James H. Knapp

Mantle earthquakes in the absence of subduction? Continental delamination in the Romanian Carpathians

The Vrancea seismogenic zone of Romania is a steeply NW-dipping volume (30 × 70 × 200 km) of intermediate-depth seismicity in the upper mantle beneath the bend zone of the Eastern Carpathians. It is widely held that the source of this seismicity is the remnant of a Miocene-age subduction zone. However, recent deep seismic-reflection data collected over the Eastern Carpathian bend zone image an orogen lacking (1) a crustal root and (2) dipping crustal-scale fabrics routinely imaged in modern and ancient subduction zones. Here, we use these data to evaluate the lithospheric structure of the Eastern Carpathians as it relates to the Vrancea seismogenic zone. Crustal architecture obtained from these data indicate the 140-km-wide orogen is only supported by ∼33-km-thick crust, while the adjacent Transylvanian and Focsani basins have ∼37- (possibly up to ∼46 km) and 42-km-thick crust, respectively. Because the Vrancea seismogenic zone is located beneath the east side of the thin orogenic crust, we infer that the lower orogenic crust was removed through continental delamination and is now represented by the mantle seismicity observed in the Vrancea seismogenic zone. These data and their interpretation suggest an alternate means of generating mantle seismicity in the absence of subduction processes.

Mapping fine-scale heterogeneities within the continental mantle lithosphere beneath Scotland: Combining active- and passive-source seismology

We analyze new observational evidence for seismic velocity discontinuities in teleseismic receiver functions in comparison to well-documented discontinuities observed in marine reflection profiles and wide-angle reflection-refraction profiles in northern Scotland. Our study establishes the viability of mapping small amplitude P to S (Ps) converted phase arrivals from the upper mantle generated in the P-wave coda of teleseismic earthquakes using well-known receiver function methods. Teleseismic earthquakes recorded by a small array of portable broadband stations and permanent short-period stations are used to ascertain the lateral extent of velocity discontinuities within the continental mantle lithosphere beneath Scotland. Radial receiver functions contain distinct Ps converted phases at ∼3.1–3.2 s (Moho) and at 4.5–5.2 s (upper mantle depths) after the direct P wave. We suggest that the upper mantle Ps phase originates from a high-velocity and/or anisotropic layer within the upper mantle. At two stations, ORE and BACA, located along the northern shoreline of Scotland, these upper mantle phases can be correlated with the W reflector, a bright, regionally extensive seismic reflector previously observed on marine deep seismic reflection and wide-angle refraction-reflection profiles. However, the variability in physical characteristics (depth, thickness, azimuthal variations in the vertical structure, velocity contrast, and anisotropic properties) suggests the possibility that there may be multiple layered reflectors in the upper mantle beneath northern Scotland and raises doubt about the global significance of some of these reflectors.

Fine crustal structure beneath the junction of the southwest Tian Shan and Tarim Basin, NW China

The geometry of the entire crust from the northern part of the Tarim Basin to the southwestern Tian Shan east of Kashi is imaged on a N-S–directed explosive-source deep seismic-reflection profile. The profile reflects the sedimentary formations in the northern part of the Tarim Basin and the fold-and-thrust belt of the southern Tian Shan. N-dipping reflectors of the lower crust, as well as fluctuations in Moho depth, below which several mantle reflectors were observed, reveal the fine crustal structure beneath the junction of the southwest Tian Shan and the Tarim Basin. Mesozoic–Cenozoic shortening of the southwestern Tian Shan occurred at a crustal scale involving detachment-related folding in the basin directed northward toward the mountains and reverse faulting in the mountains directed toward the basin. In addition, a crocodile fabric developed within the lower crust beneath the basin area. The lithospheric structure revealed by the seismic-reflection section between the Tarim Basin and the Tian Shan Mountains reflects a process of intracontinental collision.

Preserved extent of Jurassic flood basalt in the South Georgia Rift: A new interpretation of the J horizon

At the end of the Triassic, ∼200 m.y. ago, the Central Atlantic Magmatic Province (CAMP), one of the largest igneous provinces in the world, was emplaced within a very short period of time. The flows, sills, and dikes that mark the event are predominantly preserved in Triassic rift basins along the Atlantic margins. Conventional wisdom implies that the areally largest of the CAMP flows is preserved in the South Georgia Rift, a Triassic rift basin buried beneath the Atlantic Coastal Plain. The extent of this flow has been mapped on the basis of a prominent seismic reflection referred to as the J horizon. This seismic horizon has been used as a time marker for estimating the end of rifting in the southern United States and the beginning of seafloor spreading. Reanalysis of existing well and seismic data, however, shows that the extent of the flood basalt is limited to a few areas, and that the J horizon coincides with the base of the Coastal Plain. This reopens the question of how the CAMP relates to the rift-drift transition of eastern North America.

西南天山—塔里木盆地结合带浅深构造关系———深地震反射剖面的初步揭露

盆山结合部的浅-深结构样式是进行陆内造山动力学研究与讨论的重要依据.2007年，在喀什东的天山与 塔里木盆地之间的过渡带上，完成了一条近南北向的长度为121km的主动源深地震反射剖面，显示出盆山结合部 现今地壳尺度的构造格架.剖面南部呈现出10?12 km巨厚的沉积盖层，沉积盖层内发育滑脱断层；盆山结合部多 排隆起构造以及天山山前上地壳显现出向北倾斜的断裂与地表地质观察吻合；盆山结合带展现出滑脱与逆冲推覆 构造相关的断层褶皱；与塔里木盆地稳定沉积层相比，在南天山浅、中层地层受到强烈的变形改造，导致地层比较 破碎，反射变弱、连续性较差；时间剖面上可以追踪到比较连续的Moho反射，从南向北有加深的趋势.深地震反射 剖面揭露出的西南天山与塔里木盆地的这些浅-深构造，展现出塔里木盆地盖层向南天山滑脱与南天山向塔里木 盆地逆冲推覆的特征，反映出陆内汇聚下的盆山耦合关系.