沉积速率与基底蓄水层流体活动对冲绳海槽海底热流值的影响

INFLUENCES OF SEDIMENTATION RATE AND FLUID ACTIVITIES IN BASEMENT AQUIFERS ON SEAFLOOR HEAT FLOW IN OKINAWA TROUGH

冲绳海槽海底热流值高且离散性强、热液活动发育,这一地热特征主要表现在海槽南段与中段,其形成与沉积过程和浅层基底水热活动密切相关。首先依据居里点深度资料计算海底热流背景值,并与实测热流资料进行对比,分析了冲绳海槽的海底热流异常特征。然后选取一条与海槽扩张中心走向一致的模拟剖面,以实测地热资料为约束,利用热模拟的方法,探讨了沉积速率和基底流体活动对海底热流值的影响。最后,探讨了海槽热流特征形成机制。热模拟结果表明,如果南段与中、北段分别在中中新世和晚更新世开始拉张,在冲绳海槽的形成演化过程中,沉积速率可使海槽南段和中段的海底热流值分别降低7%~16%和4%~12%,沉积速率越大对热流值的影响也越大;在基底流体活动的影响下,海槽热流值表现出极大的离散性,下降流体使得海底热流值进一步降低,上升流体作用则相反,海槽中段的上升流体影响远远超过沉积速率影响,使得局部海底热流值提高到背景值的3倍。在局部上升流体与下降流体运移速率相同的前提下,当沉积速率<25cm/ka、流体运移速率为60~66cm/a时最符合海槽南段西部的热异常形成条件;当宫古断裂及海槽中段基底蓄水层内的流体运移速率约为南段流体运移速率的4~6倍,或者蓄水层之下热流背景值约为南段背景值的2~3倍时,最符合宫古断裂带附近和海槽中段的热流异常形成条件,这可能预示着在<10km深度处存在岩浆房。

The Okinawa Trough has high but variable mean heat flow values and extensive hydrothermal activities are developed there,especially in the south and north segments of the trough.These geothermal characteristics are closely related to the past and present sedimentation process and fluid activities in the shallow basement aquifers.This paper firstly calculated the background seafloor heat flow values on the basis of Curie Point Depth data,and compared them with the measured data to analyze the features of heat flow anomalies in the Okinawa Trough.Then,we selected a modeling profile along the spreading center and used the thermal modeling method to explore the influences of sedimentation rate and basement fluid activity on the seafloor heat flow.At last,we analyzed and discussed the possible genetic mechanisms of geothermal features.Modeling results indicate that,if extensional processes start at the middle Miocene for the South Okinawa Trough（SOT）and at the late Pleistocene for the Middle Okinawa Trough（MOT）and the North Okinawa Trough（NOT）,the present seafloor heat flow would be reduced by 7%~16%for SOT and 4%~12%for MOT with the influence of sediment process;the larger the sedimentation rate is,the greater the effect will be produced.Fluid activities in the basement aquifers brought about the significantly scattered heat flow values,which were reduced above the upward flow and increased above the downward flow.With more influence of upward flow than that of sediment process,some values were increased up to 3times of background values.On the premise of an equal fluid migration rate for upward flow and downward flow,a sedimentation rate of 25cm/a and a fluid migration rate of 60~66cm/aqualify the formation condition of geothermal anomalies in the western SOT.While for the formation of geothermal features nearby Gonggu fracture zone and in the MOT,it requires a migration rate of about 4~6times or a heat supply（below basement aquifers）of about 2~3times relative to that of SOT.The latter implies the existence of a magma chamber shallower than 10 km.