Precise P and S wave velocity structures in the Kitakami Massif, Northern Honshu, Japan, from a seismic refraction experiment
Precise P and S wave velocity structures in the Kitakami Massif, Northern Honshu, Japan, from a seismic refraction experiment
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日本本州北部北上地块通过地震折射实验获得的精确 P 波和 S 波速度结构
DOI：
10.1029/94jb00732
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发表时间：
1994
影响因子：
--
通讯作者：
T. Masuda
中科院分区：
文献类型：
--
作者：
T. Iwasaki;T. Yoshii;T. Moriya;A. Kobayashi;M. Nishiwaki;T. Tsutsui;T. Iidaka;A. Ikami;T. Masuda
位于日本本州北部东部的北上地块由两个地质单元组成。北部北上地体为侏罗纪增生杂岩，南部北上地体由前志留系基底和志留系-下白垩统海相沉积组成。这两个单元的边界区域被称为Hayachine构造带(HTB)，由镁铁质到超镁铁质岩石组成。北上地块在白垩纪经历了强烈的花岗岩侵入。我们给出了该地块东部的详细地壳结构模型，该模型是在一条194公里长的北S线上进行的广泛的地震折射实验得出的。最上地壳覆盖着一层很薄的表层(0.5-1公里)，速度为3.1-5.4公里/S，其下的速度结构具有明显的横向变化。北上地体北部基底顶部的P波速度为5.85~5.95公里/S，波速比为1.68~1.70。该地区地震衰减大(Qp=150-200，Qs=70-100)。相比之下，北上地体南部地壳具有高的P波速度(6.05-6.15公里/S)和Vp/VS(1.74-1.77)的特征。Qp和Qs也分别表现出300-400和150-200的高值。这种构造差异持续到14~16公里深度，P波速度增加到6.45公里/S。北上地体北部的低速度和高衰减代表了增生杂岩的高度变形结构。北上地体南部较高的P波速度和Vp/Vs值指示了相对镁铁质的地壳成分，这可能是侏罗纪末期-早白垩世洋壳拼合过程中洋壳碎屑或隆升的结果。由广角反射确定的中地壳界面显示，在HTB之下，向南的深度从25公里骤降到20公里。14~16公里深度和中地壳界面的P波速度和波速比分别为6.45~6.55公里/S和1.74~1.78。北上地体北部的莫霍面深度从34公里向南减小到32公里。在北上地体南部，莫霍面略微向南倾斜。下地壳P波速度为6.9~7.0公里/S，波速比为1.75~1.76。最上地幔的P波速度分辨率不高，但可能小于7.7公里/S，由较清晰的锡测得的S波速度为4.35-4.40公里/S，结果表明，HTB是延伸到莫霍面的一个显著的构造边界。北上地块的地壳未被白垩纪花岗岩侵入体均一化，原构造差异仍存在于上地壳。
The Kitakami massif, which is located in the eastern part of Northern Honshu, Japan, is composed of two geological units. The northern Kitakami terrane is characterized as a Jurassic accretionary complex, while the southern Kitakami terrane consists of pre-Silurian basement and Silurian-lower Cretaceous marine sediments. The boundary region of these two units, called the Hayachine tectonic belt (HTB), is composed of mafic to ultramafic rocks. The Kitakami massif experienced intense granitic intrusions in the Cretaceous. We present a detailed crustal structure model for the eastern part of the massif derived from an extensive seismic refraction experiment conducted on a 194-km N-S line. The uppermost crust is covered with a very thin (0.5–1 km) surface layer with a velocity of 3.1–5.4 km/s. The velocity structure below this layer shows remarkable lateral variation. In the northern Kitakami terrane the P wave velocity and Vp/Vs at the top of the basement are 5.85–5.95 km/s and 1.68–1.70, respectively. The seismic attenuation in this region is high (Qp = 150–200 and Qs = 70–100). In contrast, the uppermost crust in the southern Kitakami terrane is characterized by a high P wave velocity (6.05–6.15 km/s) and Vp/Vs (1.74–1.77). The Qp and Qs also show high values of 300–400 and 150–200, respectively. Such a structural difference persists to 14-to 16-km depth, at which the P wave velocity increases to 6.45 km/s. The low velocity and high attenuation in the northern Kitakami terrane represent a highly deformed structure of the accretionary complex. The high P wave velocity and Vp/Vs in the southern Kitakami terrane indicate the relatively mafic crustal composition, which may result from the fragment of the oceanic crust incorporated by the accretion process or the uplifting in the latest Jurassic-early Cretaceous. A midcrustal interface determined from wide-angle reflections shows an abrupt southward depth decrease from 25 to 20 km under the HTB. The P wave velocity and Vp/Vs between 14- and 16-km depth and the midcrustal interface are 6.45–6.55 km/s and 1.74–1.78, respectively. The Moho depth under the northern Kitakami terrane decreases southward from 34 to 32 km. In the southern Kitakami terrane the Moho dips slightly southward. The P wave velocity and the Vp/Vs ratio in the lower crust are 6.9–7.0 km/s and 1.75–1.76, respectively. The P wave velocity in the uppermost mantle is not well resolved but is probably less than 7.7 km/s. The S wave velocity derived from relatively clear Sn is 4.35–4.40 km/s. Our results show that the HTB is a prominent structural boundary extending to the Moho. The crust of Kitakami massif was not homogenized by the Cretaceous granitic intrusions, and the original structural difference remains in the upper crust.