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Thermal maturity in the Shimanto accretionary prism, southwest Japan, with the thermal change of the subducting slab: fluid inclusion and vitrinite reflectance study
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Earth and Planetary Science Letters 173 (1999) 61–74 www.elsevier.com/locate/epsl
Thermal maturity in the Shimanto accretionary prism, southwest Japan, with the thermal change of the subducting slab: fluid inclusion and vitrinite reflectance study Arito Sakaguchi * Department of Natural Environment Science, Kochi University, Kochi, 780-8520, Japan Received 29 December 1998; accepted 27 August 1999
Abstract The P –T path of the Cretaceous Shimanto accretionary complex, western Shikoku, southwest Japan, was determined by a combination of vitrinite reflectance data and homogenization temperatures of fluid inclusion analysis. Within the complex, the P –T paths of the Upper Cretaceous Yokonami Melange and surrounding coherent strata were compared. No P –T difference was found between the Yokonami Melange and surrounding strata. At least two types of fluids: methane-rich and water-rich fluids, migrated at different stages during metamorphism within the complex. The methane-rich fluid was initially trapped in vein quartz under pressures of ¾260 MPa and a geothermal gradient of ¾24ºC=km. The water-rich fluid was included in calcite at 95–125 MPa and a geothermal gradient of 50ºC=km. The complex sequence of occurrence of the vein minerals and radiometric age data suggest that the water-rich fluid was trapped during the Kula–Pacific ridge subduction into the Shimanto accretionary complex. The paleoheat flow values for the two stages of subduction are consistent with the changes in the inferred slab age. The metamorphism of the Shimanto accretionary complex was predominantly controlled by the thermal condition of the subducting plate. © 1999 Elsevier Science B.V. All rights reserved. Keywords: melange; Shimanto Group; vitrinite; reflectance; accretionary wedges
1. Introduction P –T data are required for understanding both the tectonic evolution and thermal maturity of accretionary prisms. Although P –T studies have been done in the high-pressure metamorphic belts, the P –T path for low-grade metamorphic belts has remained speculative. The Shimanto complex in southwest Japan is typical of many low-grade accretionary complexes. A Ł E-mail:
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number of studies have estimated the maximum paleotemperature using vitrinite reflectance and illite crystallinity. It has been shown that the Shimanto accretionary complex, at least in part, underwent relatively high temperature alteration [1,2]. Sakaguchi et al. [3], Underwood et al. [4] and Hibbard et al. [5] have also pointed out that the Shimanto accretionary complex probably suffered thermal overprints due to the subduction of relatively young oceanic crust. These studies, however, fail to yield unique solutions for the thermal structure because the burial depth and geothermal gradient were poorly understood.
0012-821X/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 1 9 - 8
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The P –T conditions recorded by fluid inclusions in vein minerals offer clues to paleogeothermal gradients. Vrolijk et al. [6] suggested that the warm fluid migrated along the fault zones in the Kodiak accretionary prism. Sakaguchi [7] determined that the water-rich fluid inclusions were trapped under the high-geothermal environment in the Shimanto complex. However, neither study clarifies the P –T evolution with the fluid flow. Various types of fluid, for example methane or water, occur within the vein minerals in the Shimanto complex, and may record the P –T conditions during tectonic evolution.
2. Geologic background The accretionary complex of southwest Japan has been divided into the Jurassic Chichibu, the Cretaceous Shimanto and the Tertiary Shimanto complexes, and is bounded by the BTL (Butsuzo Tectonic Line) and the ATL (Aki Tectonic Line). The Cretaceous Shimanto accretionary complex in Shikoku, furthermore, can be subdivided into two groups; the Lower Cretaceous Shinjogawa Group and Upper Cretaceous Taisho Group [8]. The complex is composed of four facies; slope basin, forearc basin, trench-fill and melange facies. Trench-fill facies are dominant, and three units of melange facies, the Yokonami, Kure and Okitsu melanges, are interleaved within the trench-fill facies [8]. The slope and forearc basin sediments are scattered; the Cretaceous Monobegawa Group and Torinosu Group overlie the Chichibu complex, and the Doganaro and Uwagumi formations and the Uwajima Group unconformably overlie the Shinjogawa Group in the Cretaceous Shimanto complex [9] (Fig. 1). The sedimentary succession of the trenchfill, forearc basin and slope facies are relatively coherent sedimentary sequences, with only locally tight fold structures. In contrast, the melange facies
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have suffered much greater deformation, and include ‘exotic’ oceanic blocks. In the melange, the age difference between the limestone overlying the basalt, and the black shale matrix indicates the time from the eruption of the basalt to the accretion. This age difference in southwest Japan decreases trenchward from over 100 m.y. in the Jurassic to perhaps as young as 10 m.y. in the latest Cretaceous and Tertiary. This trend indicates that a spreading ridge moved progressively closer to the trench [8]. The Ocean Island Basalt (OIB-type) blocks are common in most of the melanges in Japan [10,11]; however, only the uppermost Cretaceous melange in the Shimanto complex involves Mid-Oceanic Ridge Basalt (MORB-type) blocks without any pelagic sediment cover [12]. This also supports the idea that a mid-ocean ridge approached the trench and subducted beneath an accretionary prism of the Cretaceous Shimanto complex. In this paper, I derive the P –T path in the Campanian–Maastrichtian Yokonami Melange and surrounding strata of the Cenomanian Susaki and Campanian–Maastrichtian Shimotsui formations (Fig. 1). The sedimentary age of this melange, both blocks and matrix, is the best constrained of any part of the Cretaceous Shimanto accretionary complex, and the age difference between the oldest block and matrix is 60–70 Ma.
3. Vitrinite reflectance studies Sakaguchi [7] reconstructed all the formations and melanges of the Cretaceous Shimanto complex from two hundred vitrinite reflectance measurements. Vitrinite particles were collected from both the sandstone and black shale in the coherent sequences, and from the black shale matrix and the sandstone block in the melange zones. For each
Fig. 1. Index map and schematic geologic map. The Cretaceous Shimanto accretionary complex is separated from the Jurassic complex by the Butsuzo Tectonic Line (BTL). The veins for the fluid inclusion analysis are from the Yokonami Melange and surrounding strata. Samples were taken from both the Awa and Goshikigahama area within the same melange zone. The veins parallel or subparallel to the bedding and C-surface of the S–C structure were sampled. The orientation data are shown to the right. The circles indicate the poles of the layers, and the triangles show the poles of the veins. The thermal structure based on the maximum reflectance of vitrinite .Rmax / along the line from A to A0 is shown at the top right. The results of the homogenization temperature (Th) of the methane-rich and water-rich fluid inclusions are shown at the bottom to the right.
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sample, I measured one hundred vitrinite particles at three orientations; random mean (Rm ), maximum (Rmax ) and minimum reflectances. Although the Rmax value is the most reliable reflectance for the maturation studies, most methods for calculating maximum temperature are based on the Rm value [13]. Therefore, I used the Rmax for analysis of thermal structure and the Rm for estimating the maximum temperature. Two representative equations for calculation of maximum temperature from Rm are from Barker [14] and Sweeney and Burnham [15]. Although Barker’s equation is independent of the heating time, Sweeney and Burnham [15] are accurate for the duration of heating. The heating time is defined as the period between the maximum temperature and a temperature of 15ºC lower than the maximum temperature [16]; this time is typically 1 m.y. to 10 m.y. at the active margins [13,17]. This paper estimates the maximum temperature using both equations with the assumed heating times of 1 m.y. and 10 m.y. The equations of Sweeney and Burnham [15] are T (ºC) D 174 C (93 [ln percentage Rm ]) and T (ºC) D 158 C (90 [ln percentage Rm ]). These two equations are for 1 m.y. and 10.m.y. of effective time, respectively. Barker’s [14] time-independent equation is T (ºC) D 148 C (104 [ln percentage Rm ]). The error of these equations is š30ºC in temperature. The thermal structure in the Jurassic to Cretaceous complex is characterized by repeated southward increases in the Rmax , independent of the facies, ages and geologic structure [7]. For example, the Monobegawa Group, Doganaro Formation, Susaki Formation, Yokonami Melange, Shimotsui Formation and Kure Melange are included in the same thermal structure over the BTL, although they are very different in age and deformation. These results show that the primary accretion-related thermal structure was reset by a thermal overprint [7]. Additionally, both sandstone blocks and black shale matrix within the melange units have equal Rmax values (Fig. 1). The map pattern of thermal structure has a simple shape, and the Rmax increases not only southward but also eastward along the strike of the formations. For example, the value of Rmax in the Yokonami Melange and the surroundings increases eastwards from 1.6% at the Awa area to 2.8% at the Goshikigahama area. I believe that this thermal structure is independent of the geological structure
formed by the thermal overprint of the young oceanic plate during Eocene subduction [7]. The variation in Rmax may reflect the subsequent uneven uplift. The thrust between the block of the thermal structure is compared with the out-of-sequence thrust in the present Nankai trough [17]. An exception to this relatively simple thermal structure was identified for the Okitsu Melange which suffered much higher temperatures (Fig. 1). The Rmax value increases sharply from 2.2% to 3.1% on the boundary at the Okitsu Melange of 1 km in width. It was most likely that the Okitsu Melange was affected by a much higher geothermal gradient of >90ºC=km [7].
