Abstract: The Huashan piedmont fault, forming a part of the southern margin of the Weihe graben, is one of the important normal faults that control the subsidence of the intracontinental rift. Developing on the footwall of the fault, the Huashan block has experienced rapid cooling during the Cenozoic, especially since the early-middle Miocene. Mountain exhumation causes and transports a great amount of sediments to the adjacent hanging wall, setting a typical case of mountain-basin coupling system. Studies on active tectonics, historical and paleo earthquakes and field investigations reveal that the middle section(Huaxian-Huayin)of the fault is much more active than the west(Lantian-Huaxian)and east(Huayin-Lingbao)sections.
    We extracted channel profiles of rivers that originate from the main water divide of the northern flank of the Huashan Mountain. Based on the method of slope-area analysis and the integral approach, we identified knickpoints, calculated channel concavity and steepness indices, and constructed paleo river profiles. Of most rivers, the concavities are within a relatively narrow range of 0.3~0.6, with no obvious correlation with tectonics. However, channel steepness and knickpoint distribution vary spatially. In the east section, rivers are under steady-state with smooth, concave-up channels and lower steepness((104±30)m^0.9). In the other two sections, rivers are mainly under transient state with slope-break knickpoints. For the channel segments below knickpoints, steepness indices are much higher in the middle section((230±92)m^0.9)than in the west((152±53)m^0.9). Thus, the variance of fault activity can be reflected by channel steepness pattern. Above the knickpoints, channel steepness indices are much lower(middle(103±23)m^0.9, west(60±14)m^0.9). What's more, we found a statistically significant power-law scaling between knickpoint retreat distance and catchment drainage area. Thus, we attributed these knickpoints to be the results of recent rapid uplift of the Huashan block. The relief of paleo channels(middle(1000±153)m, west(751±170)m)accounts for~60%~80% of the relief of modern rivers(middle(1323±249)m, west(1057±231)m), which means that ~20%~40% of modern channel relief was caused by the episode of the rapid uplift. Assuming a balance between the rates of rock uplift and downstream river incision, a power-law function between uplift rates and channel steepness can be derived. According to the fault throw rates of the middle section 1.5~3mm/a(since late Pleistocene), we constrained slope exponent n~0.5 and channel erodibility K~1.5×10^-4m^0.55/a. Combining the knickpoint age formula, we estimated that the rapid mountain uplift/fault throw began at ~(0.55±0.25)Ma BP. Therefore, the middle of the Huashan piedmont fault is more active than the west and east sections. The fast fault throw of the west and middle sections since the middle Pleistocene has caused rapid mountain uplift and high topographic relief.

Key words: stream-power incision model, knickpoint, paleo channel projection, steepness index, Huashan

摘要： 华山山前正断层是渭河地堑南缘重要的控盆断裂, 其强烈的构造活动造成了华山的掀斜式隆升。 研究表明, 该断裂的活动性存在显著的空间差异: 中段(华县—华阴)最强, 西段(蓝田—华县)和东段(华阴—灵宝)相对较弱。 文中通过提取华山山前河流的高程剖面, 结合坡度-面积分析和积分法识别裂点, 计算河流凹度和陡峭系数, 并对古河道进行投影重建。 结果表明, 河流凹度大多为0.3~0.6, 基本不受构造活动的影响。 陡峭系数与裂点分布的空间差异显著: 在断裂东段, 河流处于稳态, 陡峭系数最低为(104±30)m^0.9。 断裂中、 西2段的河流多为瞬态, 发育slope-break型裂点。 裂点下游河道陡峭系数中断裂中段最高((230±92)m^0.9), 西段次之((152±53)m^0.9)。 结合区域构造、 岩性和河流沉积等, 分析认为河流陡峭系数的分布指示了断裂活动性的空间差异, 与活动构造研究结果基本一致。 裂点上游陡峭系数(断裂中段为(103±23)m^0.9, 西段为(60±14)m^0.9)明显低于下游, 并且裂点迁移距离与流域汇水面积存在良好的幂律关系, 因此, 河流裂点是正断活动增强、 山体加速隆升的标志。 古河道重建结果表明, 河道古起伏量(中段为(1 000±153)m, 西段为(751±170)m)约占现今起伏量(中段为(1 323±249)m, 西段为(1 057±231)m)的66%~75%, 因而现今地形起伏量的25%~34%是裂点所指示的这一期隆升加速造成的。 假设裂点下游的河流下切速率与山体隆升相平衡, 则河道陡峭系数与断裂滑动速率满足幂律关系。 根据断裂中段河流陡峭系数与前人研究得到的晚更新世晚期以来断裂的滑动速率, 且文中约束的坡度指数n约为0.5、 侵蚀系数K约为1.52×10^-4 m^0.55/a, 结合裂点年龄公式, 估算裂点的起始时限约(0.55±0.25)MaBP。 文中对华山山前河流高程剖面的研究表明, 华山山前正断层中段的活动性强于西段和东段, 而且中、 西2段在中更新世开始加速活动, 并导致山体快速抬升和山前的地形起伏显著增加。

关键词: 水力侵蚀模型, 裂点, 古河道投影, 陡峭系数, 华山