Moho Depth De rived from Grav ity Data in the Tai wan Strait Area

Terr. Atmos. Ocean. Sci., Vol. 21, No. 2, 235-241, April 2010 doi: 10.3319/TAO.2009.03.05.01(T) Moho Depth De rived from Grav ity Data in the Tai wan Strait Area Hsien-Hsiang Hsieh, Horng-Yuan Yen *, and Min-Hung Shih In sti tute of Geo phys ics, Na tional Cen tral Uni ver sity, Jhongli, Tai wan, ROC Re ceived 22 July 2008, ac cepted 5 March 2009 AB STRACT We have con structed a re gional Bouguer grav ity anom aly map us ing ma rine and land data from Tai wan and the Chi nese prov ince of Fuchien, as well as SEASAT altimetry-de rived grav ity data for the Tai wan Strait and its sur round ing area. The map shows isogals trending gen er ally in a NE-SW di rec tion, con form ing with the over all shal lower geo log i cal strike of the strait. Re mov ing grav ity ef fects gen er ated by the wa ter layer and seafloor to pog ra phy, the re gional Bouguer grav ity anom aly is ob tained, re flect ing the subsurface struc ture. Moho depth is then com puted by the Parker-Oldenburg it er a tive method from the re gional Bouguer grav ity data set. Over the strait area, the ge om e try of the Moho re lief is smooth with an av er age depth of about 30 km, ex cept for the Penghu up lift. Moho depth is shal lower in the Tai wan Strait and thick ens to ward both sides of the strait. The rel a tively shal lower Moho depth, reach ing up to 28 km, is con vex up ward un der neath the Penghu uplift. Key words: Grav ity, Moho depth, Tai wan Strait Ci ta tion: Hsieh, H. H., H. Y. Yen, and M. H. Shih, 2010: Moho depth de rived from grav ity data in the Tai wan Strait area. Terr. Atmos. Ocean. Sci., 21, 235-241, doi: 10.3319/TAO.2009.03.05.01(T) 1. INTRODUCTION The Tai wan Strait is sit u ated on the con ti nen tal shelf off the south east China coast be tween the East China Sea and the South China Sea, trending NE-SW (Fig. 1). Re gional stresses of the Tai wan Strait area can be con sid ered as a re - sult of two tec tonic forces. The first is the compressional stress re lated to the arc-con ti nent col li sion be tween the Phi - lippine Sea plate and the Eur asian plate, and the sec ond is the extensional stress re lated to the open ing of the South China Sea to the south. Un der these two mech a nisms, the crust of the Tai wan Strait is formed by a com plex struc ture of up lift ing, nor mal fault ing, and vol ca nic in tru sion (Gao and Huang 1994). A cen ter of thin ning litho sphere and asthe - nosphere upwelling has been iden ti fied on the west ern side of Tai wan based upon geo chem is try in ter pre ta tions (Chung et al. 1994). The crust of the Tai wan Strait area can be con - sid ered as a part of the Eur asian con ti nent slightly de formed due to the arc-con ti nent col li sion. The Moho dips slightly from west to east shown by P n ve loc ity study (Huang et al. 1998). From a tec tonic point of view, there are sev eral alter - nating bas ins and up lifts in the Tai wan Strait. The Kuanyin * Cor re spond ing au thor E-mail: yenhy@earth.ncu.edu.tw up lift and Peikang High al ter nate with the Taichung- Taihsi and Tainan bas ins lo cated on the east side of the strait. The Taichung-Taihsi and Tainan bas ins have ac cu - mu lated dense sed i ments of great thick ness due to rapid sub si dence (Chou 1991) and have ex tended to west ern Tai wan sep a rated by the Peikang High. The Tainan ba - sin, with sed i ments more than 6000 m thick, is char ac ter - ized by extensional nor mal faults trending mainly in the NE-SW and E-W di rec tions (Yang et al. 1991). The Taiwan Strait, as a part of the South China Sea mar gin, con sists of the Paleocene syn- rift se qu ence and the Neo cene-qua - ter nary post-rift se quence (Teng 1992). A num ber of Pale - ocene fault- bounded troughs, in clud ing the Nanjihtao ba sin and Penghu ba sin, de vel oped on the thin ning Me - so zoic base ment (Sun 1982). Both the syn-rift and postrift se quences are com posed of paralic and shal low ma - rine de pos its de rived from south east ern China. The half graben is the prin ci pal fea ture in the rift ing con ti nen tal mar gin (Rosendahl 1987). As ex pected, the early Ter - tiary sed i ments are pri mar ily con fined to the half gra bens and form a thick asym met ri cal wedge. These wedge- shaped se quences, such as Nanjihtao and Penghu bas ins along the Tai wan Strait, are com prised of thick sed i ments (Sun 1982; Hsiao et al. 1991; Yu and Huang 1994).

