忻定盆地第四纪断块活动分期研究
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摘要
忻定盆地第四纪断块活动分期研究
忻定盆地位于汾渭地堑系北段，是一个典型的新生代张性断陷盆地，其边界由北东向五台山北麓断裂、系舟山北麓断裂、恒山南麓断裂和北北东向云中山东麓断裂所围限，其中五台山北麓断裂与系舟山北麓断裂垂直错动强度较大，控制了盆地的主体地貌格局。盆地四周由持续隆升的断块山地围绕，五台山位于其东侧，最高峰北台顶海拔高度3058m，是华北地区的屋脊和著名的北台期夷平面命名地。断块山地向盆地一侧自北台期夷平面之下发育了3级夷平面和7级河流阶地，指示了多期隆升作用。忻定盆地有史以来发生了3次7级以上地震，研究地震活动有益于精细地刻画断块分期活动过程。
论文以忻定盆地周围山麓地带的层状地貌为主线，以五台山与系舟山北麓为主要研究地区，通过河流阶地与麓原面的共生关系对第四纪断块活动进行分期，通过阶地断代以及河流纵剖面的定量分析，对1.2Ma以来断块活动分期进行断代。在此基础上，进一步讨论了断块山地阶段性隆升的原因，以及晚第四纪以来断块分期活动的地震表现。取得的主要进展与认识如下：
1．提出了断层阶段性活动作用下麓原面与河流地貌的共生演化模式
在断块山地的山麓地带，同一期构造运动形成的麓原面与构造阶地有一定的共生关系。根据经典的断层崖演化理论，断块隆升形成断层崖；随后，断层崖上的冲沟下切侵蚀形成“V”字型峡谷；进入构造平静期后，沟床展宽形成“U”字型宽谷，同时断层崖遭受剥蚀而坡度降低，如果平静期足够长，断层崖后退形成麓原面；受下降盘盆地地表这一共同侵蚀基准面的控制，麓原面与宽谷谷底在山麓地带处在大致相同的高度；后期的断块隆升形成新的断层崖，原先的麓原面和宽谷被抬升，冲沟从断层崖开始溯源侵蚀形成阶地。在山麓地带，阶地与麓原面联合成一级地貌面，二者在空间上连续、在高度上可比。多个构造旋迴可以形成一系列由河流阶地和麓原面组成的多级联合地貌面。因此，可以通过联合地貌面的级数确定断块隆升的期次，并通过阶地的年代推断断块隆升的起始时间。
如果把断层活跃期理解为地震断层活动的丛集期，数十米的陡坎是多次地震位错的累积，而平静期理解为断层活动强度与频度的明显减弱，则在河流纵剖面上，阶地消失点下游会形成由一系列裂点组成的急流段，河床坡度较陡，河谷狭窄，而阶地消失点上游为缓流段，河床坡度较缓，是稳定期形成的宽谷地貌。急流段代表断层活跃期，缓流段代表稳定期。如果相邻两级阶地之间各裂点的后退速度是一致的，则可以根据急流段与缓流段的长度估算活跃期与平静期的持续时间。缓流段时间跨度T_(Si)=(T_i-T_(i-1))L_(Si)／(L_(si)+L_(Ri))，急流段时间跨度T_(Ri)=(T_i-T_(i-1))L_(Ri)／(L_(Ri)+L_(Si))。其中i为阶地级次，从低向高编号，Ti为第i级阶地的年代，L_(Ri)与L_(Si)分别代表第i级与i-1级阶地之间急流段与缓流段的长度。这样，我们可以得到断块活跃期或稳定期的起始年代与结束年代。
物质组成是制约断层崖演化的主要因素之一。松散沉积物构成的断层崖退化速度较快，地震周期的断层活动通常可留下易辨的复合断层崖与河流阶地等层状地貌。基岩断层崖则不然，地震周期内断层崖退化不明显，河床展宽也不明显，周期性地震断错仅会在冲沟中留下一系列跌水，但通常难以形成阶地系列。在坚硬的基岩山区，那些数十米乃至数百米高的复合断层崖和基座阶地形成于时间跨度更长的构造活跃期与平静期的交替，每一个构造活跃期由一系列丛集的地震位错事件组成，在上升盘的河谷中形成由一系列裂点构成的急流段和狭谷地貌，而构造平静期对应宽谷地貌与缓流段。河流溯源侵蚀古宽谷地貌形成阶地。
第四纪气候变化也是制约上述模式的主要因素之一。第一，河流对山地隆升作用的响应关系受到气候变化的制约。冰期河床的加积效应会使得河流对断块隆升的响应滞后，而间冰期的河流通常处于侵蚀状态或均衡状态，对断块隆升会有及时的下切响应。第二，气候变化可直接形成阶地。气候的周期性变化使河流发生周期性的堆积与侵蚀，会形成一系列阶地。如果山地处于稳定状态，每一次河流下切的深度变化不大，这些阶地的高度大致相当，构不成阶梯状地貌面。第三，气候变化影响裂点的后退速度。相邻阶地之间的裂点如果形成于不同气候期，各裂点之间的距离与形成的时间差之间并非线性相关，因此，用急流段与缓流段的长度估算构造期持续时间会造成一定误差。第四，气候周期性变化导致山盆地壳均衡升降效应，可能引发或触发断块山地的阶段性隆升。
第四纪气候的周期性变化加速了山地的侵蚀过程以及盆地的堆积作用，剥露作用导致的地壳均衡回弹效应成为山地隆升的驱动机制之一。间冰期，河流流量加大，山区遭受侵蚀，均衡作用导致地壳上隆；与此同时，盆地接受堆积，均衡作用导致地壳下沉。应变能的积累有利于盆地边界断裂发生倾滑错动。进入气候相对干冷的冰期，山区物质的外泄量明显减小，地壳均衡升降效应相应减弱。如果这一假设成立，气候周期性变化有可能引发断块山地的阶段性隆升。与区域构造机制或许不同的是，气候变化导致的山地隆升过程发生在间冰期，且始于间冰期的初期。
另外，尚有一些其它因素，使得上述模式复杂化。譬如，冲沟的发展会破坏麓原面在某些地段的完整性，而差异侵蚀作用或阶梯状断裂组合会在山麓地带形成同期多级地貌面，造成假麓原面地貌。岩性的差异、河床的堰塞会形成一些非构造成因裂点。支流的汇入使得下游主流河段的流量增加，裂点后退速度加快。因此，要合理的应用上述模式，需要在野外开展详细的地貌填图，仔细甄别。调查的冲沟尽可能多，以排除岩性差异与流域因素的影响。
2．麓原面的分级
在繁峙县南峪口附近的五台山北麓地带，以及定襄县湖村附近的系舟山北麓地带，自断层崖坡脚至唐县期夷平面之间共发育了6级麓原面(P1至P6)，自下向上，逐级抬高，沿山麓呈带状展布，并被横向冲沟所分割。在横向冲沟分割的山梁上，麓原面呈相对平缓的坡面，宽度数米至数百米不等，坡度5°～25°。每一级麓原面由其前缘陡坎与低一级麓原面相连，陡坎的坡度为30°～60°。第1级麓原面(P1)的前缘陡坎为最新断层崖或洪积台地，断层崖的坡脚为断裂带地表出露位置；第2级以上麓原面是先成断层崖后退和抬升的结果，其前缘陡坎为后退的断层崖。P6麓原面向上经过一个斜坡转为平缓的唐县期夷平面(Pt)。6级麓原面指示了第四纪期间6期快速构造隆升事件。
麓原面在山麓地带构成阶梯状地形，大致平行山边线展布，高度较稳定。同一级麓原面在冲沟两侧的高度基本一致，其高度沿山麓走向即使有变化，这种起伏也是连续的，除非被横向断裂所断错。向一些大冲沟(先成冲沟)的沟口，麓原面往往与阶地面或古宽谷的谷底相联合。根据这些特点可以将麓原面与差异性侵蚀造成的局部地形台阶相区分。
断裂段活动强度的差异会导致同一级麓原面在不同地区高度的不同。五台山北麓在羊眼河以东与以西分属两条左阶雁列的断裂段控制。羊眼河以东，P6至P1麓原面海拔依次为1510～1560m，1450～1500m，1350～1410m，1290～1310m，1250～1270m，1215～1230m左右。在羊眼河以西，P6至P1麓原面海拔依次为1488～1493m，1430m，1385～1390m，1320m左右，1270～1290m，1225～1250m左右。P6至P4麓原面在东段较高，P3至P1在西段偏高，反映了第四纪晚期以来西段断裂活动相对加强的趋势。这一点也表现在麓原面的相对高度上。在东段山麓，P6至P1高出山前冲积平原面依次为260～310m、200～230m、130～185m、95～120m、30～65m、10～25m；而在西段山麓则依次为283～313m、225～250m、185～205m、115～140m、70～85m、40m，从P4开始，西段明显高于东段，至P2与P1，西段高出东段近1倍。因此，麓原面的区域对比需要连续追踪，并参照对应的河流地貌面。
3．河流阶地的分级与断代
五台山与系舟山北麓地区较大的横向冲沟中发育了7级第四纪阶地，系舟山东部滹沱河峡谷内保存了较完整的5级阶地。各级阶地面上发育了厚度不等的黄土—古土壤序列，为地貌面断代与区域对比提供了便利。
羊眼河是五台山北麓一条规模较大的横向冲沟，发源于海拔3058m高的北台顶东北侧，全长35km，向北出山后汇入滹沱河上游。自海拔1600m的唐县期夷平面之下，羊眼河发育了7级第四纪阶地，其中T1与T2为堆积阶地，T3至T5为基座阶地，T6与T7为侵蚀阶地，T5以下阶地上披盖了较完整的黄土—古土壤序列。通过古地磁极性测试与磁化率测试，建立了五台山北麓羊眼河T5与T4阶地的磁性地层，并与标准黄土剖面与深海氧同位素曲线做了对比，确定T5阶地上覆的最老风尘沉积是古土壤S15，对应氧同位素期MIS 37，形成时代为1.2Ma；T4阶地上覆的最老风尘沉积是古土壤S5，对应氧同位素期MIS 15，时代为0.6Ma；T3阶地上覆的最老风尘沉积是古土壤S1，对应氧同位素期MIS 5，时代为0.13Ma；T2阶地上覆的最老风尘沉积是黄土L1，对应氧同位素期MIS 2。通过~(14)C测年与释光测年方法确定T1至T3阶地的形成时代分别是6ka、20ka和134ka BP。
系舟山北麓横向冲沟的规模较小，且高级阶地遭受侵蚀破坏较严重。湖村一带，横向冲沟发育了6级阶地，其中T1与T2为堆积阶地，T3为基座阶地，T4至T6为侵蚀阶地。T1阶地至T3阶地的释光测年依次为6ka、20ka、140ka。T2、T3与T4上覆的最老风尘堆积为L1、S1与S5。
滹沱河流经整个忻定盆地，后从盆地的东南侧穿越系舟山东段继续东流。在系舟山东段戎家庄、赵家庄至岭子底一带的峡谷地段，滹沱河发育了5级保存完整的河流阶地。其中T1至T2阶地为堆积阶地、T3至T5阶地为基座阶地。T1阶地至T3阶地的释光测年依次为7.6ka、20～26ka、130ka。T2、T3与T4上覆的最老风尘堆积为L1、S1与S5。T5上覆黄土地层遭受侵蚀。
总之，五台山北麓与系舟山山麓地区同一级河流阶地覆盖有相同的黄土—古土壤序列，阶地时代比较一致，说明该地区河流阶地的形成受统一机制的驱动。
对忻定盆地的第四纪黄土首次开展磁性地层研究，对第四纪河流阶地开展系统的断代与区域对比，是论文的主要进展。
4．麓原面与河流阶地的共生关系及其构造指示
在五台山北麓南峪口一带和系舟山北麓的湖村一带，麓原面(P1至P6)与横向冲沟的阶地面(T2至T7)在沟口一带存在逐级联合的关系，构成了6级层状地貌面。联合地貌面沿山麓走向带状延伸、两端伸向冲沟上游，平面上呈“U”字型，且逐级抬高，环环相套。麓原面与阶地面逐级对应关系具有区域一致性。
断块阶段性隆升对联合地貌面的形成起主导作用，但气候变化与横向冲沟的流域差异导致了联合地貌面的起伏。联合地貌面的起伏与山前冲洪积平原地势的起伏有一致性。羊眼河是五台山北麓调查区最大的河流，丰富的物源和较大的流量在山前形成了以山口为顶点的巨大的洪积扇，使得联合地貌面的发育保持了自山口向外侧逐渐降低的趋势。末次冰期导致了羊眼河河床的加积作用，水量的减少使大量物质停滞在扇顶附近，加大了扇顶与扇缘的高差；相比之下，羊眼河东侧的R1至R6冲沟规模小、物源匮乏、河床坡降大，沟床加积作用不明显，山前洪积扇不发育。冰期与流域因素的联合作用加剧了山前地形的差异，导致了第一级联合地貌面的大起伏。
第一级联合地貌面(P1与T2构成)在基岩山麓仅表现为断层崖坡折，而第二级以上联合地貌面有明显的麓原面，说明足够长的构造平静期是形成联合地貌面的必要条件。断层崖的后退与河流的演化是两类不同的地貌过程。从野外调查结果来看，河流的溯源侵蚀速度通常远大于断层崖的后退速度。如果平静期非常短暂，尽管冲沟达到了均衡状态并发生了河床的侧蚀展宽，但断层崖没有明显的变缓或后退，随之而来的构造隆升会导致河流下切并形成阶地，但不会有对应的麓原面或断层崖坡折。T1阶地在出山口没有对应的麓原面，其在断层横跨的地段形成数米的陡坎，下降盘的同期沉积被埋藏于现代洪积扇之下，其形成既体现了断层活动，又与全新世气候最适宜期相一致，构造与气候因素是如何作用的，值得深入研究。
根据山麓横向冲沟阶地的沉积结构及阶地面上覆的最老风尘堆积，T3至T5阶地形成时河流处于侵蚀状态或均衡状态，因此断块隆升会导致河流的及时快速下切，阶地的年代可以作为快速构造隆升的起始年代。T2阶地形成时河流处于加积状态(在大冲沟)或侵蚀与均衡状态(在中、小冲沟)，而此时末次冰期最盛期的干旱气候未能阻止河流的下切，可以推断气候因素对构造隆升导致的下切作用影响不大，如果下切时间稍有滞后的话，也不会太长。所以，T2阶地的年代可以近似作为最新快速构造隆升的起始年代。
基于麓原面与河流阶地的共生关系研究，认为五台山断块山地第四纪期间发生了6期较强的构造隆升过程，其中最后4次隆升过程的起始时代分别为距今1.2Ma、0.60Ma、0.13Ma和0.02Ma。该研究结果与前人对汾渭地堑系南部和青藏高原东北部的认识比较一致。