4. Fluid inclusions: occurrence and methods of analysis Seventeen quartz and calcite veins were sampled from the same outcrops from which the vitrinite samples were obtained in the Yokonami Melange, and two in the surrounding coherent sequence. All of the samples are composed of many small veins that are a few mm or less in width, and are parallel or sub-parallel to bedding (Fig. 2a). Some of the small syntectonic quartz veins show an en echelon structure. The quartz and calcite veins are complicated; the quartz precipitated earlier than the calcite because some quartz crystals are euhedral with a comb structure in the veins, and the calcite crystals tend to be anhedral and occur in the central part of the veins (Fig. 2b,c). Fluid inclusions within the double polished rock wafer were analyzed using the heating and cooling stage of a Linkam THM-600 with an error of š5ºC. The heating and cooling stage was calibrated by the melting temperatures of water (0ºC), dyphenylamine (54ºC), phenolphthalein (262ºC) and potassium dichromate (396ºC) standards. Two types of fluid inclusions were identified with the microscope: the one-phase fluid inclusions and the two-phase fluid inclusions (consisting of vapor and liquid at room temperature). The one-phase inclusions also look darker under the microscope, and they are segregated into two phases at temperatures below 100ºC. The one-phase inclusions occur only within the quartz veins, whereas the two-phase inclusions occur only in the calcite veins. Secondary
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Fig. 2. All of the veins are composed of many small quartz and calcite veins parallel or sub-parallel to the bedding and the C-surface of the S–C structure in the melange zone. The quartz crystals tend to be euhedral with a comb structure, and the calcite crystal tends to be anhedral (b,c).
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Fig. 3. Method of determination of P –T conditions. In the heating experiment, the internal pressure of the fluid inclusion increases along the isochore after homogenization. The P –T condition during fluid trapping was somewhere along the isochore, therefore the intersection of the isochore and maximum temperature determined from vitrinite reflectance indicates the maximum pressure of the fluid. The goethermal gradient can be limited by this method, even if the fluid was not trapped at peak heating.
two-phase type inclusions, however, locally occur within the quartz veins. The two types of fluid inclusions differ in composition; the one-phase inclusions are methane-rich fluid and the two-phase inclusions are water-rich fluid. Most of the fluid inclusions are of these two types with the exception of very minor fluid inclusions that homogenized around 40ºC and are of unknown composition. This paper examines only the methane and water inclusions to determine the P –T path of the Cretaceous Shimanto complex. The P –T conditions and geothermal gradients during fluid trapping are estimated by the homogenization temperature of the fluid inclusion and the vitrinite reflectance data. The liquid and vapor within any fluid inclusion are homogenized by heating. As shown in Fig. 3, the internal pressure of the fluid inclusion increases along the isochore after homogenization [18]. The P –T condition during fluid trapping should plot somewhere along the isochore, so the point of intersection between the isochore and the maximum temperature based on the vitrinite reflectance indicates the upper limit of
the fluid pressure (Fig. 3). Similarly, if the isochore crosses the axis of the 0ºC (minimum temperature of the seafloor), the intersection indicates the minimum limit of the fluid pressure at the trapping. The geothermal gradient during fluid trapping can be estimated from the difference of the P –T condition between the seafloor and the fluid (Fig. 3). Therefore, this is true even if the fluid had not been trapped during peak heating. The P –T condition and geothermal gradient during fluid trapping still can be limited by this method. The isochores are calculated from the basic data of Shepherd et al. [19] for waterrich fluid, and Saxena and Fei [20] for methane-rich fluid, together with the computer program of Brown and Hagemann [21]. The inclination of each isochore is also dependent upon the fluid composition and salinity. I determined these parameters by the infrared spectrum and by the melting temperature of the fluid, respectively. The Fourier-transform infrared microspectroscopy yields the molecular structural information on individual fluid inclusions larger than 50 μm in size.
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Fig. 4. Infrared spectrum of the water-rich fluid inclusion (a) and the methane-rich fluid inclusion (b). (a) The infrared radiation is absorbed in the water bands (H2 O). (b) Absorption occurs in the methane band (CH4 ) with little absorption in the H2 O and carbon dioxide (CO2 ) bands. This indicates that this type of fluid inclusion is rich in CH4 and poor in H2 O and CO2 .
Some spectral ranges are not distinguishable due to masking with strong absorption of the host mineral (Fig. 4). Unfortunately, it is a difficult task to detect carbon dioxide within the fluid inclusion in detail, because the device suffers from the effect of the gas fluctuation of carbon dioxide in the laboratory atmosphere. The spectrum indicates that the fluid inclusions of the one-phase and two-phase types are predominantly methane and water, respectively (Fig. 4), and I infer that the carbon dioxide content is low in both types of fluid inclusions. This is because the negative absorption peak was identified near the
band of the carbon dioxide. The spectrometer subtracts the spectrum data of the host mineral from the primary data. Therefore the negative absorption peak indicates that either the carbon dioxide within the inclusions are poorer than the host mineral, or poorer than the amount of the fluctuation in the laboratory during measuring. In either case, the carbon dioxide contents are very low. Moreover, the carbon dioxide bearing fluid inclusion is generally observed in the geothermal field. The liquid or clathrate of the carbon dioxide could not be recognized during cooling experiments. This means that the density of carbon
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dioxide has a maximum of 1.4 mole percentage [22] which would not effect the P –T estimation below. The salinity of each water-rich fluid inclusion can be estimated from the melting temperatures. Dissolved salt should reduce the melting point of the water, thus the salinity can be calculated when assuming that the salt is 100% NaCl. Melting temperatures of the ice within the inclusions are 1.4 to 0.2ºC corresponding to 24‰ to 30‰ concentrations for equivalent NaCl [23] indicating that the salinities are lower than that of normal sea water. Thus, the salinity and concentration of impurities in both types of fluid inclusion are low enough to estimate the isochores for a pure system.
5. Results 5.1. Temperature of homogenization (Th) of the methane- and water-rich fluid inclusions In the water-rich fluid inclusions, the Th values range from 92ºC to 239ºC and are different from location to location. In general, all fluid inclusions have similar Th values at the same location without relation to the orientation of the vein, and the fluid inclusions in the areas with higher Rmax values have higher values of Th (Fig. 1). The modal values of Th are 120ºC in the Susaki Formation, 140ºC in the Awa area of the Yokonami Melange, 180ºC in the Goshikigahama area of the Yokonami Melange, and 150ºC in the Shimotsui Formation (Fig. 1). There is a relationship between the Th and Rmax . An area with high Rmax has a higher Th value. In contrast, a clear relationship between Th and Rmax in the methane-rich fluid inclusions is not found. Modal Th values of the methane-rich fluid inclusions vary from 130ºC to 85ºC, although the peak values of the histogram range from 120ºC to 100ºC. The local differences among the Th values of the methane-rich inclusions are smaller than those of the water-rich inclusions. 5.2. The estimation of the P –T conditions and paleogeothermal gradient As mentioned above, the fluid pressure was obtained from the intersection between the isochores
and upper limits of fluid temperatures (Fig. 5 and Table 1) as estimated from Rmax , and the intersection between the isochores and the axis of the 0ºC or the saturation curve of vapor. The isochores of the methane-rich fluid crosses the axis of 0ºC, and the isochores of the water-rich fluid crosses the saturation curve of vapor. The minimum limits of the water-rich fluid pressures are not shown below, because the value of the saturation pressure of the vapor at the homogenization temperature is known. The results of the P –T conditions are shown in Table 1. The results calculated from the different equations are similar. In an extensive study, Laughland and Underwood [13] determined that the equation of Barker [14] is most accurate for the Shimanto complex. Therefore, the following discussion uses the results of Barker’s method. The maximum P –T conditions of the water-rich fluid are 190 (š30)ºC=110 (š50) MPa in the Susaki Formation, 190 (š30)ºC=95 (š50) MPa in the Awa area of the Yokonami Melange, 250 (š30)ºC=125 (š40) MPa in the Goshikigahama area of the Yokonami Melange, and 215 (š30)ºC=100 (š50) MPa in the Shimotsui Formation. On the other hand, the P –T condition during trapping of the methane-rich fluid ranges from 0ºC=90 MPa to 190 (š30)ºC=155 (š20) MPa in the Susaki Formation, 0ºC=75 MPa to 190 (š30)ºC=155 (š20) MPa in the Awa area of the Yokonami Melange, 0ºC=110 MPa to 250 (š30)ºC=260 (š20) MPa in the Goshikigahama area of the Yokonami Melange and 0ºC=75 MPa to 215 (š30)ºC=165 (š20) MPa in the Shimotsui Formation (Table 1). The trapping pressure of the methanerich fluid was higher than that of the water-rich fluid, and the Yokonami Melange and its surroundings do not show distinct differences in P –T conditions. The geothermal gradient is estimated from the difference in P –T conditions between the seafloor and the deeper horizon below the seafloor. The geopressure gradient was estimated from the inferred value of the bulk density of the sediment, the pore-fluid pressure and the ancient water depth as below. The bulk density of the strata within the present-day Nankai accretionary prism ranging from seafloor to 1300 mbsf (meter below seafloor) increases from 1.5 to 2.5 g=cm3 [24]. The bulk density below 1300 mbsf is presumed to be 2.5 g=cm3 , close to values of rocks from the Shimanto Belt (approximately 2.6 g=cm3 [25]).