236 Hsieh et al. sume a den sity con trast be tween the two me dia, i.e., crust and man tle, to ob tain the gross fea ture re lated to the depth of the in ter face. In this study, Moho depth in the Tai wan Strait area is in verted us ing a MATLAB source code 3DINVER.M (Gómez-Ortiz and Agarwal 2005) from an in te grated new grav ity data set. The re sult gives us a pre lim i nary un der - stand ing of crustal strc ture and de for ma tion be neath the Tai wan Strait area. Fig. 1. Map show ing the geo log i cal and tec tonic set tings near Tai wan. The Phil ip pine Sea plate moves to ward the north west rel a tive to the Eur asia plate at about 8 cm yr -1 (big red ar row). Dashed lines show the ma jor geo log i cal struc tures of the Tai wan Strait (by a se ries of bas ins: the Taichung-Taihsi, Nanjihtao, Penghu, and Tainan Bas ins; the Kuanyin and Penghu Up lifts; and the Peikang High). As men tioned, not a lot of geo log i cal and geo phys i cal in ves ti ga tions in parts of the Tai wan Strait have been re - ported in the past (Sun 1985; Chen and Nakamura 1992; Yu 1993; Chung et al. 1994; Gao and Huang 1994; Kao and Wu 1996; Huang et al. 1998, Chen et al. 2005). Be cause ac cess to the Tai wan Strait as a whole has been lim ited for years due to po lit i cal rea sons, the over all tec ton ics of the strait, de rived from to mog ra phy re sults and de tailed seis mic pro files, re - mains ob scure, even though it forms an im por tant part close to the col li sion bound ary. Al though three lo cal net works, Fuchien Seis mic Net work (FJN), Broad band Ar ray in Tai - wan for Seis mol ogy (BATS), and the Cen tral Weather Bu - reau Seis mic Net work (CWBSN) are lo cated on both sides of the strait, very few seis mic con straints on the tec tonic struc ture of this area are avail able. Re cently, Ai et al. (2007) uti lized seis mic wave forms re corded by the above three net - works to il lus trate the 2-D crustal and up per man tle struc - tures of two par al lel pro files across the south east ern China re gion by us ing com mon con verted point stack ing of re - ceiver func tions. Re gret ta bly, the shal lower struc tures be - neath the Tai wan Strait within those two pro files could not be solved due to sparse seis mic data. Up to now, grav ity data over the Tai wan Strait are cur - rently the most in te grated and pro vide the best geo phys i cal data cov er age. Fun da men tal in for ma tion re gard ing struc - tural con fig u ra tion, such as Moho re lief, gen er ally can be in - verted by us ing es sen tial grav ity data. For sim plic ity, we as - 2. DATA COM PI LA TION AND BOUGUER ANOMALY MAP Grav ity mea sure ments and anal y ses are par tic u larly use ful as a re con nais sance tool for a large tec tonic re gion. A high-res o lu tion re gional grav ity map over the Tai wan Strait (21-27 N and 116-122.5 E) can help us un der stand its tectonic sig na ture. There is, there fore, a need to com pile a gravity map that en com passes the en tire strait re gion. In this study, the grav ity data are com piled from four sources: (1) 603 Is land-wide grav ity sta tions have been es tab lished by the In sti tute of Earth Sci ences, Ac a de mia Sinica (Yen et al. 1995b) since 1980. Sev eral tens of mea sure ments from the off shore is lands, such as Penghu, Lutao, Lanyu, and Peng - chiayu (Fig. 1), have since been added to the data set. (2) There are sev eral tens of ship tracks in the Na tional Geo - phys i cal Data Cen ter (NGDC) ar chive of the NOAA, USA, with con tin u ing grav ity ob ser va tions of off shore Tai wan. Those data with ob vi ous noise, large time gaps and sus pi - cious glitches or drifts in the dig i tal re cords are first re jected. Any cruise with out a base tie is use ful only if it yields con sis - tent crossover differences with all intercepting, calibrated cruises. This means that for the cross over dif fer ences, the root-mean-squares de vi a tion from the mean must be small com pared to the mean it self. Thir teen such ship tracks plus sim i lar two from the Chi nese Pe tro leum Com pany are re - tained as suit able for the study. Three cruises men tioned above