首次提出了麓原面与横向冲沟阶地之间的共生关系，并通过这种共生关系重建断块山地隆升的期次，有效地降低了单一地貌类型在隆升期次识别上的不确定性，是论文的主要进展。
5．横向冲沟纵剖面的特点与山地隆升的节律
根据五台山北麓3条冲沟和系舟山北麓4条冲沟纵剖面的测量结果，河床的纵剖面由一系列坡度较陡的急流段和坡度较缓的缓流段连接而成，其中急流段由一系列密集的大裂点联合而成，而缓流段发育的裂点高度小、间距大。阶地消失点正好对应河床坡折点，其下游为急流段，河谷狭窄，而上游为缓流段，河谷宽阔。鉴于断块阶段性隆升对横向河流下切起到主导作用，认为河流纵剖面的这种特点难以用岩性差异导致的差异性侵蚀来解释，应当是断裂分期活动的结果，急流段形成于断块快速隆升期(活跃期)，多级裂点可能是多次地表位错的结果，而缓流段形成于稳定期，断层活动的强度与频率都较低。
假若相邻两级阶地之间的河段内各裂点后退的速度是一致的，根据阶地的时代、以及急流段与缓流段的长度估算各河段所代表的时段长度，进而估算各构造期(活跃期或平静期)的持续时间。据此，可将五台山断块山地近120Ma以来隆升的节律划分为，快速隆升期为距今1200至1056ka、600至522ka、130至99ka、20至0ka，其他时段为稳定期。可将系舟山断块山地近120Ma以来隆升的节律划分为，快速隆升期为距今1200至1063ka、600至486ka、130至106ka、20至0ka，其他时段为稳定期。两个地区的估算结果可比，但有差异，认为这种差异是统计样本偏少，局部岩性与流域差异等因素所致，随着调查地区与统计样本的增加，局部因素可以进一步消除。统计表明，活跃期的持续时间比其相邻稳定期小的多，是其前一个稳定期的1／2.4～1／18.8，是其后一个稳定期的1／2.0～1／6.7。且稳定期持续时间越长，其后一个活跃期的持续时间也长。
五台山北麓唐县期夷平面、P6与P4麓原面高出山前洪积扇分别为400m、270～290m、130～180m，系舟山北麓唐县期夷平面、P6与P4麓原面高出山前洪积扇也分别为400～430m、350～370m、145～160m，说明第四纪早期与晚期山地的隆升幅度大致相当(以1.2Ma为界)，但包括唐县期夷平面解体的构造事件在内，上新世末至距今1.2Ma之间共有3个快速隆升期，而距今1.2Ma以来有4个隆升期(不包括T1阶地的下切事件)。说明尽管第四纪以来山地的长期平均隆升速率变化不大，但阶段性隆升的频度在1.2Ma以来增大了，且近1.2Ma以来具有隆升作用频度增大、隆升期持续时间变短、隆升速率变大的趋势。
阶地消失点附近的某些河段存在坡度的过渡带，即缓流段与急流段构成“S”型坡折，说明构造活动的转型有时可能存在过渡期，但需要在统计样本增加的情况下进一步核实。
从近2万年以来的古地震研究结果来看，地震地表位错的次数与T2阶地消失点以下的裂点数并不能较好的对应，裂点数一般少于地震的期次，原因尚需探索。
在明确了断块隆升在河流阶地形成中的主导作用前提下，通过阶地断代与河流纵剖面分析估算各构造期的持续时间，提高了构造期的断代精度，是论文的主要进展。
6．断块山地分期隆升的成因探索
忻定盆地周缘断块山地阶段性隆升存在两种可能的机制：区域构造活动机制与气候变化机制。区域地壳拉张作用下盆地的阶段性裂陷，会导致边界断裂的多期活动和山地的相对隆升。第四纪气候变冷、变化幅度增大的趋势使得山地经受了越来越强烈的周期性侵蚀，由此引发的周期性地壳均衡回弹效应，也是断块山地阶段性隆升过程中不可忽视的动力因素。
区域地壳拉张裂陷导致盆地一侧地壳减薄与沉陷，与裂谷翼部较厚的地壳和较高的地势形成反差，是山盆地壳均衡升降效应的前提条件。尔后，裂谷翼部的剥露作用通过山盆地壳均衡升降效应不断消减山盆之间地势与地壳厚度的差异，直至实现地壳厚度均一化，地形夷平化，山地相对隆升过程终止，进入麓原化阶段。因此，地壳均衡回弹是与Davis侵蚀旋回相伴生的地壳动力过程。
阶梯状麓原面的相对高差表明，上新世晚期以来忻定盆地周缘断块山地的长期平均隆升速率变化不大，但隆升的频度在1.2Ma以来明显增大了，且隆升期具有持续时间变短、隆升速率变大的趋势。这一现象难以用单一的区域构造活动机制解释。另一方面，近1.2Ma以来的快速隆升期与气候变化周期并不能完全吻合，说明气候变化并非主导因素，但统共4个隆升期中有3个始于间冰期初期，2个完全发生在间冰期，显示隆升作用与气候变化在时间上似乎有一定的关系。
气候变化导致的地壳均衡回弹效应自第四纪以来、尤其是1.2Ma以来趋于加强，在山地隆升过程中扮演越来越重要的角色。从间冰期到冰期，地壳均衡回弹效应具有强弱交替的特点，并周期性地施加于地壳动力过程中，有利于促进山体的阶段性隆升，这或许是第四纪中期以来山地隆升频度加大的原因。由于山地的侵蚀作用在冰期向间冰期转换时最强，山盆地壳均衡升降效应因而最显著，使得山地快速隆升多开始于间冰期的初期。第四纪气候变化使得山地的侵蚀作用趋于加强，加速了山地隆升期的进程，这或许是第四纪中期以来隆升期持续时间变短以及隆升速率加大的原因。
总之，经历了白垩纪晚期至第三纪早期长期广泛的北台期夷平作用之后，忻定盆地周缘地区处于地壳厚度均一化、地形夷平化状态。区域地壳拉张裂陷作用形成了裂谷盆地，并与气候变化共同控制了盆地周缘断块山地的第四纪阶段性隆升。第四纪以来，气候变化导致的山地侵蚀作用趋于加强，强化了地壳均衡回弹效应，并周期性地施加于地壳动力过程中，有可能是导致第四纪以来山地隆升作用频度加大、隆升期持续时间变短、隆升速率加大的关键因素。
初步探索了区域构造活动与气候变化在断块山地第四纪阶段性隆升中的作用，是论文的主要进展。
7．晚第四纪以来断块分期活动的地震表现
系舟山北麓断裂在距今100ka至22ka之间活动不明显，未发现地震地表位错事件；距今22ka以来垂直位移量达16～18m，发生了6～7次地表位错型地震事件。地震活跃期与最新断块活跃期相吻合，而地震平静期与末次断块稳定期相吻合。
近22ka以来，系舟山北麓断裂在早期和晚期的地震活动相对丛集，中间相对稀疏。震级也存在从大到小、再由小到大的变化趋势。五台山北麓断裂在距今6～7ka之前经历了相对平静期，之后进入地震活跃期。冲沟纵剖面揭示最新断块活跃期可划分为几乎等长的3个活动亚期，活动性依次为强、弱和强，体现了不平稳的特点。从系舟山北麓断裂的古地震研究来看，从末次稳定期转入最新活跃期是突然的、强烈的。
从古地震和微构造地貌方面对晚第四纪以来断块分期活动做了精细研究，是论文的主要进展。
Xinding Basin is situated in the northern part of Fenwei Graben system; it is a typical Cenozoic extensional fault-depression basin. The basin is bounded by the NE-trending North Piedmont Fault of Wutai Mountain, North Piedmont Fault of Xizhou Mountain, South Piedmont Fault of Hengshan Mountain, and the NNE-trending East Piedmont Fault of Yunzhong Mountain. The vertical displacements of Wutai and Xizhou Mountain North Piedmont Faults are relatively stronger, which control the primary geomorphic feature of the basin. The basin is surrounded by continuously uprising fault-block mountains. The Wutai Mountain is on the east side of the basin. It is the roof of North China region, its highest peak Beitai is 3058m above sea level, after which the famous Beitai-stage planation surface is named. On the basin side of the fault-block mountains 3 levels of planation surfaces and 7 steps of river terraces were developed below the Beitai-stage planation surface, indicating multi-stage uplifting. In the history three earthquakes above magnitude 7 took place in the Xinding Basin. A study on the responses of earthquakes to episodic tectonism helps to reveal the dynamic mechanism of earthquakes.
This dissertation took the stepped landforms of the piedmont zone surrounding Xinding Basin as the main thread, the north piedmont of Wutai and Xizhou mountains as the study area, and divided the Quaternary tectonic movement into stages based on the paragenetic relation between river terraces and pediments. By dating the river terraces and quantitatively analyzing the longitudinal river profile the division and timing of tectonic stages since 1.2 Ma were carried out. On this basis the responses of earthquakes to tectonism in the Late Quaternary were further discussed. The major achievements and new understandings are described as follows.
1. The paragenetic evolution model of pediments and fluvial morphology under the action of episodic fault movement
In the piedmont zone of fault-block mountains the pediments and tectonic terraces developed during the same tectonic movement are possessed of a certain paragenetic relation. According to the classic fault scarp evolution theory, a fault block was uplifted to form a fault scarp; afterwards the gully on the scarp eroded and cut down to form a V-shaped valley. Upon entering a quiescent period of tectonic activity, the riverbed was widened to form a U-shaped wide valley, meanwhile the slope of fault scarp decreased because of denudation. If the quiescent period was sufficiently long, a pediment was formed by fault scarp recession. Under the control of downthrown basin ground as the common base level of erosion, the pediment surface and the valley bed were approximately situated at the same altitude. Later-stage block uplift formed new fault scarp, the previous pediment surface and the wide valley were uplifted, and the gully started retrogressive erosion from the scarp to produce a terrace. In the piedmont zone the terrace and pediment surface jointly formed a stair of surface; they were contiguous in space with comparable elevation. Multiple tectonic cycles may produce a multi-stepped landforms consisting of a series of pediments and corresponding river terraces. Therefore the history of tectonic uplifting can be determined from the stepped landforms, and the starting time of rapid uplift, i.e., the commencing of active tectonic period, can be determined by dating the tectonic terraces (the time of initial river downcutting).