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Fig. 5. P –T path in the Cretaceous Yokonami Melange and surrounding strata. The CH4 -rich fluid was trapped at a pressure of 75–260 MPa with a low geothermal gradient of ¾33ºC=km. Subsequently the H2 O-rich fluid was trapped in a geothermal environment of 50ºC=km. In both stages, no P –T difference was found between the Yokonami Melange and surrounding coherent strata.
The line through the points of the upper P –T limit of the water-rich fluid cuts the 0ºC axis at ¾20 MPa (Fig. 5). This result might be explained by the pressure exerted by the ¾2000 m column of sea water over the sediment. Though the fluid pressure at a given depth below the seafloor may have been anywhere between the hydrostatic and lithostatic pressures, it is probably close to the lithostatic pressure. The pore-fluid pressure increases with burial depth and reaches >80% of lithostatic pressure in the deeper portions of the Barbados accretionary prism [26]. The pore-fluid pressure reaches >90% of lithostatic pressure near the Barbados decollement zone with a total pressure only 8 MPa [27]. Moreover, it seems likely that the vein-filled fracture opened as a result of hydrofracturing. Therefore, I presume that the pore-fluid pressure during trapping was very close to the lithostatic pressure. The error of the geothermal gradient increases ex-
ponentially with reduction of pressure; therefore, I show the two differential errors for the fluid decompression (C) and for the fluid compression (). The lower limits of the paleogeothermal gradients during water-rich fluid trapping are estimated as 45 (C31=11)ºC=km for the Susaki Formation, 54 (C60=12)ºC=km for the Awa area, 52 (C29=12)ºC=km for the Goshikigahama area of the Yokonami Melange and 52 (C40=14)ºC=km for the Shimotsui Formation (Fig. 5 and Table 1). The geothermal gradients during methane-rich fluid inclusions are estimated as 25 (C3=2) to 0ºC=km for the Susaki Formation, 30 (C4=2) to 0ºC=km for the Awa area, 24 (C1=2) to 0ºC=km for the Goshikigahama area of the Yokonami Melange and 33 (C3=4) to 0ºC=km for the Shimotsui Formation (Fig. 5 and Table 1). Clearly each type of fluid was trapped at a very different P –T condition. The water-rich fluid was
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Temp. (max)
Methane-rich fluid
Water-rich fluid
pressure (max)
depth (max)
gradient (max)
pressure (max)
depth (max)
gradient (min)
188 (š10) MPa 155 (š8) MPa 260 (š18) MPa 165 (š8) MPa
7.5 (š0.4) km 6.0 (š0.3) km 10.5 (š0.8) km 6.5 (š0.3) km
25 (C3=2)ºC=km 30 (C4=2)ºC=km 24 (C1=2)ºC=km 33 (C3=4)ºC=km
110 (š50) MPa 95 (š50) MPa 125 (š50) MPa 100 (š50) MPa
4.2 km (š2.2) 3.5 km (š2.2) 4.8 km (š2.2) 4.1 km (š2.2)
45 (C31=11)ºC=km 54 (C60=16)ºC=km 52 (C27=12)ºC=km 52 (C56=11)ºC=km
Sweeney and Burnham [15], effective time: 10 m.y. Susaki Fm. 195 (š30)ºC 154 (š10) MPa Yokonami Mel. (Awa) 195 (š30)ºC 154 (š10) MPa Yokonami Mel. (Goshiki.) 245 (š30)ºC 262 (š15) MPa Shimotsui Fm. 216 (š30)ºC 162 (š10) MPa
6.0 (š0.4) km 6.0 (š0.4) km 10.5 (š0.4) km 6.5 (š0.4) km
32 (C3=2)ºC=km 33 (C3=2)ºC=km 24 (C2=1)ºC=km 33 (C2=3)ºC=km
108 (š45) MPa 92 (š45) MPa 120 (š45) MPa 99 (š45) MPa
4.2 (š2.0) km 3.5 (š2.0) km 4.7 (š2.0) km 3.8 (š2.0) km
46 (C29=10)ºC=km 56 (C54=14)ºC=km 52 (C28=10)ºC=km 57 (C47=14)ºC=km
Sweeney and Burnham [15], effective time: 1 m.y. Susaki Fm. 210 (š30)ºC 160 (š10) MPa Yokonami Mel. (Awa) 210 (š30)ºC 160 (š10) MPa Yokonami Mel. (Goshiki.) 265 (š30)ºC 270 (š15) MPa Shimotsui Fm. 235( š30)ºC 170 (š10) MPa
6.5 (š0.4) km 6.5 (š0.4) km 11.0 (š0.4) km 7.0 (š0.4) km
33 (C3=2)ºC=km 33 (C3=2)ºC=km 24 (C1=1)ºC=km 34 (C2=2)ºC=km
135 (š45) MPa 120 (š45) MPa 145 (š45) MPa 130 (š45) MPa
5.1 (š2.0) km 4.7 (š2.0) km 5.7 (š2.0) km 5.1 (š2.0) km
41 (C22=8)ºC=km 45 (C22=8)ºC=km 47 (C15=8)ºC=km 46 (C18=8)ºC=km
Barker [14], effective time: independent Susaki Fm. 190 (š30)ºC Yokonami Mel. (Awa) 190 (š30)ºC Yokonami Mel. (Goshiki.) 250 (š30)ºC Shimotsui Fm. 215 (š30)ºC
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Table 1 List of the estimated depths and geothermal gradients during the methane-rich and water-rich fluid trapping events
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trapped at a higher geothermal gradient than the ordinary value of 30ºC=km. This high-temperature condition was not confined to a local zone around each vein. High Th values occur in areas with high Rmax values, and the paleogeothermal gradients in the both Awa and Goshikigahama areas are very similar to each other, although the inclusions were trapped under different P –T conditions. These observations suggest that the high heat flow was regional, and thermal equilibrium between the fluid and its surrounding rock was attained. In other words, the trapping of the water-rich fluid and thermal maturation of the vitrinite happened concurrently under similar thermal conditions. Furthermore, I infer that the absolute P –T condition during the water-rich fluid trapping was very close to the upper limit of the Rm data. It is more difficult to determine an accurate geothermal gradient during methane-rich fluid trapping, because maximum P –T conditions of each locality do not plot on a single line of the geothermal gradient. This might be caused by the difference of timing between the methane-rich fluid trapping and peak heating. Therefore, the P –T condition during methane-rich fluid trapping is not as strictly constrained as in the case of water-rich fluid trapping. The results suggest geothermal gradients from 0ºC=km to 33ºC=km. This range of possible geothermal gradients can be narrowed. Two areas of the same tectonic zone would have similar values of geothermal gradient. Within the Yokonami Melange zone, the upper limit of the geothermal gradient is 24ºC=km in the Goshikigahama area. This is lower than the value of 31ºC=km at the Awa area. Assuming thermal equilibrium, the lower value in the Goshikigahama area should be near to the true paleogeothermal gradient rather than the higher value in the Awa area. Additionally, a geothermal gradient of <5ºC=km is unknown anywhere. Thus, the paleogeothermal gradient during the methane-rich fluid trapping is probably in the range of 5ºC=km to 24ºC=km with an error of 3ºC=km.
6. Discussion The methane-rich fluid was trapped in quartz before the calcite crystals trapped the water-rich fluid,
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because textural evidence indicates that the quartz crystals formed first (Fig. 2b). Furthermore, the calcite crystals have only the water-rich fluid inclusions, and quartz crystals have primary methane-rich fluid inclusions and secondary water-rich fluid inclusions. This indicates that the fluid was water-rich during calcite precipitation, and water-rich fluid healed cracks in the existing quartz crystals. Judging from the relationship between the Th for water and Rmax , the timing of the water-rich fluid trapping was close to the timing of the peak heating. The interpreted P –T path is shown in Fig. 5. During the first stage of fluid trapping, the rock reached the highest pressures of <260 MPa at depths shallower than approximately 10.5 km. During the second stage of fluid trapping, the geothermal gradient increased to <50ºC=km with pressures of 95– 125 MPa at depths shallower than approximately 3.5–5.0 km. This result is consistent with the idea that the thermal overprint occurred after the juxtaposition of tectono-stratigraphic units within the complex [7]. It is likely that the thermal development took place with the change of the P –T conditions of the subducting oceanic plate. Higher-grade metamorphic rocks may undergo two stages of metamorphism during passage through the inverted isotherm region deep in the subduction zone. These rocks may suffer different geothermal gradients without any change of the thermal condition of the slab. However, in the lower-grade Shimanto example discussed here, all the coherent formations and melanges, including forearc basin-fill sediment, show the same thermal structure. In such a shallow-level accretionary prism, the heat from the oceanic lithosphere is probably the most influential factor for the thermal structure [28] as compared to factors such as friction across the plate boundary [29] and cooling by the old plate subduction [30]. Zircon fission track dates indicate that the whole Cretaceous Shimanto accretionary complex cooled below 230ºC after 70 Ma [31,32]. This cooling age may have occurred by episodic exhumation in the whole Cretaceous Shimanto complex; however, this interpretation disagrees with the uplift history of forearc basin strata based on foraminifera assemblage data. The Doganaro Formation, for example, overlying the northern Shinjogawa Group, was deposited above the CCD during Aptian time, and the
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Fig. 6. Thermal history in the Cretaceous Shimanto accretionary prism. Initially, the rocks were buried to 7–10 km depth under a geothermal gradient of ¾24ºC=km; subsequently the rock reached a peak temperature of 190 to 270ºC at 3–5 km depth under a geothermal gradient of 50ºC=km. The paleoheat flow of both stages corresponds with the inferred heat flow, based on the age of the subducting oceanic slab.