are an a lyzed through the cross over anal y sis. Fol low - ing this strat egy, most grav ity lines are tied to the MW9006 in the south ern area and to the POP 1 and 2 to the north east of Tai wan (Yen et al. 1995a). (3) To main tain a rea son able ex - trap o la tion of the strait area where no ship tracks were pre - sent, the SEASAT grav ity data was used to fill data gaps during the over all com pi la tion (Haxby 1987). These data, given at 1/4 grids, are de rived from the SEASAT sat el lite geo detic mis sion ver i fied with the grav ity data at the Penghu islands, and are therefore characterized by relatively smooth vari a tions. (4) The grav ity data of Fuchien Prov ince, China was dig i tized from the Bouguer grav ity anom aly map of the South and East China Seas and ad ja cent re gions, as com - piled by the Min is try of Ge ol ogy and Min eral Re sources, Peo ple s Re pub lic of China and pub lished by the Geo log i cal Pub lish ing House (Liu 1992). It is also time-con sum ing, te dious and dif fi cult work to in te grate grav ity data from so many sources (in land, off -

Moho Depth De rived from Grav ity Data in Tai wan Strait 237 shore is lands, ma rine, and sat el lite data). De tails of grav ity com pi la tion pro ce dure and data qual ity for items (1) to (3) have been de scribed by Yen et al. (1995a, b). To en sure the re li abil ity of the Bouguer anom aly of Fuchien Prov ince, we car ried out a grav ity sur vey at the off shore is lands of Kin - men, Wuchiu, and Matsu (Fig. 1) lo cated nearby Fuchien s pro vin cial coast to cor rect their Bouguer anom aly value. These data sets were ac cu rately cal i brated us ing the Inter - national Grav ity Stan dard iza tion Net 1971, and the grav ity anom aly were ref er enced to Geo detic Ref er ence Sys tem 1967, thus fa cil i tat ing the con struc tion of a re gional map that in cludes both land grav ity data (Bouguer anom aly) and ma - rine grav ity data (free-air anom aly). The com piled grav ity anom aly map for the Tai wan Strait is shown in Fig. 2a. The isogals trend gen er ally in a NE-SW di rec tion, in con so nance with the over all shal lower geo log i cal strike of the strait. Con spic u ous neg a tive anom a lies cover Fuchien Prov ince and West ern Tai wan, whereas pos i tive anom a lies dom i nate the Tai wan Strait. An anom aly high over the Penghu is lands and their south ern part reaches a max i mum value of 60 mgal. Taichung-Taihsi, Tainan, Nanjihtao, and Penghu bas ins show rel a tive grav ity lows due to thick sed i ments. It is ob - vi ous that dominants in flu ence over those grav ity anom a lies are the sed i ment and wa ter lay ers. To better sup press grav ity ef fects due to the shorter wave length at trib ut able to the wa ter layer and sub ma rine to pog ra phy, Bouguer, and ter rain cor rec tions should be con sid ered for the ma rine grav ity data. For tu nately, the av er age depth of the wa ter layer is about 100 m, and the to pog ra phy of the seafloor is not rug ged over the Tai wan Strait. The wa ter layer can be re placed di rectly by the crust with a den sity of 2.67 g cm -3 by us ing a sim ple in te gra tion pro cess, and the ter rain cor rec tion of the seafloor to pog ra phy can be ne glected. The den sity of the Penghu is - lands, com posed of ex ten sive ba salt flows, is close to the av er age crustal den sity (2.67 g cm -3 ). To ob tain Moho depth, cor re spond ing lon ger wave length anom a lies, the re gional Bouguer grav ity anom aly (Fig. 2b) is cal cu lated by the Grif - fin method (1949). A semi cir cu lar grav ity high, with a max i - mum value of 180 mgal, man i fests it self as a prom i nent, con - tin u ous high ex tend ing north erly to the Penghu is lands from the south ern bor der of the map. This anom aly is most pro - b a bly caused by the oce anic crust be neath the south ern strait and high-den sity ex tru sive ba salt out crop on the Penghu is - lands. and den sity con trast be tween the two me dia and the geo - logical con trol depths de scribed above, the 3-D ge om e try of the in ter face is iteratively cal cu lated. (a) (b) 3. THE ORY AND AL GO RITHM The pro