If an active tectonic stage is considered as a period of intense earthquake activities, then a scarp of tens meters high is the result of cumulated multiple earthquake dislocations, and a quiet tectonic stage means obviously decreased strength and frequency of fault slips, then downstream from the end of terrace is a rapid-flow reach consisting of a series of knickpoints, where the river bed is steep and the valley is narrow. On the other hand, upstream from the end of terrace is a slow-flow reach, where the riverbed is broad and gentle. The rapid reach represents an active tectonic period, while the slow reach means a stable tectonic period. If the retreating speeds of the knickpoints between two neighboring terraces are uniform, then the durations of active and quiet period can be estimated from the length of rapid and slow reach respectively. The time span of slow reach is T_(Si) = (T_i - T_(i-1)) L_(Si)/(L_(Si) + L_(Ri)), the time span of rapid reach is T_(Ri) = (T_i - T_(i-1)) L_(Ri)/(L_(Ri) + L_(Si)), where i is terrace stair numbered from bottom upward, T_i is the age of i-th terrace stair, L_(Ri) and L_(Si) represents respectively the length of rapid and slow reach between the i-th and i-1-th terrace. In this way we can obtain the starting and ending times of the active and stable tectonic periods.
2. The step division of pediments
On the north piedmont of Wutai Mountain nearby Nanyukou of Fanshi County, as well as on the north piedmont of Xizhou Mountain nearby Hu Village of Dingxiang County, 6 pediments (P1 to P6) were developed between the fault scarp foot and the Tangxian-stage planation surface. They rise step by step, extend as belts along the piedmont, and are cut by transverse gullies. On the interfluves, the pediment surfaces are relatively flat with widths from meters to hundreds meters and slopes between 5°and 25°. Each step of pediment is connected to the next step via a frontal bank whose slope is between 30°and 60°. The frontal riser of the first pediment surface (P1) is the most recent fault scarp or diluvial platform; at the foot of pediment P1 outcrops the fault. The higher pediments resulted from recession and uplift of antecedent fault scarps, their frontal risers are retreating scarps. Above the P6 pediment there is a slope that transits into the flat Tangxian-stage planation surface (Pt). The six steps of pediments indicate six stages of rapid tectonic uplift during the Quaternary.
The spatial pattern of pediments is relatively stable. Near the mouth of some large gullies (antecedent gullies), pediments are often united with the terraces or the bottoms of ancient wide valley. The altitude of the same stair pediment is basically identical on both sides of a gully. It changes little along the piedmont; if it does the undulation is continuous unless the pediment is offset by transverse faults. These features make it possible to distinguish pediments from local landform steps caused by differential erosion.
The differentiation of activity along fault may result in different altitudes of pediments of the same stair in different regions. To the east and west of Yangyan River the north piedmont of Wutai Mountain is respectively controlled by two left-step en echelon faults. To the east of Yangyan River the altitudes of pediment P6 to P1 are successively 1510～1560m, 1450～1500m, 1350～1410m, 1290～1310m, 1250～1270m, and 1215～1230m above sea level. To the west they are 1488～1493m, about 1430m, 1385～1390m, 1320m, 1270～1290m, and 1225～1250m. Pediments P6 to P4 are higher on the east, while P3 to P1 are higher on the west, indicating that the western fault is more active in Late Quaternary. This is also seen in the relative heights of pediments. On the east piedmont P6 to P1 are successively 260～310m, 200～230m, 130～185m, 95～120m, 30～65m, and 10～25m above the alluvial plain on the basin side; while on the west piedmont they are successively 283～313m, 225～250m, 185～205m, 115～140m, 70～85m, and 40m. Starting from P4 the west is obviously higher than the east. P2 and P1 in west are nearly twice as high as that in the east. Therefore, a regional correlation of pediments needs to trace continuously , and the corresponding fluvial geomorphology should be referred to.
3. Division and dating of river terraces
On the north piedmont zone of Wutai and Xizhou Mountains 7 steps of Quaternary terraces were developed in large transverse gullies. Along the valley of Hutuo River in east Xizhou Mountain 5 steps of terraces are rather completely preserved. Each terrace is covered by a loess-paleosol sequence of different thickness, which facilitates dating the geomorphic surfaces and making a regional correlation.