Uwajima Group, overlying the southern Shinjogawa Group, was deposited above the CCD during Coniacian time [9]. These data indicate that the Lower Cretaceous basement of the Shinjogawa Group was already placed at the CCD level by mid-Cretaceous time, and it was exhumed throughout the Cretaceous. This is not a local uplift history because the Doganaro Formation is scattered along the BTL through Shikoku. I deduce that the episodic cooling shown by fission track dates was caused by a single regional thermal overprint. Other dates of post-Cretaceous age have been reported such as the K–Ar age of 55 Ma [33] in the Okitsu Melange. That melange is characterized by a lack of pelagic sediment, and the rocks suffered much higher geothermal gradients of over 90ºC=km [7]. This age is almost coincident with the timing of the close approach between the Kula–Pacific ridge and the Shimanto accretionary prism as the subduction margin of Japan from latest Cretaceous to
Eocene [8,34,35]. It is likely that the water-rich fluid trapping occurred during this peak heating event in the Cretaceous Shimanto complex (Fig. 6). Although the whole Cretaceous complex probably suffered the effects of high temperature, the Okitsu Melange underwent a much higher geothermal gradient of 90ºC=km. This local effect might have been caused by its location within the accretionary prism. The high geothermal gradient may have been produced by either the warm fluid passing through the Okitsu Melange similar to the Barbados decollement zone, or due to the thermal gradient becoming higher in the frontal portion of the accretionary prism because of the young plate subduction. The present-day subduction setting and high heat flow of 50–130 mW=m2 in the frontal portion of the present Nankai accretionary prism are very similar to what has been inferred for the Cretaceous Shimanto complex, though the subducting slab is as old as 15 Ma [24,36,37].
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As mentioned above, the occurrence of a thermal imprint after formation of the tectonostratigraphic architecture might have occurred, because of a ridge– trench collision or very young plate subduction beneath the Shimanto accretionary prism. In view of the heat flow, let us then consider the thermal relation between the slab and the accretionary prism. The paleoheat flow can be estimated from the thermal conductivity of the sedimentary rock and paleogeothermal gradient. I assume a thermal conductivity value of 2.2 W m1 ºC1 , which is equivalent to the deepest sequence in the Nankai prism [24]. Consequently, the calculated heat flow during the stage of the water-rich fluid trapping is 95 (C70=20) to 120 (C110=40) mW=m2 . On the other hand, the calculated heat flow corresponding to the stage of the methane-rich fluid trapping is 11–53 mW=m2 . The heat flow of the subducting oceanic crust can be estimated roughly from the age of the plate [38]. The age difference between the limestone block overlying the basalt and black shale matrix within the Yokonami Melange is 60–70 Ma [24]. If the basalt was of the MORB-type, then the heat flow should have been approximately 57–61 mW=m2 by the equation of Parsons and Sclater [38]. However, MORB-type basalt blocks within the melange are rare in Japan, and most of the rocks are of the OIBtype [10,11]. Therefore, there is a possibility that this age is younger than the true age of the plate, and the heat flow of the slab becomes even lower. In this case, the heat flow estimation of the slab becomes closer to the prism’s heat flow at the stage of the methane-rich fluid trapping. This indicates that the thermal condition of the slab definitively affects the heat flow in the accretionary prism.
7. Conclusions The P –T history of the Cretaceous Shimanto complex from the Yokonami Melange and surrounding strata records at least two distinct stages of metamorphism. Initially, these rocks were buried to 6.5–10.5 km depth in methane-rich fluids and under a geothermal gradient of ¾24ºC=km. Subsequently, they reached peak temperatures of 190 to 270ºC at depths of 3.5–5.0 km in water-rich fluids under a higher geothermal gradient of approximately
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50ºC=km. The overall thermal peak was reached during the second stage. At that time, the rocks were in thermal equilibrium with the fluid. This peak heating might have occurred when the Kula–Pacific ridge was subducted during the Eocene. During both stages, the complex has a heat flow value as expected from the subducting oceanic crust. The metamorphism and heat flow in this accretionary complex are controlled largely by the thermal condition of the subducting slab.
Acknowledgements I thank Professor Yujiro Ogawa of the University of Tsukuba for reading the entire text in its original form and making helpful suggestions throughout the work. Professors Atsuo Aihara and Tatsushi Murae of Kyushu University greatly assisted with the vitrinite reflectance and Fourier-transform infrared microspectroscopic studies. Dr. Mamoru Enjoji of the University of Tsukuba provided guidance for fluid inclusion analysis. Professor J. Casey Moore of the University of California and Associate Professor Wonn Soh of Kyushu University assisted in revision of the manuscript. Professor Michael B. Underwood of the University of Missouri, Professor Tim Byrne of the University of Connecticut and Dr. Peter Vrolijk reviewed the manuscript and provided critical comments. [MK]
References [1] K. Mori, K. Taguchi, Examination of the low-grade metamorphism in the Shimanto Belt by vitrinite reflectance, Mod. Geol. 12 (1988) 325–339. [2] L. DiTullio, M.M. Laughland, T. Byrne, Thermal maturity and constraints on deformation from illite crystallinity and vitrinite reflectance in shallow levels of an accretionary prism. Eocene–Oligocene Shimanto Belt, southwest Japan, Geol. Soc. Am. Spec. Pap. 273 (1993) 63–82. [3] A. Sakaguchi, K. Ohmori, H. Yamamoto, A. Aihara, M. Okamura, Paleo-geothermal structure of the Cretaceous Shimanto Belt analyzed by vitrinite reflectance, central shikoku, southwest Japan (in Japanese with English abstract), Res. Rep. Kochi Univ. 41 (1992) 29–47. [4] M.B. Underwood, M.M. Laughland, T. Byrne, J.P. Hibbard, L. DiTullio, Thermal evolution of the Tertiary Shimanto Belt, Muroto peninsula, Shikoku, Japan, Island Arc 1 (1992) 116–132.
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[5] J.P. Hibbard, M.M. Laughland, S.M. Kang, D. Karig, The thermal imprint of the ridge subduction on the upper structural levels of an accretionary prism, southwest Japan, Geol. Soc. Am. Spec. Pap. 273 (1993) 83–101. [6] P. Vrolijk, G. Myers, J.C. Moore, Warm fluid migration along tectonic melanges in the Kodiak accretionary complex, Alaska, J. Geophys. Res. 93 (1988) 10313–10324. [7] A. Sakaguchi, High geothermal gradient with ridge subduction beneath Cretaceous Shimanto accretionary prism, southwest Japan, Geology 24 (1996) 795–798. [8] A. Taira, J. Katto, M. Tashiro, M. Okamura, K. Kodama, The Shimanto Belt in Shikoku, Japan — evolution of Cretaceous to Miocene accretionary prism, Mod. Geol. 12 (1988) 5–46. [9] M. Okamura, Cretaceous Radiolaria from Shikoku, Japan (Part 1), Mem. Fac. Sci, Kochi Univ. 13 (1992) 21–164. [10] Y. Ogawa, H. Taniguchi, Origin and emplacement of basaltic rocks in the accretionary complexes in SW Japan, Ofioliti 14 (1989) 177–193. [11] T. Sakamoto, Y. Ogawa, S. Nakada, Origin of the greenstones in the Setogawa accretionary complex and their tectonic significance (in Japanese with English abstract), J. Geol. Soc. Jpn. 99 (1993) 9–28. [12] K. Kiminami, N. Kashiwagi, S. Miyashita, Occurrence and significance of in-situ greenstones from the Mugi Formation in the Upper Cretaceous Shimanto Supergroup, eastern Shikoku, Japan (in Japanese with English abstract), J. Geol. Soc. Jpn. 98 (1992) 867–883. [13] M.M. Laughland, M.B. Underwood, Vitrinite reflectance and estimates of paleotemperature within the Upper Shimanto Group, Muroto Peninsula, Shikoku, Japan, Geol. Soc. Am. Spec. Pap. 273 (1993) 103–114. [14] C.E. Barker, Geothermics of petroleum systems. Implications of the stabilization of kerogen thermal maturation after a geologically brief heating duration at peak temperature, U.S. Geol. Surv. Bull. 1870 (1988) 26–29. [15] J.J. Sweeney, A.K. Burnham, Evaluation of a simple model of vitrinite reflectance based on chemical kinetics, Am. Assoc. Pet. Geol. Bull. 74 (1990) 1559–1570. [16] K. Sekiguchi, A. Hirai, Estimation of maturation level of organic matter (in Japanese with English abstract), J. Jpn. Assoc. Pet. Technol. 45 (1980) 39–47. [17] K. Ohmori, A. Taira, H. Tokuyama, A. Sakaguchi, M. Okamura, A. Aihara, Paleothermal structure of the Shimanto accretionary prism, Shikoku, Japan: role of an out-ofsequence thrust, Geology 25, 327–330. [18] E. Roeder, Fluid inclusions, Min. Soc. Am., Rev. Mineral. 12 (1984) 646. [19] T.J. Shepherd, A. Rankin, D.H. Alderton, A Practical Guide to Fluid Inclusion Studies, Blackie, Glasgow, 1985, 239 pp. [20] S.K. Saxena, Y. Fei, Fluids at crustal pressure and temperatures, I: pure species, Contrib. Mineral. Petrol. 95 (1987) 370–375. [21] E.P. Brown, FLINCOR: A microcomputer program for the reduction and investigation of fluid-inclusion data, Am.