gram, based on the Parker-Oldenburg method, has been pre sented by Gómez-Ortiz and Agarwal (2005) for com put ing 3D ge om e try of the den sity in ter face from a gridded grav ity anom aly. This pro ce dure is based on a re la - tion ship be tween the Fou rier trans form of the grav ity anom - aly and the sum of the Fou rier trans form of the in ter face to - pog ra phy. Given the av er age depth of the in ter face bound ary Fig. 2. (a) Grav ity anom aly map of the Tai wan Strait area with a 20 mgal con tour in ter val. The map is ob tained by join ing the in ter po lated land grav ity data (Bouguer anom aly) and the ma rine data (free-air anom aly) on a 1/12 grid. The isogals trend gen er ally to the NE-SW di rec tion, con sis tent with the over all shal lower geo log i cal strike of the strait. (b) Af ter re plac ing the sea-wa ter layer to the crust, the re gional Bouguer grav ity anom aly map was ob tained. The con tour in ter val is 20 mgal.

238 Hsieh et al. The equa tion de scribed by Parker (1973) is used to cal - cu late the ho mo ge neous layer with an un even bound ary ge - nerated by den sity con trasts. Us ing the Fou rier trans form of the grav ity anom aly, we have: where F( g) is the Fou rier trans form of the grav ity ano - maly; G is the grav i ta tional con stant; is the den sity con - trast across the in ter face; k is the wave num ber; h(x) is the depth to the in ter face (pos i tive down wards); and z 0 is the av er age depth of the hor i zon tal in ter face. We com pute the un du lat ing in ter face depth from grav ity anom aly data, fol low ing Oldenburg (1974) in a re ar ranged Eq. (1): The first term of the equa tion is com puted by assigning h(x) = 0 (Oldenburg 1974) and the first ap prox i ma tion value h(x) from its in verse Fou rier trans form. The value of h(x) is iteratively used in Eq. (2) to es ti mate a new value until a rea son able so lu tion is achieved. Be cause the Parker- Oldenburg method is un sta ble at high fre quen cies, Gómez- Ortiz and Agarwal (2005) add a co sine low-pass fil ter to con straint the high-fre quency sig nals. Af ter a sim ple test, we set 0.012 Hz as the cor ner fre quency to deal with the anom a - lous data. The above al go rithm is ap plied to com pute Moho depth in the Tai wan Strait area (21-27 N and 116-122.5 E) from the re gional Bouguer grav ity anom aly (Fig. 2), with grid spac ing of 5 km. Be cause of the in her ent non-unique ness of grav ity in ter pre ta tion and a pau city of deeper geo log i - cal cross-sec tions, seis mic pro files, and ve loc ity to mog ra - phy in the strait area, an at tempt was made to con sider the rel a tively sim ple as sump tion that the two pa ram e ters of depth and den sity con trast be tween the man tle and crust in ter face. Re ferred to as the global crustal model (CRUST 5.1), a crustal thick ness of 30 km is taken from the world - wide crustal thick ness con tour map (Moo ney et al. 1998). The as sumed crustal thick ness of 30 km in this study is in ba sic agree ment with pre vi ous lit er a ture re ports (Kim et al. 2004; Hwang and Yu 2005; Ai et al. 2007), de scrib ing a typ i cal old and sta ble con ti nen tal crust. In other words, the in verted Moho s re lief rel a tive to the av er age Moho depth of 30 km is prac ti ca ble. The com mon den sity con - trast of 0.6 g cm -3 be tween the man tle and crust in ter face is ap pro pri ate (Dehlinger 1978; Gao and Huang 1994; Yen et al. 1998). (1) (2) 4. MOHO DEPTH AND DIS CUS SIONS Convergence of the iterative procedure has been achieved at the sixth it er a tion, with an RMS er ror of 0.0157 km. This RMS er ror, re ferred to as the RMS er ror of 0.0157 km from Gómez-Ortiz and Agarwal (2005), in ter prets that the in ver - sion re sult can be ac cept able. Fig ure 3 shows the Moho depth dis tri bu tion in the Taiwan Strait area. Its vari a tion is ba si cally con sis tent with two seis mic ob ser va tions. One is the seis mic re frac tion pro - file (A-A ) off shore of Fuchien Prov ince (Liao et al. 1988). Here, the av er age Moho depth is about 31 km ex cept for a change that takes place at 150 km where a fault cuts through the en