Yangyan River is a large transverse gully on the north piedmont of Wutai Mountain. It originates from northeast Beitai peak of 3058m above sea level and is 35km long. It flows northwards out of the mountain then into the upper reach of Hutuo River. Below the Tangxian-stage planation surface of altitude 1600m there are 7 steps of Quaternary terraces along the Yangyan River(T1 to T7, T1 is the youngest). Among them T1 and T2 are aggradational terraces, T3 to T5 are strath terraces, T6 and T7 are degradational terraces. Below T5 the terraces are overlain by relatively complete loess-paleosol sequences. By means of paleomagnetic polarity and susceptibility tests the paleomagnetic stratigraphy of terraces T5 and T4 was established and correlated to the standard loess profile and abyssal oxygen isotope curve. It is determined that the oldest aeolian deposit on terrace T5 is paleosol S15, corresponding to oxygen isotope stage MIS 37, and the age of the terrace is 1.2 Ma.The oldest aeolian deposit on terrace T4 is paleosol S5, corresponding to oxygen isotope stage MIS 15, and the age is 0.6 Ma. The oldest aeolian deposit on terrace T3 is paleosol S1, corresponding to oxygen isotope stage MIS 5, and the age is 0.13 Ma. The oldest aeolian deposit on terrace T2 is loess L1, corresponding to oxygen isotope time MIS2. The ages of terrace T1, T2 and T3 given by ~(14)C and luminescence dating are 6ka, 20ka and 134ka BP respectively.
The gullies on the north piedmont of Xizhou Mountain are of smaller scale, and the higher terraces are rather severely damaged by erosion. Nearby Hu Village 6 stairs of terraces were developed in the transverse gully, among which T1 and T2 are aggradational terraces, T3 is strath terrace, T4 to T6 are degradational terraces. The age for terrace T1, T2, and T3 from luminescence dating is respectively 6ka, 20ka, and 140ka BP. The oldest aeolian deposit on T2, T3, and T4 is respectively L1, S1, and S5.
The Hutuo River flows through the entire Xinding Basin, then crosses the east segment of Xizhou Mountain on the southeast side of the basin to continue eastward. In the gorge area along Rongjiazhuang, Zhaojiazhuang, to Lingzidi in the eastern Xizhou Mountain, 5 stairs of river terraces are well preserved along the river. Among them T1 and T2 are aggradational terraces, T3 to T5 are strath terraces. Luminescence dating resulted the ages of T1 to T3 terraces as 7.6ka, 20～26ka, and 130ka BP. The oldest aeolian deposit on T2, T3, and T4 is L1, S1, and S5 respectively. The loess stratum on T5 was eroded.
In summary the river terraces of the same stair on the north piedmont of Wutai Mountain and on Xizhou Mountain piedmont are overlain by identical loess-paleosol sequences, the ages of terraces are generally consistent, indicating that the river terraces in this region were formed by the same mechanism.
Geomagnetic stratigraphic study of the Quaternary loess in Xinding Basin was carried out for the first time, age determination and regional correlation of the Quaternary river terraces were done systematically. This is a major progress achieved by this dissertation.
4. The paragenetic relation between pediments and river terraces and its tectonic implications
Nearby Nanyukou on the northern piedmont of Wutai Mountain and Hu Village on the north piedmont of Xizhou Mountain, the pediments (P1 to P6) and the terraces of lateral gullies (T2 to T7) are united at the gully mouth step by step, constructing six levels of stepped surfaces. These unified geomorphic surfaces extend as belts along the piedmont, with two ends extending toward the upper reach of gullies. In a plan view they are "U"-shaped and rise and encircle step by step. This step-by-step correspondence of pediments and terraces is consistent in the region.
The episodic uplift of fault block played the leading role in the formation of the unified surfaces, however, the climatic change and drainage difference of gullies resulted in the undulation of the surfaces which is accordant with the landform of the alluvial plain on the basin side. The Yangyan River is the largest river in the study area of north Wutai piedmont, abundant material source and relatively large discharge produced the huge alluvial fan with the valley mouth as its vertex, and made the united surface to maintain a descending trend from near the vertex outward in its development. The last glacial stage caused aggradation of the Yangyan River and the reduction of discharge made a large quantity of material stagnant nearby the fan vertex, thus increased the height difference between the fan vertex and the edge. By contrast, the gullies R1 to R6 on the east side of Yangyan River are of small size, insufficient material sources and steep riverbed, aggradation in glacial stage was not significant, and alluvial fans were not well developed. Glacial and local factors jointly enhanced the altitude difference of alluvial fans on the basin side, resulted in the large undulation of the first-step unified surface.
The first-step unified surface (consisting of P1 and T2) appears merely as a slope break of the fault scarp on the bedrock piedmont, while the unified surfaces of second and above steps have obvious pediment surfaces, indicating that a sufficiently long quiet tectonic period is necessary to form a unified geomorphic surface. Fault recession and river evolution are two different kinds of geomorphologic processes. According to field investigation, the headward erosion of the river was generally much quicker than the recession of fault scarp. If the quiescent period was short, even though the gully reached an equilibrium state and the riverbed was widened by lateral erosion, but the fault scarp did not obviously retreat or become less steep, the following tectonic uplift might cause river incision to form terrace, but there would not be corresponding pediment or slope break of the fault scarp. Terrace T1 did not develop a corresponding pediment at the valley mouth, but formed a scarplet several meters high across the fault, its synchronous sediments on the downthrown wall are buried below the contemporary alluvial fan. Its formation was a manifestation of fault activity, and was also accordant with the optimal climate period in Holocene. What roles the tectonic and climatic factors played is worthy of further investigation.
According to the sedimentary texture of alluvium and the oldest aeolian deposit overlying, terraces T3 to T5 were formed when the river was in an erosion or equilibrium state, so fault-block uplift caused synchronous and rapid river incision, the age of terrace can be considered as the starting time of speedy tectonic uplift. Terrace T2 was formed when the river was in an aggradation state (in large gullies) or in an erosion and equilibrium state (in medium and small gullies), but the arid climate of the last glacial maximum did not prevent river incision. It can be inferred that climatic factors did not significantly affect the incision caused by tectonic uplift. Even if the incision was postponed slightly, the time lag should not be very long. Therefore, the age of T2 can be approximately considered as the starting time of the last speedy tectonic uplift.
Based on the study of paragenetic relation of pediments and river terraces, it is deduced that Wutai Fault-block Mountain experienced 6 relatively strong tectonic uplifting events in the Quaternary. The last 4 events started at 1.2Ma, 0.6Ma, 0.13Ma, and 0.02Ma B.P. respectively. This result is consistent with previous studies in southern Fenwei Graben system and in the northeastern part of Tibetan Plateau.
Paragenetic relation of pediments and terraces of transverse rivers was put forward for the first time, then uplifting stages of fault block was rebuilt by the paragenesis, thus effectively reducing the uncertainty of uplifting-event identification with single landform type. This is another major progress achieved by this dissertation.
5. Longitudinal profile of gully and the rhythm of mountain uplifting
Based on the surveyings of longitudinal profile along 3 gullies on the north piedmont of Wutai Mountain and 4 gullies on the north piedmont of Xizhou Mountain, it is found that the riverbed consists of serial rapid reaches with steep slope and slow reaches with gentle slope. The rapid reach is composed of a series of dense knickpoints, while on the slow reach the knickppoints have small height and large interval. The extinction point of terrace corresponds to the slope-break of riverbed, from which downstream there is narrow rapid reach and upstream there is broad slow reach. Because the episodic uplifting of fault-block plays a leading role in the incision of transverse river, the above features in the river profile are difficult to explain by differential erosion due to lithological differences. Instead they should be the result of episodic fault slippings. The rapid reachs were formed in active tectonic periods, the knickpoints resulted from many times of large-displacement surface faultings at short recurrence intervals; while the slow reachs were formed in stable tectonic periods when the displacement and frequency of faulting were low.
If the retreating speeds of the knickpoints between two neighboring terraces were uniform, then the duration time of active and quiet tectonic periods can be estimated from the ages of terraces and the lengths of rapid and slow reachs. In this way the uplifting rhythm of Wutai fault-block mountain in the recent 120Ma can be described as follows. The rapid uplifting periods are 1200 to 1056ka, 600 to 522ka, 130 to 99ka, and 20 to Oka; the rest times are quiet periods. The uplifting rhythm of Xizhou fault-block mountain in the recent 120Ma can be described as follows. The rapid uplifting periods are 1200 to 1063ka, 600 to 486ka, 130 to 106ka, and 20 to 0ka; the rest times are quiet periods. The results from two regions are comparable but with some differences, which are considered due to insufficient statistic samples and local lithological and drainage differences. With the increase of surveying gullies and statistic samples, the local factors can be further suppressed. The statistics indicates that the duration of an active period is much shorter than the consecutive stable period, being about l/2.4～1/18.8 of the previous stable period and 1/2.0～1/6.7 of the following stable period. Moreover, the longer the stable period lasted, the longer the following active period was.
Nearby the extinction point of terrace the river may sometimes have a transition zone of gradient, i.e., the rapid reach and slow reach form an S-shaped slope break zone, which means that the change of tectonic mode may have a transition period. But not all cases are like this, further verification is needed.
Comparing to the result of paleo-earthquake studies for the recent 20ka, the number of surface-rupture earthquakes is not consistent with the number of knickpoints below terrace T2, the latter is generally less than the former. The reason should be further investigated.
On the premise that fault-block uplifting plays a leading role in the formation of river terraces, the durations of tectonic stages can be estimated by dating the terraces and analyzing the longitudinal profile of the river, thus improving the accuracy of age of tectonic stages. This is the third major progress achieved by the dissertation.