Mineral. 74 (1989) 1390–1393. [22] M. Sasada, Microthermometry of fluid inclusions in minerals from geothermal fields (in Japanese), Geotherm. Energy 14 (1989) 27–42. [23] R.W. Potter, M.A. Clynne, D.L. Brown, Freezing point depression of aqueous sodium chloride solutions, Econ. Geol. 73 (1978) 284–285. [24] A. Taira, I. Hill, J. Fish et al., Proceedings of the Ocean Drilling Program, Initial Reports 131, College Station, Texas, Ocean Drilling Program, pp. 71–269, 1991. [25] S. Hada, Physical and mechanical properties of sedimentary rocks in the Cretaceous Shimanto Belt, Mod. Geol. 12 (1988) 341–359. [26] G.K. Westbrook, Geophysical evidence for the role of fluids in accretionary wedge tectonics, in: J.I. Tarney (Ed.), The Behaviour and Influence of Fluid on Subduction Zones, Royal Society, London, pp. 1–16, 1991. [27] J.C. Moore et al., Abnormal fluid pressures and fault-zone dilation in the Barbados accretionary prism: evidence from logging while drilling, Geology 23 (1995) 605–608. [28] T.S. James, S. Hollister, W.J. Morgan, Thermal modeling of the Chugach metamorphic complex, J. Geophys. Res. 94 (1989) 4411–4423. [29] T.A. Dumitru, Effects of subduction parameter on geothermal gradients in Forearcs, with an application to Franciscan subduction in California, J. Geophys. Res. 96 (1991) 621– 641. [30] M.N. Toksoz, J.W. Minear, B.R. Julian, Temperature field and geophysical effects of a downgoing slab, J. Geophys. Res. 76 (1971) 1113–1138. [31] N. Hasebe, T. Tagami, N. Nishimura, Evolution of the Shimanto accretionary complex; a fission-track thermochronologic study, Geol. Soc. Am. Spec. Pap. 273 (1993) 121– 136. [32] T. Tagami, N. Hasebe, C. Shimada, Episodic exhumation of accretionary complex: fission-track thermochronologic evidence from the Shimanto Belt and its vicinities, southwest Japan, Island Arc 4 (1995) 209–230. [33] S.M. Agar, I.R. Cliff, D.C. Rex, Accretion and uplift in the Shimanto Belt, SW Japan, J. Geol. Soc. 146 (1989) 893–896. [34] D.C. Engebretson, A. Cox, R.G. Gordon, Relative motions between oceanic and continental plates in the Pacific basin, Geol. Soc. Am. Spec. Pap. 206 (1985) 59. [35] S. Maruyama, T. Seno, Orogeny and relative plate motions: example of the Japanese islands, Tectonophysics 127 (1986) 305–329. [36] M. Yamano, S. Honda, S. Uyeda, Nankai Trough: a hot trench?, Mar. Geophys. Res. 6 (1984) 187–203. [37] J. Ashi, A. Taira, Thermal structure of the Nankai accretionary prism as inferred from the distribution of gas hydrate BSRs, Geol. Soc. Am. Spec. Pap. 273 (1993) 137– 149. [38] B. Parsons, J.G. Sclater, An analysis of the variation of ocean floor with age, J. Geophys. Res. 82 (1977) 803–827.
Earth and Planetary Science Letters 173 (1999) 61–74 www.elsevier.com/locate/epsl
Thermal maturity in the Shimanto accretionary prism, southwest Japan, with the thermal change of the subducting slab: fluid inclusion and vitrinite reflectance study Arito Sakaguchi * Department of Natural Environment Science, Kochi University, Kochi, 780-8520, Japan Received 29 December 1998; accepted 27 August 1999
Abstract The P –T path of the Cretaceous Shimanto accretionary complex, western Shikoku, southwest Japan, was determined by a combination of vitrinite reflectance data and homogenization temperatures of fluid inclusion analysis. Within the complex, the P –T paths of the Upper Cretaceous Yokonami Melange and surrounding coherent strata were compared. No P –T difference was found between the Yokonami Melange and surrounding strata. At least two types of fluids: methane-rich and water-rich fluids, migrated at different stages during metamorphism within the complex. The methane-rich fluid was initially trapped in vein quartz under pressures of ¾260 MPa and a geothermal gradient of ¾24ºC=km. The water-rich fluid was included in calcite at 95–125 MPa and a geothermal gradient of 50ºC=km. The complex sequence of occurrence of the vein minerals and radiometric age data suggest that the water-rich fluid was trapped during the Kula–Pacific ridge subduction into the Shimanto accretionary complex. The paleoheat flow values for the two stages of subduction are consistent with the changes in the inferred slab age. The metamorphism of the Shimanto accretionary complex was predominantly controlled by the thermal condition of the subducting plate. © 1999 Elsevier Science B.V. All rights reserved. Keywords: melange; Shimanto Group; vitrinite; reflectance; accretionary wedges
1. Introduction P –T data are required for understanding both the tectonic evolution and thermal maturity of accretionary prisms. Although P –T studies have been done in the high-pressure metamorphic belts, the P –T path for low-grade metamorphic belts has remained speculative. The Shimanto complex in southwest Japan is typical of many low-grade accretionary complexes. A Ł E-mail:
[email protected]
number of studies have estimated the maximum paleotemperature using vitrinite reflectance and illite crystallinity. It has been shown that the Shimanto accretionary complex, at least in part, underwent relatively high temperature alteration [1,2]. Sakaguchi et al. [3], Underwood et al. [4] and Hibbard et al. [5] have also pointed out that the Shimanto accretionary complex probably suffered thermal overprints due to the subduction of relatively young oceanic crust. These studies, however, fail to yield unique solutions for the thermal structure because the burial depth and geothermal gradient were poorly understood.
0012-821X/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 1 9 - 8
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The P –T conditions recorded by fluid inclusions in vein minerals offer clues to paleogeothermal gradients. Vrolijk et al. [6] suggested that the warm fluid migrated along the fault zones in the Kodiak accretionary prism. Sakaguchi [7] determined that the water-rich fluid inclusions were trapped under the high-geothermal environment in the Shimanto complex. However, neither study clarifies the P –T evolution with the fluid flow. Various types of fluid, for example methane or water, occur within the vein minerals in the Shimanto complex, and may record the P –T conditions during tectonic evolution.
2. Geologic background The accretionary complex of southwest Japan has been divided into the Jurassic Chichibu, the Cretaceous Shimanto and the Tertiary Shimanto complexes, and is bounded by the BTL (Butsuzo Tectonic Line) and the ATL (Aki Tectonic Line). The Cretaceous Shimanto accretionary complex in Shikoku, furthermore, can be subdivided into two groups; the Lower Cretaceous Shinjogawa Group and Upper Cretaceous Taisho Group [8]. The complex is composed of four facies; slope basin, forearc basin, trench-fill and melange facies. Trench-fill facies are dominant, and three units of melange facies, the Yokonami, Kure and Okitsu melanges, are interleaved within the trench-fill facies [8]. The slope and forearc basin sediments are scattered; the Cretaceous Monobegawa Group and Torinosu Group overlie the Chichibu complex, and the Doganaro and Uwagumi formations and the Uwajima Group unconformably overlie the Shinjogawa Group in the Cretaceous Shimanto complex [9] (Fig. 1). The sedimentary succession of the trenchfill, forearc basin and slope facies are relatively coherent sedimentary sequences, with only locally tight fold structures. In contrast, the melange facies
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have suffered much greater deformation, and include ‘exotic’ oceanic blocks. In the melange, the age difference between the limestone overlying the basalt, and the black shale matrix indicates the time from the eruption of the basalt to the accretion. This age difference in southwest Japan decreases trenchward from over 100 m.y. in the Jurassic to perhaps as young as 10 m.y. in the latest Cretaceous and Tertiary. This trend indicates that a spreading ridge moved progressively closer to the trench [8]. The Ocean Island Basalt (OIB-type) blocks are common in most of the melanges in Japan [10,11]; however, only the uppermost Cretaceous melange in the Shimanto complex involves Mid-Oceanic Ridge Basalt (MORB-type) blocks without any pelagic sediment cover [12]. This also supports the idea that a mid-ocean ridge approached the trench and subducted beneath an accretionary prism of the Cretaceous Shimanto complex. In this paper, I derive the P –T path in the Campanian–Maastrichtian Yokonami Melange and surrounding strata of the Cenomanian Susaki and Campanian–Maastrichtian Shimotsui formations (Fig. 1). The sedimentary age of this melange, both blocks and matrix, is the best constrained of any part of the Cretaceous Shimanto accretionary complex, and the age difference between the oldest block and matrix is 60–70 Ma.
