er ence field is calcu lated by: (1) Here, V is the the o ret i cal earth's mag netic in ten sity by IGRF; R is the earth's av er age ra dius; r is the dis tance from the mea sur ing point to the earth's cen ter; g and h are the Gauss co ef fi cients of de gree n and or der m; f is the lon gitude; q is the geo cen tric co-lat i tude; and P(cos q) are called Legendre poly no mi als (m = 0). In this study, the spher i cal har monic co ef fi cients up to n = 13 are based on IGRF version 9 to cal cu late the nor mal mag netic value (http://www .ngdc.noaa.gov/IAGA/vmod/igrf.html, 2003). The ver ti cal gra di ent clearly de pends on the lat i tude of the mea sur ing point. The el e va tion cor rec tion, es pe cially in ar eas of rugged to pog ra phy, must be con sid ered to be about 0.023 nT m -1 for Tai wan (Chen 2005).

MAG NETIC ANOM ALY MAP
Af ter cor rec tions, a mag netic anom aly map (Fig. 3) was con structed from the newly col lected data. This map dif fers sig nif i cantly from pre vi ous maps, which were based on insuf fi cient data from the Cen tral Range. The map is gen er ally more com plex and vari a tions in field more er ratic and lo calized than the grav ity map (Yen et al. 1998). As Fig. 3 shows, the range of the mag netic anom aly dis trib utes be tween -400 400 nT over Tai wan. Anom a lies are much higher in the south than in the north; and those of the west ern plains area are higher than those in the moun tain ranges.
Al though such an in ter pre ta tion of the mag netic anomaly dis tri bu tion is still far from unique, it does pro vide several in ter est ing and sig nif i cant in sights about Tai wan within the new map. A high-fre quency, pos i tive, mag netic anom aly trending NE-SW man i fests as a prom i nent (Zone A of Fig. 3), con tin u ous high from the Peikang base ment high, ex tend ing as far as cen tral Tai wan (around 121°E and 24°N). It is well known that an anom a lous area over the Coastal Plain is as so ci ated with the Peikang high. This anom aly belt re lates to the shal low ness of the Cre ta ceous base ment, something that is well doc u mented in drill ing data (Tang 1977). This pos i tive mag netic anom aly belt also ex tends south westward to the Penghu is lands (Hsu et al. 1998), formed by extru sive bas alts with strong thermoremanent mag ne ti za tion (Gao and Huang 1994). Fur ther north east of the Peikang base ment high, one would ex pect the mag netic anom aly to de crease, since low-grade meta mor phosed pre-Neo gene sand stone and slate with low mag netic sus cep ti bil ity, are exposed, but in fact, a high-fre quency anom aly pres ents around the mid dle part of the West ern Foot hills and Hsuehshan Range. This some what in con gru ent re sult is also found on the Bouguer anom aly dis tri bu tion of this area (Yen et al. 1995). As a re sult of this find ing, the tec tonic im pli ca tions for cen tral Tai wan and its ac tual mech a nism will be in vestigated fur ther us ing seis mic ex per i ments. An other highfrequency anom aly is also found in south west ern Tai wan (Zone B of Fig. 3), an area filled with nat u ral greigite/ pyrrhotite-bear ing sed i ments (Horng et al. 1992;Torii et al. 1996).
Neg a tive anom a lies cover west cen tral Tai wan, over Ter tiary and Qua ter nary sed i men tary bas ins. Two mi nor mag netic anom a lies ap pear on the north ern ex trem ity of the is land. One is lo cated over the Kuanying base ment high, a shal low pre-Ter tiary base ment char ac ter ized by tec tonic stabil ity. The other is over the Tatun Vol cano Group, cov ered with andesitic lavas and pyroclastics. In ad di tion, a mag netic anom aly is found in the Coastal Range where the tele scoped Luzon arc is com posed of young ig ne ous ma te ri als.
The smooth and gen tle mag netic anom a lies over a ma jor part of the Back bone Range and the Hsuehshan Range in di cate that the meta mor phic rocks of those re gions are com posed of the low-sus cep ti bil ity ma te ri als, such as slates, mar bles, am phibo lites and quartzites. It is in ter est ing to note that the south ern seg ment of the Cen tral Range (Zone C of Fig. 3), an area of roughly 50 ´ 50 km 2 , ex hib its a con spic u ous mag netic anom aly with about a 500-nT anom a lous range. This un ex pected magnetic anom aly seems to con tra dict an te ced ent un der stand ings.
In the past, few geo phys i cal and geo log i cal ob ser va tions of the south ern seg ment of the Cen tral Range were re ported due to ex tremely dif fi cult ac ces si bil ity. There fore, the over all tec tonic struc ture of the south ern seg ment of the Cen tral Range re mains ob scure and the sub ject of wide de bate. The heat flow map in Tai wan is ba si cally con structed ac cord ing to mea sured heat flow near the sur face, mostly over the Coastal Plain and the West ern Foot hills (Lee and Cheng 1986). The heat flow in creases from west to east, reach ing its peak over 200 mW m -2 in the Cen tral Range, par tic u larly be tween 23 and 24°N. One of the most sig nif i cant fea tures is a per sis tent aseismic area, more or less the same as the mag netic anom aly re gion. This lack of seis mic ity here has been attrib ut ed to enhanced duc til ity of rocks in the Central Range, an im por tant pa ram e ter in the geodynamics of oro g eny (Wu et al. 1997). The higher tem per a tures pro duced by the ex humed crust exposed on the south ern seg ment of the Cen tral Range are consis tent with the higher heat flow ob served on the sur face and the aseismic zone within the up per crust (Lin 2000). The tectonic com pres sion, es pe cially in the Cen tral Range, involves the litho sphere and is char ac ter ized by a strong co herent de for ma tion of the up per man tle and the crust, based on ex am ined shear wave split ting in SKS phases from a large event re corded by a tem po rary ar ray across south ern Taiwan (Huang et al. 2006).
In ad di tion, on geo logic and tec tonic maps of Tai wan, the larg est ex po sure of ultramafite (N-S trending, 30 km in length and 2 km wide) is lo cated along the bound ary be tween Fig. 3. Mag netic anom aly map of Tai wan with a con tour of 100 nT. Inter est ingly, the south ern seg ment of the Cen tral Range is in a con spic uous mag netic anom aly with a 500-nT anom a lous range, over an area of roughly 50 ´ 50 km 2 . (A re gion: from the Peikang base ment high, ending in cen tral Tai wan; B re gion: south west ern Tai wan; C re gion: the south ern seg ment of the Cen tral Range). the base ment com plex and the slate belt (Ho 1982). Al though mag netic sus cep ti bil ity can have a wide range of val ues in rocks, the sus cep ti bil ity of igneous rock is gen er ally larger than that of meta morphic and sed i men tary rocks (Langel and Hinze 1988). Con sid er ing the am pli tude of the mag netic anom a lies and their cov er age, we pro pose a bold con jec ture that a broad ex tru sive ultramafite body prob a bly ex ists un derneath the south ern seg ment of the Cen tral Range. More over, this new as ser tion could pro vide im por tant in for ma tion for mean ing ful plan ning of large-scale geo phys i cal and geo log ical sur veys in in ac ces si ble moun tain ous re gions. It is pos si ble that the pre vi ous con cept of the orog eny of Tai wan, i.e., an alter nat ing pro cess be tween the Phil ip pine Sea and Eur asia Plates, might be sup planted as a re sult of better and more com pre hen sive data.