6. Mechanism discussion of episodic block uplifting
Two possible mechanisms are responsible for the episodic block uplifting around Xinding basin—regional tectonism and climate changes. Regional crust pulling-apart induce sinking of basin, multiperiodic activity of boundary faults and uplifting of mountain. Cooling and greater variability of Quaternary climate cause intense periodical erosion and isostasy rebound of the mountain, which is a factor which cannot be neglected either in the episodic uplifting of block mountains.
The heights of pediment series indicate that the long term average uplifting rate of the block mountains around Xinding basin changed little since late Pliocene, but the uplifting frequency increased distinctly since the last 1.2Ma, and the uplifting has a tendency of shorter duration, shorter recurrent time and bigger uplifting rate, which cannot be ascribed to pure regional tectonism. On the other hand, quick uplifting stages did not accord completely with the fluctuation of climate, which favor that fluctuation of climate is not the dominant agent. However three of four uplifting stage started from early interglacial, and two are completely in interglacial stage, which suggest some temporal relation between uplifts and climate changes.
The greater variability of climate since 1.2Ma ago enhance the interglacial exhumation of mountain and the accumulation of basin, intensifying the isostasy rebound and favoring the episodic uplift of mountains, which maybe the reason why the mode of mountain uplifting changed. The erosion of mountain in the switch from glacial to interglacial is the strongest, so the isostasy is the most prominent, which may be the reason why mountain uplifts mostly start at early interglacial.
In a word, some combination of regional tectonism and climate changes has been responsible for episodic uplifting of block mountain in Quaternary. Crust isostasy rebound induced by fluctuation of climate has been intensifying since Quaternary, and periodically acting on crustal dynamic process, which may be the key mechanism for more frequent uplift of mountain, shorter duration and higher rate of uplift since Quaternary.
Rules of regional tectonism and climate changes in episodic uplift of block mountains since Quaternary has been discussed preliminarily. This is the fourth major progress achieved by the dissertation.
7. Responses of earthquakes to tectonic episodes in late Quaternary
The Xizhou Mountain north piedmont fault was not active between 100ka and 22ka BP, no surface-faulting event was found; on the other hand, the vertical displacement since 22ka amounts to 16～18m, 6～7 faulting events have taken place. The active period of earthquakes accords to the most recent active tectonic stage, while the time of seismic quiescence consists with the last stable tectonic stage.
Since the last 22ka, the earthquakes were clustering in the forepart and the terminal. but relative sparse in the middle stage. The magnitudes of earthquakes waned in the early stage, then largened in the late stage. The Wutai north piedmont fault underwent a quiet period 6 to 7 ka before, then entered a active period of earthquakes until now. The longitudinal profiles of transverse gullies indicates that since the last 20ka the two fault have experienced three inferior tectonic stages near equally long which are in turn active, quiet and active. Paleoearthquakes show that switch from the last stable to the recent active tectonic stage was abrupt and intense.
The responses of earthquakes to tectonic episodes in late Quaternary have been explored initially, which helps to unveil the mechanism of earthquake clustering of fault segment in long time span. This is the fifth major progress achieved by the dissertation.
忻定盆地位于汾渭地堑系北段，是一个典型的新生代张性断陷盆地，其边界由北东向五台山北麓断裂、系舟山北麓断裂、恒山南麓断裂和北北东向云中山东麓断裂所围限，其中五台山北麓断裂与系舟山北麓断裂垂直错动强度较大，控制了盆地的主体地貌格局。盆地四周由持续隆升的断块山地围绕，五台山位于其东侧，最高峰北台顶海拔高度3058m，是华北地区的屋脊和著名的北台期夷平面命名地。断块山地向盆地一侧自北台期夷平面之下发育了3级夷平面和7级河流阶地，指示了多期隆升作用。忻定盆地有史以来发生了3次7级以上地震，研究地震活动有益于精细地刻画断块分期活动过程。
论文以忻定盆地周围山麓地带的层状地貌为主线，以五台山与系舟山北麓为主要研究地区，通过河流阶地与麓原面的共生关系对第四纪断块活动进行分期，通过阶地断代以及河流纵剖面的定量分析，对1.2Ma以来断块活动分期进行断代。在此基础上，进一步讨论了断块山地阶段性隆升的原因，以及晚第四纪以来断块分期活动的地震表现。取得的主要进展与认识如下：
1．提出了断层阶段性活动作用下麓原面与河流地貌的共生演化模式
在断块山地的山麓地带，同一期构造运动形成的麓原面与构造阶地有一定的共生关系。根据经典的断层崖演化理论，断块隆升形成断层崖；随后，断层崖上的冲沟下切侵蚀形成“V”字型峡谷；进入构造平静期后，沟床展宽形成“U”字型宽谷，同时断层崖遭受剥蚀而坡度降低，如果平静期足够长，断层崖后退形成麓原面；受下降盘盆地地表这一共同侵蚀基准面的控制，麓原面与宽谷谷底在山麓地带处在大致相同的高度；后期的断块隆升形成新的断层崖，原先的麓原面和宽谷被抬升，冲沟从断层崖开始溯源侵蚀形成阶地。在山麓地带，阶地与麓原面联合成一级地貌面，二者在空间上连续、在高度上可比。多个构造旋迴可以形成一系列由河流阶地和麓原面组成的多级联合地貌面。因此，可以通过联合地貌面的级数确定断块隆升的期次，并通过阶地的年代推断断块隆升的起始时间。
如果把断层活跃期理解为地震断层活动的丛集期，数十米的陡坎是多次地震位错的累积，而平静期理解为断层活动强度与频度的明显减弱，则在河流纵剖面上，阶地消失点下游会形成由一系列裂点组成的急流段，河床坡度较陡，河谷狭窄，而阶地消失点上游为缓流段，河床坡度较缓，是稳定期形成的宽谷地貌。急流段代表断层活跃期，缓流段代表稳定期。如果相邻两级阶地之间各裂点的后退速度是一致的，则可以根据急流段与缓流段的长度估算活跃期与平静期的持续时间。缓流段时间跨度T_(Si)=(T_i-T_(i-1))L_(Si)／(L_(si)+L_(Ri))，急流段时间跨度T_(Ri)=(T_i-T_(i-1))L_(Ri)／(L_(Ri)+L_(Si))。其中i为阶地级次，从低向高编号，Ti为第i级阶地的年代，L_(Ri)与L_(Si)分别代表第i级与i-1级阶地之间急流段与缓流段的长度。这样，我们可以得到断块活跃期或稳定期的起始年代与结束年代。
物质组成是制约断层崖演化的主要因素之一。松散沉积物构成的断层崖退化速度较快，地震周期的断层活动通常可留下易辨的复合断层崖与河流阶地等层状地貌。基岩断层崖则不然，地震周期内断层崖退化不明显，河床展宽也不明显，周期性地震断错仅会在冲沟中留下一系列跌水，但通常难以形成阶地系列。在坚硬的基岩山区，那些数十米乃至数百米高的复合断层崖和基座阶地形成于时间跨度更长的构造活跃期与平静期的交替，每一个构造活跃期由一系列丛集的地震位错事件组成，在上升盘的河谷中形成由一系列裂点构成的急流段和狭谷地貌，而构造平静期对应宽谷地貌与缓流段。河流溯源侵蚀古宽谷地貌形成阶地。
第四纪气候变化也是制约上述模式的主要因素之一。第一，河流对山地隆升作用的响应关系受到气候变化的制约。冰期河床的加积效应会使得河流对断块隆升的响应滞后，而间冰期的河流通常处于侵蚀状态或均衡状态，对断块隆升会有及时的下切响应。第二，气候变化可直接形成阶地。气候的周期性变化使河流发生周期性的堆积与侵蚀，会形成一系列阶地。如果山地处于稳定状态，每一次河流下切的深度变化不大，这些阶地的高度大致相当，构不成阶梯状地貌面。第三，气候变化影响裂点的后退速度。相邻阶地之间的裂点如果形成于不同气候期，各裂点之间的距离与形成的时间差之间并非线性相关，因此，用急流段与缓流段的长度估算构造期持续时间会造成一定误差。第四，气候周期性变化导致山盆地壳均衡升降效应，可能引发或触发断块山地的阶段性隆升。