3. Vitrinite reflectance studies Sakaguchi [7] reconstructed all the formations and melanges of the Cretaceous Shimanto complex from two hundred vitrinite reflectance measurements. Vitrinite particles were collected from both the sandstone and black shale in the coherent sequences, and from the black shale matrix and the sandstone block in the melange zones. For each
Fig. 1. Index map and schematic geologic map. The Cretaceous Shimanto accretionary complex is separated from the Jurassic complex by the Butsuzo Tectonic Line (BTL). The veins for the fluid inclusion analysis are from the Yokonami Melange and surrounding strata. Samples were taken from both the Awa and Goshikigahama area within the same melange zone. The veins parallel or subparallel to the bedding and C-surface of the S–C structure were sampled. The orientation data are shown to the right. The circles indicate the poles of the layers, and the triangles show the poles of the veins. The thermal structure based on the maximum reflectance of vitrinite .Rmax / along the line from A to A0 is shown at the top right. The results of the homogenization temperature (Th) of the methane-rich and water-rich fluid inclusions are shown at the bottom to the right.
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sample, I measured one hundred vitrinite particles at three orientations; random mean (Rm ), maximum (Rmax ) and minimum reflectances. Although the Rmax value is the most reliable reflectance for the maturation studies, most methods for calculating maximum temperature are based on the Rm value [13]. Therefore, I used the Rmax for analysis of thermal structure and the Rm for estimating the maximum temperature. Two representative equations for calculation of maximum temperature from Rm are from Barker [14] and Sweeney and Burnham [15]. Although Barker’s equation is independent of the heating time, Sweeney and Burnham [15] are accurate for the duration of heating. The heating time is defined as the period between the maximum temperature and a temperature of 15ºC lower than the maximum temperature [16]; this time is typically 1 m.y. to 10 m.y. at the active margins [13,17]. This paper estimates the maximum temperature using both equations with the assumed heating times of 1 m.y. and 10 m.y. The equations of Sweeney and Burnham [15] are T (ºC) D 174 C (93 [ln percentage Rm ]) and T (ºC) D 158 C (90 [ln percentage Rm ]). These two equations are for 1 m.y. and 10.m.y. of effective time, respectively. Barker’s [14] time-independent equation is T (ºC) D 148 C (104 [ln percentage Rm ]). The error of these equations is š30ºC in temperature. The thermal structure in the Jurassic to Cretaceous complex is characterized by repeated southward increases in the Rmax , independent of the facies, ages and geologic structure [7]. For example, the Monobegawa Group, Doganaro Formation, Susaki Formation, Yokonami Melange, Shimotsui Formation and Kure Melange are included in the same thermal structure over the BTL, although they are very different in age and deformation. These results show that the primary accretion-related thermal structure was reset by a thermal overprint [7]. Additionally, both sandstone blocks and black shale matrix within the melange units have equal Rmax values (Fig. 1). The map pattern of thermal structure has a simple shape, and the Rmax increases not only southward but also eastward along the strike of the formations. For example, the value of Rmax in the Yokonami Melange and the surroundings increases eastwards from 1.6% at the Awa area to 2.8% at the Goshikigahama area. I believe that this thermal structure is independent of the geological structure
formed by the thermal overprint of the young oceanic plate during Eocene subduction [7]. The variation in Rmax may reflect the subsequent uneven uplift. The thrust between the block of the thermal structure is compared with the out-of-sequence thrust in the present Nankai trough [17]. An exception to this relatively simple thermal structure was identified for the Okitsu Melange which suffered much higher temperatures (Fig. 1). The Rmax value increases sharply from 2.2% to 3.1% on the boundary at the Okitsu Melange of 1 km in width. It was most likely that the Okitsu Melange was affected by a much higher geothermal gradient of >90ºC=km [7].
4. Fluid inclusions: occurrence and methods of analysis Seventeen quartz and calcite veins were sampled from the same outcrops from which the vitrinite samples were obtained in the Yokonami Melange, and two in the surrounding coherent sequence. All of the samples are composed of many small veins that are a few mm or less in width, and are parallel or sub-parallel to bedding (Fig. 2a). Some of the small syntectonic quartz veins show an en echelon structure. The quartz and calcite veins are complicated; the quartz precipitated earlier than the calcite because some quartz crystals are euhedral with a comb structure in the veins, and the calcite crystals tend to be anhedral and occur in the central part of the veins (Fig. 2b,c). Fluid inclusions within the double polished rock wafer were analyzed using the heating and cooling stage of a Linkam THM-600 with an error of š5ºC. The heating and cooling stage was calibrated by the melting temperatures of water (0ºC), dyphenylamine (54ºC), phenolphthalein (262ºC) and potassium dichromate (396ºC) standards. Two types of fluid inclusions were identified with the microscope: the one-phase fluid inclusions and the two-phase fluid inclusions (consisting of vapor and liquid at room temperature). The one-phase inclusions also look darker under the microscope, and they are segregated into two phases at temperatures below 100ºC. The one-phase inclusions occur only within the quartz veins, whereas the two-phase inclusions occur only in the calcite veins. Secondary
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Fig. 2. All of the veins are composed of many small quartz and calcite veins parallel or sub-parallel to the bedding and the C-surface of the S–C structure in the melange zone. The quartz crystals tend to be euhedral with a comb structure, and the calcite crystal tends to be anhedral (b,c).
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Fig. 3. Method of determination of P –T conditions. In the heating experiment, the internal pressure of the fluid inclusion increases along the isochore after homogenization. The P –T condition during fluid trapping was somewhere along the isochore, therefore the intersection of the isochore and maximum temperature determined from vitrinite reflectance indicates the maximum pressure of the fluid. The goethermal gradient can be limited by this method, even if the fluid was not trapped at peak heating.
two-phase type inclusions, however, locally occur within the quartz veins. The two types of fluid inclusions differ in composition; the one-phase inclusions are methane-rich fluid and the two-phase inclusions are water-rich fluid. Most of the fluid inclusions are of these two types with the exception of very minor fluid inclusions that homogenized around 40ºC and are of unknown composition. This paper examines only the methane and water inclusions to determine the P –T path of the Cretaceous Shimanto complex. The P –T conditions and geothermal gradients during fluid trapping are estimated by the homogenization temperature of the fluid inclusion and the vitrinite reflectance data. The liquid and vapor within any fluid inclusion are homogenized by heating. As shown in Fig. 3, the internal pressure of the fluid inclusion increases along the isochore after homogenization [18]. The P –T condition during fluid trapping should plot somewhere along the isochore, so the point of intersection between the isochore and the maximum temperature based on the vitrinite reflectance indicates the upper limit of
the fluid pressure (Fig. 3). Similarly, if the isochore crosses the axis of the 0ºC (minimum temperature of the seafloor), the intersection indicates the minimum limit of the fluid pressure at the trapping. The geothermal gradient during fluid trapping can be estimated from the difference of the P –T condition between the seafloor and the fluid (Fig. 3). Therefore, this is true even if the fluid had not been trapped during peak heating. The P –T condition and geothermal gradient during fluid trapping still can be limited by this method. The isochores are calculated from the basic data of Shepherd et al. [19] for waterrich fluid, and Saxena and Fei [20] for methane-rich fluid, together with the computer program of Brown and Hagemann [21]. The inclination of each isochore is also dependent upon the fluid composition and salinity. I determined these parameters by the infrared spectrum and by the melting temperature of the fluid, respectively. The Fourier-transform infrared microspectroscopy yields the molecular structural information on individual fluid inclusions larger than 50 μm in size.
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Fig. 4. Infrared spectrum of the water-rich fluid inclusion (a) and the methane-rich fluid inclusion (b). (a) The infrared radiation is absorbed in the water bands (H2 O). (b) Absorption occurs in the methane band (CH4 ) with little absorption in the H2 O and carbon dioxide (CO2 ) bands. This indicates that this type of fluid inclusion is rich in CH4 and poor in H2 O and CO2 .
Some spectral ranges are not distinguishable due to masking with strong absorption of the host mineral (Fig. 4). Unfortunately, it is a difficult task to detect carbon dioxide within the fluid inclusion in detail, because the device suffers from the effect of the gas fluctuation of carbon dioxide in the laboratory atmosphere. The spectrum indicates that the fluid inclusions of the one-phase and two-phase types are predominantly methane and water, respectively (Fig. 4), and I infer that the carbon dioxide content is low in both types of fluid inclusions. This is because the negative absorption peak was identified near the
band of the carbon dioxide. The spectrometer subtracts the spectrum data of the host mineral from the primary data. Therefore the negative absorption peak indicates that either the carbon dioxide within the inclusions are poorer than the host mineral, or poorer than the amount of the fluctuation in the laboratory during measuring. In either case, the carbon dioxide contents are very low. Moreover, the carbon dioxide bearing fluid inclusion is generally observed in the geothermal field. The liquid or clathrate of the carbon dioxide could not be recognized during cooling experiments. This means that the density of carbon
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dioxide has a maximum of 1.4 mole percentage [22] which would not effect the P –T estimation below. The salinity of each water-rich fluid inclusion can be estimated from the melting temperatures. Dissolved salt should reduce the melting point of the water, thus the salinity can be calculated when assuming that the salt is 100% NaCl. Melting temperatures of the ice within the inclusions are 1.4 to 0.2ºC corresponding to 24‰ to 30‰ concentrations for equivalent NaCl [23] indicating that the salinities are lower than that of normal sea water. Thus, the salinity and concentration of impurities in both types of fluid inclusion are low enough to estimate the isochores for a pure system.