tire crust (Fig. 4a). The other seis mic ob ser va tion (B-B ) along the west ern coast of Tai wan is ob tained from tomographic in ver sion by jointly us ing the seis mic data set of the CWBSN and two tem po rary seis mic ar rays in Hualien and Pingtung (Kim et al. 2005). We as sume that the Vp = 7.5 km s -1, taken as the av er age ve loc ity of three seis mic tomography re sults (Rau and Wu 1995; Ma et al. 1996; Wu et al. 2007), is the dis con tin ued in ter face be tween the lower crust and up per man tle. The depth of in ter face es ti mated from the ve loc ity struc tures range from 32 to 34 km (Fig. 4b). They gen er ally agree with our re sults cal cu lated from the grav ity anom aly. The Moho depth of Penghu up lift is shal - lower in the Tai wan Strait and thick ens to ward both sides of Fig. 3. Moho depth dis tri bu tion in the Tai wan Strait area de rived from the re gional Bouguer grav ity anom aly data. To avoid edge ef fects caused by the cal cu la tion pro cess, the map is only shown 22-26 N and 117-121.5 E. (Con tour in ter val: 1 km)

Moho Depth De rived from Grav ity Data in Tai wan Strait 239 the strait (Fig. 4c). The rel a tively shal lower Moho depth, reach ing up to 28 km, is con vex up ward un der neath the Penghu up lift. Fig ure 5 shows the dif fer ence be tween the observed re gional grav ity anom aly and the grav ity anomaly due to the com puted 3D Moho re lief (by means of the for - ward mod el ing al go rithm). These dif fer ences are at most in the range of -5 to 5 mgal and can be tol er ated for the whole (a) strait area. In short, Moho depth dis tri bu tion in the Tai wan Strait area ob tained from the grav ity anom aly is ac cept able in that the re sults are gen er ally con sis tent with two seis mic con straints and front eval u a tion. Over the strait area, the ge om e try of the Moho re lief is rel a tively flat with an av er age depth about 30 km, ex cept for the Penghu up lift. It is con ceiv able that the crust of the Tai - wan Strait can be re garded as a typ i cal old and sta ble con - tinental crust in te rior of the south east ern Chi nese main land (part of Eur asian plate) and that it is only slightly de formed as it is far away from the col li sion bound ary. The rel a tively shal lower Moho depth, reach ing up to 28 km, is con vex up - ward un der neath the Penghu up lift, which may have been caused by magma upwelling. This is con sis tent with the ex - tensive ba salt flows that form the Penghu Is land group. Moho depth thins rap idly for south west ern off shore Tai wan (i.e., the south ern bor der of the map). The Moho depth dis tri bu tion is shal lower for the strait it self and thick ens to ward both the Fuchien and Tai wan coastal zones. The trend of the con vex axis is roughly in a N-S di rec tion and nearly per pen dic u lar to the rel a tive mo - tion between the Phil ip pine Sea plate and the Eur asian plate. Accord ing to Chung et al. (1994), in the litho sphere sec tion across the Tai wan Strait, the thin nest crust is lo cated at the west ern part of strait (i.e., be tween Penghu is lands and Fuchien Prov ince). Kao and Wu (1996) pro pose that ex - tensional N-S man tle flow un der the crust in the Tai wan Strait may have been able to cre ate fa vor able con di tions for (b) (c) Fig. 4. Lo ca tions of three Moho depth pro files (pur ple lines A-A, B-B, and C-C at the in dex map). (a) The av er age Moho depth (black line) from the seis mic re frac tion sur vey is about 31 km, ex cept for a fault cut ting through the en tire crust at a dis tance of 150 km (Liao et al. 1988); (b) the Moho depth (cor re sponds to Vp = 7.5 km s -1, black line) vari a tions re ferred from the ve loc ity struc tures (Kim et al. 2005). The Moho depth along the A-A and B-B pro files cal cu lated from the grav - ity data (red line) are ba si cally con sis tent with two seis mic ob ser va - tions; (c) The Moho depth along the C-C pro file across Is land Penghu taken from Fig. 3 (red line). Fig. 5. Difference between the gravity anomaly forward computation of 3D Moho re lief and the ob served grav ity map pre sented in Fig. 2b. (Contour interval 5 mgal.)