第四纪气候的周期性变化加速了山地的侵蚀过程以及盆地的堆积作用，剥露作用导致的地壳均衡回弹效应成为山地隆升的驱动机制之一。间冰期，河流流量加大，山区遭受侵蚀，均衡作用导致地壳上隆；与此同时，盆地接受堆积，均衡作用导致地壳下沉。应变能的积累有利于盆地边界断裂发生倾滑错动。进入气候相对干冷的冰期，山区物质的外泄量明显减小，地壳均衡升降效应相应减弱。如果这一假设成立，气候周期性变化有可能引发断块山地的阶段性隆升。与区域构造机制或许不同的是，气候变化导致的山地隆升过程发生在间冰期，且始于间冰期的初期。
另外，尚有一些其它因素，使得上述模式复杂化。譬如，冲沟的发展会破坏麓原面在某些地段的完整性，而差异侵蚀作用或阶梯状断裂组合会在山麓地带形成同期多级地貌面，造成假麓原面地貌。岩性的差异、河床的堰塞会形成一些非构造成因裂点。支流的汇入使得下游主流河段的流量增加，裂点后退速度加快。因此，要合理的应用上述模式，需要在野外开展详细的地貌填图，仔细甄别。调查的冲沟尽可能多，以排除岩性差异与流域因素的影响。
2．麓原面的分级
在繁峙县南峪口附近的五台山北麓地带，以及定襄县湖村附近的系舟山北麓地带，自断层崖坡脚至唐县期夷平面之间共发育了6级麓原面(P1至P6)，自下向上，逐级抬高，沿山麓呈带状展布，并被横向冲沟所分割。在横向冲沟分割的山梁上，麓原面呈相对平缓的坡面，宽度数米至数百米不等，坡度5°～25°。每一级麓原面由其前缘陡坎与低一级麓原面相连，陡坎的坡度为30°～60°。第1级麓原面(P1)的前缘陡坎为最新断层崖或洪积台地，断层崖的坡脚为断裂带地表出露位置；第2级以上麓原面是先成断层崖后退和抬升的结果，其前缘陡坎为后退的断层崖。P6麓原面向上经过一个斜坡转为平缓的唐县期夷平面(Pt)。6级麓原面指示了第四纪期间6期快速构造隆升事件。
麓原面在山麓地带构成阶梯状地形，大致平行山边线展布，高度较稳定。同一级麓原面在冲沟两侧的高度基本一致，其高度沿山麓走向即使有变化，这种起伏也是连续的，除非被横向断裂所断错。向一些大冲沟(先成冲沟)的沟口，麓原面往往与阶地面或古宽谷的谷底相联合。根据这些特点可以将麓原面与差异性侵蚀造成的局部地形台阶相区分。
断裂段活动强度的差异会导致同一级麓原面在不同地区高度的不同。五台山北麓在羊眼河以东与以西分属两条左阶雁列的断裂段控制。羊眼河以东，P6至P1麓原面海拔依次为1510～1560m，1450～1500m，1350～1410m，1290～1310m，1250～1270m，1215～1230m左右。在羊眼河以西，P6至P1麓原面海拔依次为1488～1493m，1430m，1385～1390m，1320m左右，1270～1290m，1225～1250m左右。P6至P4麓原面在东段较高，P3至P1在西段偏高，反映了第四纪晚期以来西段断裂活动相对加强的趋势。这一点也表现在麓原面的相对高度上。在东段山麓，P6至P1高出山前冲积平原面依次为260～310m、200～230m、130～185m、95～120m、30～65m、10～25m；而在西段山麓则依次为283～313m、225～250m、185～205m、115～140m、70～85m、40m，从P4开始，西段明显高于东段，至P2与P1，西段高出东段近1倍。因此，麓原面的区域对比需要连续追踪，并参照对应的河流地貌面。
3．河流阶地的分级与断代
五台山与系舟山北麓地区较大的横向冲沟中发育了7级第四纪阶地，系舟山东部滹沱河峡谷内保存了较完整的5级阶地。各级阶地面上发育了厚度不等的黄土—古土壤序列，为地貌面断代与区域对比提供了便利。
羊眼河是五台山北麓一条规模较大的横向冲沟，发源于海拔3058m高的北台顶东北侧，全长35km，向北出山后汇入滹沱河上游。自海拔1600m的唐县期夷平面之下，羊眼河发育了7级第四纪阶地，其中T1与T2为堆积阶地，T3至T5为基座阶地，T6与T7为侵蚀阶地，T5以下阶地上披盖了较完整的黄土—古土壤序列。通过古地磁极性测试与磁化率测试，建立了五台山北麓羊眼河T5与T4阶地的磁性地层，并与标准黄土剖面与深海氧同位素曲线做了对比，确定T5阶地上覆的最老风尘沉积是古土壤S15，对应氧同位素期MIS 37，形成时代为1.2Ma；T4阶地上覆的最老风尘沉积是古土壤S5，对应氧同位素期MIS 15，时代为0.6Ma；T3阶地上覆的最老风尘沉积是古土壤S1，对应氧同位素期MIS 5，时代为0.13Ma；T2阶地上覆的最老风尘沉积是黄土L1，对应氧同位素期MIS 2。通过~(14)C测年与释光测年方法确定T1至T3阶地的形成时代分别是6ka、20ka和134ka BP。
系舟山北麓横向冲沟的规模较小，且高级阶地遭受侵蚀破坏较严重。湖村一带，横向冲沟发育了6级阶地，其中T1与T2为堆积阶地，T3为基座阶地，T4至T6为侵蚀阶地。T1阶地至T3阶地的释光测年依次为6ka、20ka、140ka。T2、T3与T4上覆的最老风尘堆积为L1、S1与S5。
滹沱河流经整个忻定盆地，后从盆地的东南侧穿越系舟山东段继续东流。在系舟山东段戎家庄、赵家庄至岭子底一带的峡谷地段，滹沱河发育了5级保存完整的河流阶地。其中T1至T2阶地为堆积阶地、T3至T5阶地为基座阶地。T1阶地至T3阶地的释光测年依次为7.6ka、20～26ka、130ka。T2、T3与T4上覆的最老风尘堆积为L1、S1与S5。T5上覆黄土地层遭受侵蚀。
总之，五台山北麓与系舟山山麓地区同一级河流阶地覆盖有相同的黄土—古土壤序列，阶地时代比较一致，说明该地区河流阶地的形成受统一机制的驱动。
对忻定盆地的第四纪黄土首次开展磁性地层研究，对第四纪河流阶地开展系统的断代与区域对比，是论文的主要进展。
4．麓原面与河流阶地的共生关系及其构造指示
在五台山北麓南峪口一带和系舟山北麓的湖村一带，麓原面(P1至P6)与横向冲沟的阶地面(T2至T7)在沟口一带存在逐级联合的关系，构成了6级层状地貌面。联合地貌面沿山麓走向带状延伸、两端伸向冲沟上游，平面上呈“U”字型，且逐级抬高，环环相套。麓原面与阶地面逐级对应关系具有区域一致性。
断块阶段性隆升对联合地貌面的形成起主导作用，但气候变化与横向冲沟的流域差异导致了联合地貌面的起伏。联合地貌面的起伏与山前冲洪积平原地势的起伏有一致性。羊眼河是五台山北麓调查区最大的河流，丰富的物源和较大的流量在山前形成了以山口为顶点的巨大的洪积扇，使得联合地貌面的发育保持了自山口向外侧逐渐降低的趋势。末次冰期导致了羊眼河河床的加积作用，水量的减少使大量物质停滞在扇顶附近，加大了扇顶与扇缘的高差；相比之下，羊眼河东侧的R1至R6冲沟规模小、物源匮乏、河床坡降大，沟床加积作用不明显，山前洪积扇不发育。冰期与流域因素的联合作用加剧了山前地形的差异，导致了第一级联合地貌面的大起伏。
第一级联合地貌面(P1与T2构成)在基岩山麓仅表现为断层崖坡折，而第二级以上联合地貌面有明显的麓原面，说明足够长的构造平静期是形成联合地貌面的必要条件。断层崖的后退与河流的演化是两类不同的地貌过程。从野外调查结果来看，河流的溯源侵蚀速度通常远大于断层崖的后退速度。如果平静期非常短暂，尽管冲沟达到了均衡状态并发生了河床的侧蚀展宽，但断层崖没有明显的变缓或后退，随之而来的构造隆升会导致河流下切并形成阶地，但不会有对应的麓原面或断层崖坡折。T1阶地在出山口没有对应的麓原面，其在断层横跨的地段形成数米的陡坎，下降盘的同期沉积被埋藏于现代洪积扇之下，其形成既体现了断层活动，又与全新世气候最适宜期相一致，构造与气候因素是如何作用的，值得深入研究。
根据山麓横向冲沟阶地的沉积结构及阶地面上覆的最老风尘堆积，T3至T5阶地形成时河流处于侵蚀状态或均衡状态，因此断块隆升会导致河流的及时快速下切，阶地的年代可以作为快速构造隆升的起始年代。T2阶地形成时河流处于加积状态(在大冲沟)或侵蚀与均衡状态(在中、小冲沟)，而此时末次冰期最盛期的干旱气候未能阻止河流的下切，可以推断气候因素对构造隆升导致的下切作用影响不大，如果下切时间稍有滞后的话，也不会太长。所以，T2阶地的年代可以近似作为最新快速构造隆升的起始年代。
基于麓原面与河流阶地的共生关系研究，认为五台山断块山地第四纪期间发生了6期较强的构造隆升过程，其中最后4次隆升过程的起始时代分别为距今1.2Ma、0.60Ma、0.13Ma和0.02Ma。该研究结果与前人对汾渭地堑系南部和青藏高原东北部的认识比较一致。
首次提出了麓原面与横向冲沟阶地之间的共生关系，并通过这种共生关系重建断块山地隆升的期次，有效地降低了单一地貌类型在隆升期次识别上的不确定性，是论文的主要进展。
5．横向冲沟纵剖面的特点与山地隆升的节律
根据五台山北麓3条冲沟和系舟山北麓4条冲沟纵剖面的测量结果，河床的纵剖面由一系列坡度较陡的急流段和坡度较缓的缓流段连接而成，其中急流段由一系列密集的大裂点联合而成，而缓流段发育的裂点高度小、间距大。阶地消失点正好对应河床坡折点，其下游为急流段，河谷狭窄，而上游为缓流段，河谷宽阔。鉴于断块阶段性隆升对横向河流下切起到主导作用，认为河流纵剖面的这种特点难以用岩性差异导致的差异性侵蚀来解释，应当是断裂分期活动的结果，急流段形成于断块快速隆升期(活跃期)，多级裂点可能是多次地表位错的结果，而缓流段形成于稳定期，断层活动的强度与频率都较低。
假若相邻两级阶地之间的河段内各裂点后退的速度是一致的，根据阶地的时代、以及急流段与缓流段的长度估算各河段所代表的时段长度，进而估算各构造期(活跃期或平静期)的持续时间。据此，可将五台山断块山地近120Ma以来隆升的节律划分为，快速隆升期为距今1200至1056ka、600至522ka、130至99ka、20至0ka，其他时段为稳定期。可将系舟山断块山地近120Ma以来隆升的节律划分为，快速隆升期为距今1200至1063ka、600至486ka、130至106ka、20至0ka，其他时段为稳定期。两个地区的估算结果可比，但有差异，认为这种差异是统计样本偏少，局部岩性与流域差异等因素所致，随着调查地区与统计样本的增加，局部因素可以进一步消除。统计表明，活跃期的持续时间比其相邻稳定期小的多，是其前一个稳定期的1／2.4～1／18.8，是其后一个稳定期的1／2.0～1／6.7。且稳定期持续时间越长，其后一个活跃期的持续时间也长。
五台山北麓唐县期夷平面、P6与P4麓原面高出山前洪积扇分别为400m、270～290m、130～180m，系舟山北麓唐县期夷平面、P6与P4麓原面高出山前洪积扇也分别为400～430m、350～370m、145～160m，说明第四纪早期与晚期山地的隆升幅度大致相当(以1.2Ma为界)，但包括唐县期夷平面解体的构造事件在内，上新世末至距今1.2Ma之间共有3个快速隆升期，而距今1.2Ma以来有4个隆升期(不包括T1阶地的下切事件)。说明尽管第四纪以来山地的长期平均隆升速率变化不大，但阶段性隆升的频度在1.2Ma以来增大了，且近1.2Ma以来具有隆升作用频度增大、隆升期持续时间变短、隆升速率变大的趋势。
阶地消失点附近的某些河段存在坡度的过渡带，即缓流段与急流段构成“S”型坡折，说明构造活动的转型有时可能存在过渡期，但需要在统计样本增加的情况下进一步核实。
从近2万年以来的古地震研究结果来看，地震地表位错的次数与T2阶地消失点以下的裂点数并不能较好的对应，裂点数一般少于地震的期次，原因尚需探索。
在明确了断块隆升在河流阶地形成中的主导作用前提下，通过阶地断代与河流纵剖面分析估算各构造期的持续时间，提高了构造期的断代精度，是论文的主要进展。
6．断块山地分期隆升的成因探索
忻定盆地周缘断块山地阶段性隆升存在两种可能的机制：区域构造活动机制与气候变化机制。区域地壳拉张作用下盆地的阶段性裂陷，会导致边界断裂的多期活动和山地的相对隆升。第四纪气候变冷、变化幅度增大的趋势使得山地经受了越来越强烈的周期性侵蚀，由此引发的周期性地壳均衡回弹效应，也是断块山地阶段性隆升过程中不可忽视的动力因素。
区域地壳拉张裂陷导致盆地一侧地壳减薄与沉陷，与裂谷翼部较厚的地壳和较高的地势形成反差，是山盆地壳均衡升降效应的前提条件。尔后，裂谷翼部的剥露作用通过山盆地壳均衡升降效应不断消减山盆之间地势与地壳厚度的差异，直至实现地壳厚度均一化，地形夷平化，山地相对隆升过程终止，进入麓原化阶段。因此，地壳均衡回弹是与Davis侵蚀旋回相伴生的地壳动力过程。
阶梯状麓原面的相对高差表明，上新世晚期以来忻定盆地周缘断块山地的长期平均隆升速率变化不大，但隆升的频度在1.2Ma以来明显增大了，且隆升期具有持续时间变短、隆升速率变大的趋势。这一现象难以用单一的区域构造活动机制解释。另一方面，近1.2Ma以来的快速隆升期与气候变化周期并不能完全吻合，说明气候变化并非主导因素，但统共4个隆升期中有3个始于间冰期初期，2个完全发生在间冰期，显示隆升作用与气候变化在时间上似乎有一定的关系。
气候变化导致的地壳均衡回弹效应自第四纪以来、尤其是1.2Ma以来趋于加强，在山地隆升过程中扮演越来越重要的角色。从间冰期到冰期，地壳均衡回弹效应具有强弱交替的特点，并周期性地施加于地壳动力过程中，有利于促进山体的阶段性隆升，这或许是第四纪中期以来山地隆升频度加大的原因。由于山地的侵蚀作用在冰期向间冰期转换时最强，山盆地壳均衡升降效应因而最显著，使得山地快速隆升多开始于间冰期的初期。第四纪气候变化使得山地的侵蚀作用趋于加强，加速了山地隆升期的进程，这或许是第四纪中期以来隆升期持续时间变短以及隆升速率加大的原因。