5. Results 5.1. Temperature of homogenization (Th) of the methane- and water-rich fluid inclusions In the water-rich fluid inclusions, the Th values range from 92ºC to 239ºC and are different from location to location. In general, all fluid inclusions have similar Th values at the same location without relation to the orientation of the vein, and the fluid inclusions in the areas with higher Rmax values have higher values of Th (Fig. 1). The modal values of Th are 120ºC in the Susaki Formation, 140ºC in the Awa area of the Yokonami Melange, 180ºC in the Goshikigahama area of the Yokonami Melange, and 150ºC in the Shimotsui Formation (Fig. 1). There is a relationship between the Th and Rmax . An area with high Rmax has a higher Th value. In contrast, a clear relationship between Th and Rmax in the methane-rich fluid inclusions is not found. Modal Th values of the methane-rich fluid inclusions vary from 130ºC to 85ºC, although the peak values of the histogram range from 120ºC to 100ºC. The local differences among the Th values of the methane-rich inclusions are smaller than those of the water-rich inclusions. 5.2. The estimation of the P –T conditions and paleogeothermal gradient As mentioned above, the fluid pressure was obtained from the intersection between the isochores
and upper limits of fluid temperatures (Fig. 5 and Table 1) as estimated from Rmax , and the intersection between the isochores and the axis of the 0ºC or the saturation curve of vapor. The isochores of the methane-rich fluid crosses the axis of 0ºC, and the isochores of the water-rich fluid crosses the saturation curve of vapor. The minimum limits of the water-rich fluid pressures are not shown below, because the value of the saturation pressure of the vapor at the homogenization temperature is known. The results of the P –T conditions are shown in Table 1. The results calculated from the different equations are similar. In an extensive study, Laughland and Underwood [13] determined that the equation of Barker [14] is most accurate for the Shimanto complex. Therefore, the following discussion uses the results of Barker’s method. The maximum P –T conditions of the water-rich fluid are 190 (š30)ºC=110 (š50) MPa in the Susaki Formation, 190 (š30)ºC=95 (š50) MPa in the Awa area of the Yokonami Melange, 250 (š30)ºC=125 (š40) MPa in the Goshikigahama area of the Yokonami Melange, and 215 (š30)ºC=100 (š50) MPa in the Shimotsui Formation. On the other hand, the P –T condition during trapping of the methane-rich fluid ranges from 0ºC=90 MPa to 190 (š30)ºC=155 (š20) MPa in the Susaki Formation, 0ºC=75 MPa to 190 (š30)ºC=155 (š20) MPa in the Awa area of the Yokonami Melange, 0ºC=110 MPa to 250 (š30)ºC=260 (š20) MPa in the Goshikigahama area of the Yokonami Melange and 0ºC=75 MPa to 215 (š30)ºC=165 (š20) MPa in the Shimotsui Formation (Table 1). The trapping pressure of the methanerich fluid was higher than that of the water-rich fluid, and the Yokonami Melange and its surroundings do not show distinct differences in P –T conditions. The geothermal gradient is estimated from the difference in P –T conditions between the seafloor and the deeper horizon below the seafloor. The geopressure gradient was estimated from the inferred value of the bulk density of the sediment, the pore-fluid pressure and the ancient water depth as below. The bulk density of the strata within the present-day Nankai accretionary prism ranging from seafloor to 1300 mbsf (meter below seafloor) increases from 1.5 to 2.5 g=cm3 [24]. The bulk density below 1300 mbsf is presumed to be 2.5 g=cm3 , close to values of rocks from the Shimanto Belt (approximately 2.6 g=cm3 [25]).
A. Sakaguchi / Earth and Planetary Science Letters 173 (1999) 61–74
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Fig. 5. P –T path in the Cretaceous Yokonami Melange and surrounding strata. The CH4 -rich fluid was trapped at a pressure of 75–260 MPa with a low geothermal gradient of ¾33ºC=km. Subsequently the H2 O-rich fluid was trapped in a geothermal environment of 50ºC=km. In both stages, no P –T difference was found between the Yokonami Melange and surrounding coherent strata.
The line through the points of the upper P –T limit of the water-rich fluid cuts the 0ºC axis at ¾20 MPa (Fig. 5). This result might be explained by the pressure exerted by the ¾2000 m column of sea water over the sediment. Though the fluid pressure at a given depth below the seafloor may have been anywhere between the hydrostatic and lithostatic pressures, it is probably close to the lithostatic pressure. The pore-fluid pressure increases with burial depth and reaches >80% of lithostatic pressure in the deeper portions of the Barbados accretionary prism [26]. The pore-fluid pressure reaches >90% of lithostatic pressure near the Barbados decollement zone with a total pressure only 8 MPa [27]. Moreover, it seems likely that the vein-filled fracture opened as a result of hydrofracturing. Therefore, I presume that the pore-fluid pressure during trapping was very close to the lithostatic pressure. The error of the geothermal gradient increases ex-
ponentially with reduction of pressure; therefore, I show the two differential errors for the fluid decompression (C) and for the fluid compression (). The lower limits of the paleogeothermal gradients during water-rich fluid trapping are estimated as 45 (C31=11)ºC=km for the Susaki Formation, 54 (C60=12)ºC=km for the Awa area, 52 (C29=12)ºC=km for the Goshikigahama area of the Yokonami Melange and 52 (C40=14)ºC=km for the Shimotsui Formation (Fig. 5 and Table 1). The geothermal gradients during methane-rich fluid inclusions are estimated as 25 (C3=2) to 0ºC=km for the Susaki Formation, 30 (C4=2) to 0ºC=km for the Awa area, 24 (C1=2) to 0ºC=km for the Goshikigahama area of the Yokonami Melange and 33 (C3=4) to 0ºC=km for the Shimotsui Formation (Fig. 5 and Table 1). Clearly each type of fluid was trapped at a very different P –T condition. The water-rich fluid was
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Temp. (max)
Methane-rich fluid
Water-rich fluid
pressure (max)
depth (max)
gradient (max)
pressure (max)
depth (max)
gradient (min)
188 (š10) MPa 155 (š8) MPa 260 (š18) MPa 165 (š8) MPa
7.5 (š0.4) km 6.0 (š0.3) km 10.5 (š0.8) km 6.5 (š0.3) km
25 (C3=2)ºC=km 30 (C4=2)ºC=km 24 (C1=2)ºC=km 33 (C3=4)ºC=km
110 (š50) MPa 95 (š50) MPa 125 (š50) MPa 100 (š50) MPa
4.2 km (š2.2) 3.5 km (š2.2) 4.8 km (š2.2) 4.1 km (š2.2)
45 (C31=11)ºC=km 54 (C60=16)ºC=km 52 (C27=12)ºC=km 52 (C56=11)ºC=km
Sweeney and Burnham [15], effective time: 10 m.y. Susaki Fm. 195 (š30)ºC 154 (š10) MPa Yokonami Mel. (Awa) 195 (š30)ºC 154 (š10) MPa Yokonami Mel. (Goshiki.) 245 (š30)ºC 262 (š15) MPa Shimotsui Fm. 216 (š30)ºC 162 (š10) MPa
6.0 (š0.4) km 6.0 (š0.4) km 10.5 (š0.4) km 6.5 (š0.4) km
32 (C3=2)ºC=km 33 (C3=2)ºC=km 24 (C2=1)ºC=km 33 (C2=3)ºC=km
108 (š45) MPa 92 (š45) MPa 120 (š45) MPa 99 (š45) MPa
4.2 (š2.0) km 3.5 (š2.0) km 4.7 (š2.0) km 3.8 (š2.0) km
46 (C29=10)ºC=km 56 (C54=14)ºC=km 52 (C28=10)ºC=km 57 (C47=14)ºC=km
Sweeney and Burnham [15], effective time: 1 m.y. Susaki Fm. 210 (š30)ºC 160 (š10) MPa Yokonami Mel. (Awa) 210 (š30)ºC 160 (š10) MPa Yokonami Mel. (Goshiki.) 265 (š30)ºC 270 (š15) MPa Shimotsui Fm. 235( š30)ºC 170 (š10) MPa
6.5 (š0.4) km 6.5 (š0.4) km 11.0 (š0.4) km 7.0 (š0.4) km
33 (C3=2)ºC=km 33 (C3=2)ºC=km 24 (C1=1)ºC=km 34 (C2=2)ºC=km
135 (š45) MPa 120 (š45) MPa 145 (š45) MPa 130 (š45) MPa
5.1 (š2.0) km 4.7 (š2.0) km 5.7 (š2.0) km 5.1 (š2.0) km
41 (C22=8)ºC=km 45 (C22=8)ºC=km 47 (C15=8)ºC=km 46 (C18=8)ºC=km
Barker [14], effective time: independent Susaki Fm. 190 (š30)ºC Yokonami Mel. (Awa) 190 (š30)ºC Yokonami Mel. (Goshiki.) 250 (š30)ºC Shimotsui Fm. 215 (š30)ºC
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Table 1 List of the estimated depths and geothermal gradients during the methane-rich and water-rich fluid trapping events
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trapped at a higher geothermal gradient than the ordinary value of 30ºC=km. This high-temperature condition was not confined to a local zone around each vein. High Th values occur in areas with high Rmax values, and the paleogeothermal gradients in the both Awa and Goshikigahama areas are very similar to each other, although the inclusions were trapped under different P –T conditions. These observations suggest that the high heat flow was regional, and thermal equilibrium between the fluid and its surrounding rock was attained. In other words, the trapping of the water-rich fluid and thermal maturation of the vitrinite happened concurrently under similar thermal conditions. Furthermore, I infer that the absolute P –T condition during the water-rich fluid trapping was very close to the upper limit of the Rm data. It is more difficult to determine an accurate geothermal gradient during methane-rich fluid trapping, because maximum P –T conditions of each locality do not plot on a single line of the geothermal gradient. This might be caused by the difference of timing between the methane-rich fluid trapping and peak heating. Therefore, the P –T condition during methane-rich fluid trapping is not as strictly constrained as in the case of water-rich fluid trapping. The results suggest geothermal gradients from 0ºC=km to 33ºC=km. This range of possible geothermal gradients can be narrowed. Two areas of the same tectonic zone would have similar values of geothermal gradient. Within the Yokonami Melange zone, the upper limit of the geothermal gradient is 24ºC=km in the Goshikigahama area. This is lower than the value of 31ºC=km at the Awa area. Assuming thermal equilibrium, the lower value in the Goshikigahama area should be near to the true paleogeothermal gradient rather than the higher value in the Awa area. Additionally, a geothermal gradient of <5ºC=km is unknown anywhere. Thus, the paleogeothermal gradient during the methane-rich fluid trapping is probably in the range of 5ºC=km to 24ºC=km with an error of 3ºC=km.