CON CLU SIONS
The newly con structed mag netic anom aly map of Taiwan pro vides in for ma tion con cern ing geo log i cal struc tures not pre vi ously avail able, es pe cially in the moun tain ranges. The map shows the range of mag netic anom a lies mostly distrib ut ing be tween -400 to 400 nT in Tai wan. The ano m a lies in the south ern part of Tai wan are much higher than that of in the north ern part whilst anom a lies in the west ern plains area are higher than those of the moun tain ous ranges.
A high-fre quency, pos i tive mag netic anom aly trending NE-SW man i fests as a prom i nent, con tin u ous high from the Peikang base ment high, ex tend ing as far as cen tral Tai wan. An other high-fre quency anom aly is found in south west ern Tai wan. Over a ma jor part of the Back bone Range and the Hsuehshan Range shows smooth and gen tle mag netic anomalies. Mag netic anom aly dis tri bu tions of the moun tain ranges re flect the meta mor phic rocks of those re gions, com posed of low-sus cep ti bil ity ma te ri als. In ter est ingly, the south ern seg ment of the Cen tral Range is in a con spic u ous mag netic anom aly with about a 500-nT anom a lous range, over an area of roughly 50 ´ 50 km 2 . Ac cord ing to the amplitude of mag netic anom a lies and their cov er age, this find ing im plies that a broad ex tru sive ultramafite body probably ex ists un der neath the south ern seg ment of the Central Range. This re sult clearly re quires fur ther study, which we will un der take us ing seis mic ex per i ments to better understand the mech a nisms and the na ture of the mag netic anomaly.

Ac knowl edge ments
We are in debted to Ta-Liang Teng, Bor-Shouh Huang, Chau-Huei Chen, Chien-Hsin Chang, and many field-work ers. The au thors ex press their sin cere ap pre ci a tion to Chao-Hsing Lee, Lun-Tao Tong, and an anon y mous re viewer for crit i cal re views and ex cel lent sugges tions on this manu script. This study was spon sored by the Min is try of Ed u ca tion Grant un der the iSTEP pro ject at the Na tional Cen tral Uni ver sity. This re search was par tially sup ported by the In sti tute of Earth Sci ences, Ac a de mia Sinica and the Tai wan Earth quake Re search Cen ter (TEC) funded through Na tional Sci ence Coun cil. The TEC contribution num ber for this ar ti cle is 00031.