总之，经历了白垩纪晚期至第三纪早期长期广泛的北台期夷平作用之后，忻定盆地周缘地区处于地壳厚度均一化、地形夷平化状态。区域地壳拉张裂陷作用形成了裂谷盆地，并与气候变化共同控制了盆地周缘断块山地的第四纪阶段性隆升。第四纪以来，气候变化导致的山地侵蚀作用趋于加强，强化了地壳均衡回弹效应，并周期性地施加于地壳动力过程中，有可能是导致第四纪以来山地隆升作用频度加大、隆升期持续时间变短、隆升速率加大的关键因素。
初步探索了区域构造活动与气候变化在断块山地第四纪阶段性隆升中的作用，是论文的主要进展。
7．晚第四纪以来断块分期活动的地震表现
系舟山北麓断裂在距今100ka至22ka之间活动不明显，未发现地震地表位错事件；距今22ka以来垂直位移量达16～18m，发生了6～7次地表位错型地震事件。地震活跃期与最新断块活跃期相吻合，而地震平静期与末次断块稳定期相吻合。
近22ka以来，系舟山北麓断裂在早期和晚期的地震活动相对丛集，中间相对稀疏。震级也存在从大到小、再由小到大的变化趋势。五台山北麓断裂在距今6～7ka之前经历了相对平静期，之后进入地震活跃期。冲沟纵剖面揭示最新断块活跃期可划分为几乎等长的3个活动亚期，活动性依次为强、弱和强，体现了不平稳的特点。从系舟山北麓断裂的古地震研究来看，从末次稳定期转入最新活跃期是突然的、强烈的。
从古地震和微构造地貌方面对晚第四纪以来断块分期活动做了精细研究，是论文的主要进展。
Xinding Basin is situated in the northern part of Fenwei Graben system; it is a typical Cenozoic extensional fault-depression basin. The basin is bounded by the NE-trending North Piedmont Fault of Wutai Mountain, North Piedmont Fault of Xizhou Mountain, South Piedmont Fault of Hengshan Mountain, and the NNE-trending East Piedmont Fault of Yunzhong Mountain. The vertical displacements of Wutai and Xizhou Mountain North Piedmont Faults are relatively stronger, which control the primary geomorphic feature of the basin. The basin is surrounded by continuously uprising fault-block mountains. The Wutai Mountain is on the east side of the basin. It is the roof of North China region, its highest peak Beitai is 3058m above sea level, after which the famous Beitai-stage planation surface is named. On the basin side of the fault-block mountains 3 levels of planation surfaces and 7 steps of river terraces were developed below the Beitai-stage planation surface, indicating multi-stage uplifting. In the history three earthquakes above magnitude 7 took place in the Xinding Basin. A study on the responses of earthquakes to episodic tectonism helps to reveal the dynamic mechanism of earthquakes.
This dissertation took the stepped landforms of the piedmont zone surrounding Xinding Basin as the main thread, the north piedmont of Wutai and Xizhou mountains as the study area, and divided the Quaternary tectonic movement into stages based on the paragenetic relation between river terraces and pediments. By dating the river terraces and quantitatively analyzing the longitudinal river profile the division and timing of tectonic stages since 1.2 Ma were carried out. On this basis the responses of earthquakes to tectonism in the Late Quaternary were further discussed. The major achievements and new understandings are described as follows.
1. The paragenetic evolution model of pediments and fluvial morphology under the action of episodic fault movement
In the piedmont zone of fault-block mountains the pediments and tectonic terraces developed during the same tectonic movement are possessed of a certain paragenetic relation. According to the classic fault scarp evolution theory, a fault block was uplifted to form a fault scarp; afterwards the gully on the scarp eroded and cut down to form a V-shaped valley. Upon entering a quiescent period of tectonic activity, the riverbed was widened to form a U-shaped wide valley, meanwhile the slope of fault scarp decreased because of denudation. If the quiescent period was sufficiently long, a pediment was formed by fault scarp recession. Under the control of downthrown basin ground as the common base level of erosion, the pediment surface and the valley bed were approximately situated at the same altitude. Later-stage block uplift formed new fault scarp, the previous pediment surface and the wide valley were uplifted, and the gully started retrogressive erosion from the scarp to produce a terrace. In the piedmont zone the terrace and pediment surface jointly formed a stair of surface; they were contiguous in space with comparable elevation. Multiple tectonic cycles may produce a multi-stepped landforms consisting of a series of pediments and corresponding river terraces. Therefore the history of tectonic uplifting can be determined from the stepped landforms, and the starting time of rapid uplift, i.e., the commencing of active tectonic period, can be determined by dating the tectonic terraces (the time of initial river downcutting).
If an active tectonic stage is considered as a period of intense earthquake activities, then a scarp of tens meters high is the result of cumulated multiple earthquake dislocations, and a quiet tectonic stage means obviously decreased strength and frequency of fault slips, then downstream from the end of terrace is a rapid-flow reach consisting of a series of knickpoints, where the river bed is steep and the valley is narrow. On the other hand, upstream from the end of terrace is a slow-flow reach, where the riverbed is broad and gentle. The rapid reach represents an active tectonic period, while the slow reach means a stable tectonic period. If the retreating speeds of the knickpoints between two neighboring terraces are uniform, then the durations of active and quiet period can be estimated from the length of rapid and slow reach respectively. The time span of slow reach is T_(Si) = (T_i - T_(i-1)) L_(Si)/(L_(Si) + L_(Ri)), the time span of rapid reach is T_(Ri) = (T_i - T_(i-1)) L_(Ri)/(L_(Ri) + L_(Si)), where i is terrace stair numbered from bottom upward, T_i is the age of i-th terrace stair, L_(Ri) and L_(Si) represents respectively the length of rapid and slow reach between the i-th and i-1-th terrace. In this way we can obtain the starting and ending times of the active and stable tectonic periods.