6. Discussion The methane-rich fluid was trapped in quartz before the calcite crystals trapped the water-rich fluid,
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because textural evidence indicates that the quartz crystals formed first (Fig. 2b). Furthermore, the calcite crystals have only the water-rich fluid inclusions, and quartz crystals have primary methane-rich fluid inclusions and secondary water-rich fluid inclusions. This indicates that the fluid was water-rich during calcite precipitation, and water-rich fluid healed cracks in the existing quartz crystals. Judging from the relationship between the Th for water and Rmax , the timing of the water-rich fluid trapping was close to the timing of the peak heating. The interpreted P –T path is shown in Fig. 5. During the first stage of fluid trapping, the rock reached the highest pressures of <260 MPa at depths shallower than approximately 10.5 km. During the second stage of fluid trapping, the geothermal gradient increased to <50ºC=km with pressures of 95– 125 MPa at depths shallower than approximately 3.5–5.0 km. This result is consistent with the idea that the thermal overprint occurred after the juxtaposition of tectono-stratigraphic units within the complex [7]. It is likely that the thermal development took place with the change of the P –T conditions of the subducting oceanic plate. Higher-grade metamorphic rocks may undergo two stages of metamorphism during passage through the inverted isotherm region deep in the subduction zone. These rocks may suffer different geothermal gradients without any change of the thermal condition of the slab. However, in the lower-grade Shimanto example discussed here, all the coherent formations and melanges, including forearc basin-fill sediment, show the same thermal structure. In such a shallow-level accretionary prism, the heat from the oceanic lithosphere is probably the most influential factor for the thermal structure [28] as compared to factors such as friction across the plate boundary [29] and cooling by the old plate subduction [30]. Zircon fission track dates indicate that the whole Cretaceous Shimanto accretionary complex cooled below 230ºC after 70 Ma [31,32]. This cooling age may have occurred by episodic exhumation in the whole Cretaceous Shimanto complex; however, this interpretation disagrees with the uplift history of forearc basin strata based on foraminifera assemblage data. The Doganaro Formation, for example, overlying the northern Shinjogawa Group, was deposited above the CCD during Aptian time, and the
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Fig. 6. Thermal history in the Cretaceous Shimanto accretionary prism. Initially, the rocks were buried to 7–10 km depth under a geothermal gradient of ¾24ºC=km; subsequently the rock reached a peak temperature of 190 to 270ºC at 3–5 km depth under a geothermal gradient of 50ºC=km. The paleoheat flow of both stages corresponds with the inferred heat flow, based on the age of the subducting oceanic slab.
Uwajima Group, overlying the southern Shinjogawa Group, was deposited above the CCD during Coniacian time [9]. These data indicate that the Lower Cretaceous basement of the Shinjogawa Group was already placed at the CCD level by mid-Cretaceous time, and it was exhumed throughout the Cretaceous. This is not a local uplift history because the Doganaro Formation is scattered along the BTL through Shikoku. I deduce that the episodic cooling shown by fission track dates was caused by a single regional thermal overprint. Other dates of post-Cretaceous age have been reported such as the K–Ar age of 55 Ma [33] in the Okitsu Melange. That melange is characterized by a lack of pelagic sediment, and the rocks suffered much higher geothermal gradients of over 90ºC=km [7]. This age is almost coincident with the timing of the close approach between the Kula–Pacific ridge and the Shimanto accretionary prism as the subduction margin of Japan from latest Cretaceous to
Eocene [8,34,35]. It is likely that the water-rich fluid trapping occurred during this peak heating event in the Cretaceous Shimanto complex (Fig. 6). Although the whole Cretaceous complex probably suffered the effects of high temperature, the Okitsu Melange underwent a much higher geothermal gradient of 90ºC=km. This local effect might have been caused by its location within the accretionary prism. The high geothermal gradient may have been produced by either the warm fluid passing through the Okitsu Melange similar to the Barbados decollement zone, or due to the thermal gradient becoming higher in the frontal portion of the accretionary prism because of the young plate subduction. The present-day subduction setting and high heat flow of 50–130 mW=m2 in the frontal portion of the present Nankai accretionary prism are very similar to what has been inferred for the Cretaceous Shimanto complex, though the subducting slab is as old as 15 Ma [24,36,37].
A. Sakaguchi / Earth and Planetary Science Letters 173 (1999) 61–74
As mentioned above, the occurrence of a thermal imprint after formation of the tectonostratigraphic architecture might have occurred, because of a ridge– trench collision or very young plate subduction beneath the Shimanto accretionary prism. In view of the heat flow, let us then consider the thermal relation between the slab and the accretionary prism. The paleoheat flow can be estimated from the thermal conductivity of the sedimentary rock and paleogeothermal gradient. I assume a thermal conductivity value of 2.2 W m1 ºC1 , which is equivalent to the deepest sequence in the Nankai prism [24]. Consequently, the calculated heat flow during the stage of the water-rich fluid trapping is 95 (C70=20) to 120 (C110=40) mW=m2 . On the other hand, the calculated heat flow corresponding to the stage of the methane-rich fluid trapping is 11–53 mW=m2 . The heat flow of the subducting oceanic crust can be estimated roughly from the age of the plate [38]. The age difference between the limestone block overlying the basalt and black shale matrix within the Yokonami Melange is 60–70 Ma [24]. If the basalt was of the MORB-type, then the heat flow should have been approximately 57–61 mW=m2 by the equation of Parsons and Sclater [38]. However, MORB-type basalt blocks within the melange are rare in Japan, and most of the rocks are of the OIBtype [10,11]. Therefore, there is a possibility that this age is younger than the true age of the plate, and the heat flow of the slab becomes even lower. In this case, the heat flow estimation of the slab becomes closer to the prism’s heat flow at the stage of the methane-rich fluid trapping. This indicates that the thermal condition of the slab definitively affects the heat flow in the accretionary prism.
7. Conclusions The P –T history of the Cretaceous Shimanto complex from the Yokonami Melange and surrounding strata records at least two distinct stages of metamorphism. Initially, these rocks were buried to 6.5–10.5 km depth in methane-rich fluids and under a geothermal gradient of ¾24ºC=km. Subsequently, they reached peak temperatures of 190 to 270ºC at depths of 3.5–5.0 km in water-rich fluids under a higher geothermal gradient of approximately
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50ºC=km. The overall thermal peak was reached during the second stage. At that time, the rocks were in thermal equilibrium with the fluid. This peak heating might have occurred when the Kula–Pacific ridge was subducted during the Eocene. During both stages, the complex has a heat flow value as expected from the subducting oceanic crust. The metamorphism and heat flow in this accretionary complex are controlled largely by the thermal condition of the subducting slab.
Acknowledgements I thank Professor Yujiro Ogawa of the University of Tsukuba for reading the entire text in its original form and making helpful suggestions throughout the work. Professors Atsuo Aihara and Tatsushi Murae of Kyushu University greatly assisted with the vitrinite reflectance and Fourier-transform infrared microspectroscopic studies. Dr. Mamoru Enjoji of the University of Tsukuba provided guidance for fluid inclusion analysis. Professor J. Casey Moore of the University of California and Associate Professor Wonn Soh of Kyushu University assisted in revision of the manuscript. Professor Michael B. Underwood of the University of Missouri, Professor Tim Byrne of the University of Connecticut and Dr. Peter Vrolijk reviewed the manuscript and provided critical comments. [MK]
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