240 Hsieh et al. a nearly E-W strik ing nor mal fault to oc cur based upon analysis of seis mic data, such as the 16 Sep tem ber 1994 Tai wan Strait earth quake (Dziwonski et al. 1995; Zheng et al. 1998). The re sult for the Moho depth dis tri bu tion seems con sis tent with struc tural evo lu tion of man tle up lift and crust thin ning in the Tai wan Strait area. We ex pect that the PmP data from the forth com ing ac tive seis mic ex per i ments of the Tai wan In te grated Geodynamics Re search (TAIGER) pro ject will sub stan ti ate our grav ity find ings. 5. CONCLUSION The com piled grav ity anom aly map for the Tai wan Strait (Fig. 2a) shows that the trend of the main grav ity anom aly is in a NE-SW di rec tion, con sis tent with the over all shal lower geological strike of the strait. Conspicuous negative anomalies oc cur in Fuchien Prov ince and West ern Tai wan; pos i tive anom a lies dom i nate the Tai wan Strait. An anom aly high is over the Penghu is lands, where ba salt out crops, and their south ern part, with a max i mum value of 60 mgal. Re mov ing the grav ity ef fects due to sea-wa ter layer and sub ma rine to - pog ra phy, the re gional Bouguer grav ity anom aly is ob tained (Fig. 2b). A re mark able semi cir cu lar grav ity high trending N-S is found from the south ern part of the strait to the Penghu is lands. This anom a lous area, with a max i mum value of 180 mgal, is most likely caused by the oce anic crust in the south and high-den sity ex tru sive bas alts in the north. The Moho re lief, com puted from a re gional Bouguer grav ity data set, is rel a tively flat, ex cept for the Penghu up - lift, with an av er age depth about 30 km. The Moho depth is shal lower in the Tai wan Strait and thick ens to ward its sides. The rel a tively shal lower Moho depth, reach ing up to 28 km, is con vex up ward un der neath the Penghu up lift, which may be caused by magma upwelling. In this study, a thin ner crust is ob tained in the strait area as com pared with those in the south east ern Chi nese main - land and Tai wan. This sug gests that the crust un der neath the Tai wan Strait has been un der go ing a struc tural evo lu tion of man tle up lift ing and crustal thin ning. More over, our re sults pro vide an im por tant re gional frame work for fur ther plan - ning of a large-scale geo phys i cal sur vey (such as TAIGER pro ject) in the Tai wan Strait area and stud ies on al ter nate in - ter ac tions be tween the Phil ip pine Sea and Eur asian plates. Ac knowl edge ments We are in debted to Ta-Liang Teng, Yih-Hsiung Yeh, Guey-Kuen Yu, Cheng-Horng Lin, and Bor-Shouh Huang. The au thors ex press their sin cere ap pre - ci a tion to Da vid Gómez- Ortiz and an anon y mous re viewer for crit i cal re views and ex cel lent sug ges tions on this manu - script. 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