Topography characteristics and the origination of valleys at the upper edge of continental slope in the northern East China Sea
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摘要:
东海陆坡上缘区处于东海陆架与冲绳海槽两大地貌单元的过渡带，对其地形特征及成因的研究，对于深入理解两单元相互作用和晚更新世晚期的海平面变化有重要意义。多波束测深和浅地层剖面调查揭示了东海北部陆坡上缘沟，平均距离陆架坡折线约7 km，走向为NNE-SSW，分为单沟和双沟（局部），宽约400～
1600 m，沟深一般为20～25 m，单沟的等深线顺直，沟底地形平缓，双沟的地形比较复杂，发育顺坡次级沟、沟底频繁起伏等。浅地层剖面揭示上部10～15 m为向东倾斜的前积反射结构，属于晚更新世晚期的古长江三角洲前积层。静力触探试验表明海底表层为松散的粉砂质砂，往下3 m为粉砂/黏土与砂的互层。侧扫声呐影像显示海底沙波发育，波高约0.5 m，波长6～9 m。推断陆坡上缘沟的位置和走向受陆坡正断层的影响；结合沉积物14C测年和海平面变化曲线，推测沟主要形成于17.4～14.5 kaBP，也就是末次盛冰期海平面回升的初期阶段，为滨浅海环境，受潮流的冲刷侵蚀所致，为断裂冲沟；双沟内复杂的海底地形与滑坡有关。实测200 m等深线主要位于陆坡上缘沟内，北部处于沟的东侧附近，与沟相交；两者走向相近，与区域构造的影响有关。Abstract:The upper edge of the continental slope of the East China Sea is a transitional zone between two main geomorphological units: the continental shelf of the sea and Okinawa Trough. Research into its geomorphological characteristics and origination is meaningful for better understanding the interaction between these two units, and the sea level changes during the late period of Late Pleistocene. Multibeam sounding and subbottom profile surveys revealed sea bottom valleys at the upper edge of the continental slope in the northern East China Sea. The valleys extend in NNE-SSW direction in quasi-parallel to the continental shelf break with an average distance of about 7 km. The depth of the slope break is about 150 m on average. It is also generally parallel to normal faults in the continental slope. They can be subdivided into single valleys and locally double-valleys. The later ones consist of a main west valley and an auxiliary east valley with an interval about 2km. All the valleys have a higher west slope and a lower east one, 400～1600 m wide and 20～25 m deep in general. The single valleys have straight depth contours and gentle bottom topography, while the double-valleys have rather complicated topography, with minor gullies cutting down valley slopes and frequently uneven bottom and so on. Subbottom profiles across the west valley show the upper 10～15m strata having parallel reflection waves, forming an eastward-dipping structure, which is the paleo-Changjiang prodelta of the late period of Late Pleistocene. Cone penetration pest (CPT) measurements of 3m deep at 170 m depth west of the west valley are interpreted as surficial loose silty sands, and interbedded silt/clay with sands downward. Side-scan sonar images near the CPT site showed sea bottom sandwaves with amplitude about 0.5 m and wave length of 6～9 m. It is inferred that the location and layout of the valleys were affected by normal faults in the continental slope related to the third phase of the Okinawa Trough expansion. Taking into account the shell 14C dating of nearby borehole samples and sea level change curves, the valleys were formed mainly during 17.4～14.5 kaBP, which is the early stage of sea level rising after the last glacial maximum (LGM), when the area was in offshore-shallow sea environment and underwent tidal current scouring. Therefore, these valleys are fault-scouring valleys. The complicated topography of the double-valleys was related to submarine landslides of paleo-Changjiang predelta deposits. The measured 200 m depth contour lines of sea bottom are mostly within the west valley but partly to the east of the valley, resulting in the intersection of each other; and their similar stretching directions were considered to be resulted from the regional tectonic framework.
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图 3 典型海底地形断面
位置见图2。
Figure 3. Typical sections of sea bottom topography
Locations are shown in Fig.2, hereinafter.
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图 4 多波束测深显示的沟的地形简图
a、b： 单沟，c： 双沟，d： 西沟局部放大。
Figure 4. Simplified topography of valleys interpreted by data of multibeam sounding
a and b: Single valleys, c: double-valleys, d: enlarged part of the west valley of double-valleys.
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图 6 静力触探试验记录曲线
a：锥尖阻力， b：不排水抗剪强度。
Figure 6. CPT record curves
a: Tip resistance, b: undrained shear strength.
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