2. The step division of pediments
On the north piedmont of Wutai Mountain nearby Nanyukou of Fanshi County, as well as on the north piedmont of Xizhou Mountain nearby Hu Village of Dingxiang County, 6 pediments (P1 to P6) were developed between the fault scarp foot and the Tangxian-stage planation surface. They rise step by step, extend as belts along the piedmont, and are cut by transverse gullies. On the interfluves, the pediment surfaces are relatively flat with widths from meters to hundreds meters and slopes between 5°and 25°. Each step of pediment is connected to the next step via a frontal bank whose slope is between 30°and 60°. The frontal riser of the first pediment surface (P1) is the most recent fault scarp or diluvial platform; at the foot of pediment P1 outcrops the fault. The higher pediments resulted from recession and uplift of antecedent fault scarps, their frontal risers are retreating scarps. Above the P6 pediment there is a slope that transits into the flat Tangxian-stage planation surface (Pt). The six steps of pediments indicate six stages of rapid tectonic uplift during the Quaternary.
The spatial pattern of pediments is relatively stable. Near the mouth of some large gullies (antecedent gullies), pediments are often united with the terraces or the bottoms of ancient wide valley. The altitude of the same stair pediment is basically identical on both sides of a gully. It changes little along the piedmont; if it does the undulation is continuous unless the pediment is offset by transverse faults. These features make it possible to distinguish pediments from local landform steps caused by differential erosion.
The differentiation of activity along fault may result in different altitudes of pediments of the same stair in different regions. To the east and west of Yangyan River the north piedmont of Wutai Mountain is respectively controlled by two left-step en echelon faults. To the east of Yangyan River the altitudes of pediment P6 to P1 are successively 1510～1560m, 1450～1500m, 1350～1410m, 1290～1310m, 1250～1270m, and 1215～1230m above sea level. To the west they are 1488～1493m, about 1430m, 1385～1390m, 1320m, 1270～1290m, and 1225～1250m. Pediments P6 to P4 are higher on the east, while P3 to P1 are higher on the west, indicating that the western fault is more active in Late Quaternary. This is also seen in the relative heights of pediments. On the east piedmont P6 to P1 are successively 260～310m, 200～230m, 130～185m, 95～120m, 30～65m, and 10～25m above the alluvial plain on the basin side; while on the west piedmont they are successively 283～313m, 225～250m, 185～205m, 115～140m, 70～85m, and 40m. Starting from P4 the west is obviously higher than the east. P2 and P1 in west are nearly twice as high as that in the east. Therefore, a regional correlation of pediments needs to trace continuously , and the corresponding fluvial geomorphology should be referred to.
3. Division and dating of river terraces
On the north piedmont zone of Wutai and Xizhou Mountains 7 steps of Quaternary terraces were developed in large transverse gullies. Along the valley of Hutuo River in east Xizhou Mountain 5 steps of terraces are rather completely preserved. Each terrace is covered by a loess-paleosol sequence of different thickness, which facilitates dating the geomorphic surfaces and making a regional correlation.
Yangyan River is a large transverse gully on the north piedmont of Wutai Mountain. It originates from northeast Beitai peak of 3058m above sea level and is 35km long. It flows northwards out of the mountain then into the upper reach of Hutuo River. Below the Tangxian-stage planation surface of altitude 1600m there are 7 steps of Quaternary terraces along the Yangyan River(T1 to T7, T1 is the youngest). Among them T1 and T2 are aggradational terraces, T3 to T5 are strath terraces, T6 and T7 are degradational terraces. Below T5 the terraces are overlain by relatively complete loess-paleosol sequences. By means of paleomagnetic polarity and susceptibility tests the paleomagnetic stratigraphy of terraces T5 and T4 was established and correlated to the standard loess profile and abyssal oxygen isotope curve. It is determined that the oldest aeolian deposit on terrace T5 is paleosol S15, corresponding to oxygen isotope stage MIS 37, and the age of the terrace is 1.2 Ma.The oldest aeolian deposit on terrace T4 is paleosol S5, corresponding to oxygen isotope stage MIS 15, and the age is 0.6 Ma. The oldest aeolian deposit on terrace T3 is paleosol S1, corresponding to oxygen isotope stage MIS 5, and the age is 0.13 Ma. The oldest aeolian deposit on terrace T2 is loess L1, corresponding to oxygen isotope time MIS2. The ages of terrace T1, T2 and T3 given by ~(14)C and luminescence dating are 6ka, 20ka and 134ka BP respectively.
The gullies on the north piedmont of Xizhou Mountain are of smaller scale, and the higher terraces are rather severely damaged by erosion. Nearby Hu Village 6 stairs of terraces were developed in the transverse gully, among which T1 and T2 are aggradational terraces, T3 is strath terrace, T4 to T6 are degradational terraces. The age for terrace T1, T2, and T3 from luminescence dating is respectively 6ka, 20ka, and 140ka BP. The oldest aeolian deposit on T2, T3, and T4 is respectively L1, S1, and S5.
The Hutuo River flows through the entire Xinding Basin, then crosses the east segment of Xizhou Mountain on the southeast side of the basin to continue eastward. In the gorge area along Rongjiazhuang, Zhaojiazhuang, to Lingzidi in the eastern Xizhou Mountain, 5 stairs of river terraces are well preserved along the river. Among them T1 and T2 are aggradational terraces, T3 to T5 are strath terraces. Luminescence dating resulted the ages of T1 to T3 terraces as 7.6ka, 20～26ka, and 130ka BP. The oldest aeolian deposit on T2, T3, and T4 is L1, S1, and S5 respectively. The loess stratum on T5 was eroded.
In summary the river terraces of the same stair on the north piedmont of Wutai Mountain and on Xizhou Mountain piedmont are overlain by identical loess-paleosol sequences, the ages of terraces are generally consistent, indicating that the river terraces in this region were formed by the same mechanism.
Geomagnetic stratigraphic study of the Quaternary loess in Xinding Basin was carried out for the first time, age determination and regional correlation of the Quaternary river terraces were done systematically. This is a major progress achieved by this dissertation.
4. The paragenetic relation between pediments and river terraces and its tectonic implications
Nearby Nanyukou on the northern piedmont of Wutai Mountain and Hu Village on the north piedmont of Xizhou Mountain, the pediments (P1 to P6) and the terraces of lateral gullies (T2 to T7) are united at the gully mouth step by step, constructing six levels of stepped surfaces. These unified geomorphic surfaces extend as belts along the piedmont, with two ends extending toward the upper reach of gullies. In a plan view they are "U"-shaped and rise and encircle step by step. This step-by-step correspondence of pediments and terraces is consistent in the region.
The episodic uplift of fault block played the leading role in the formation of the unified surfaces, however, the climatic change and drainage difference of gullies resulted in the undulation of the surfaces which is accordant with the landform of the alluvial plain on the basin side. The Yangyan River is the largest river in the study area of north Wutai piedmont, abundant material source and relatively large discharge produced the huge alluvial fan with the valley mouth as its vertex, and made the united surface to maintain a descending trend from near the vertex outward in its development. The last glacial stage caused aggradation of the Yangyan River and the reduction of discharge made a large quantity of material stagnant nearby the fan vertex, thus increased the height difference between the fan vertex and the edge. By contrast, the gullies R1 to R6 on the east side of Yangyan River are of small size, insufficient material sources and steep riverbed, aggradation in glacial stage was not significant, and alluvial fans were not well developed. Glacial and local factors jointly enhanced the altitude difference of alluvial fans on the basin side, resulted in the large undulation of the first-step unified surface.
The first-step unified surface (consisting of P1 and T2) appears merely as a slope break of the fault scarp on the bedrock piedmont, while the unified surfaces of second and above steps have obvious pediment surfaces, indicating that a sufficiently long quiet tectonic period is necessary to form a unified geomorphic surface. Fault recession and river evolution are two different kinds of geomorphologic processes. According to field investigation, the headward erosion of the river was generally much quicker than the recession of fault scarp. If the quiescent period was short, even though the gully reached an equilibrium state and the riverbed was widened by lateral erosion, but the fault scarp did not obviously retreat or become less steep, the following tectonic uplift might cause river incision to form terrace, but there would not be corresponding pediment or slope break of the fault scarp. Terrace T1 did not develop a corresponding pediment at the valley mouth, but formed a scarplet several meters high across the fault, its synchronous sediments on the downthrown wall are buried below the contemporary alluvial fan. Its formation was a manifestation of fault activity, and was also accordant with the optimal climate period in Holocene. What roles the tectonic and climatic factors played is worthy of further investigation.
According to the sedimentary texture of alluvium and the oldest aeolian deposit overlying, terraces T3 to T5 were formed when the river was in an erosion or equilibrium state, so fault-block uplift caused synchronous and rapid river incision, the age of terrace can be considered as the starting time of speedy tectonic uplift. Terrace T2 was formed when the river was in an aggradation state (in large gullies) or in an erosion and equilibrium state (in medium and small gullies), but the arid climate of the last glacial maximum did not prevent river incision. It can be inferred that climatic factors did not significantly affect the incision caused by tectonic uplift. Even if the incision was postponed slightly, the time lag should not be very long. Therefore, the age of T2 can be approximately considered as the starting time of the last speedy tectonic uplift.
Based on the study of paragenetic relation of pediments and river terraces, it is deduced that Wutai Fault-block Mountain experienced 6 relatively strong tectonic uplifting events in the Quaternary. The last 4 events started at 1.2Ma, 0.6Ma, 0.13Ma, and 0.02Ma B.P. respectively. This result is consistent with previous studies in southern Fenwei Graben system and in the northeastern part of Tibetan Plateau.
Paragenetic relation of pediments and terraces of transverse rivers was put forward for the first time, then uplifting stages of fault block was rebuilt by the paragenesis, thus effectively reducing the uncertainty of uplifting-event identification with single landform type. This is another major progress achieved by this dissertation.
5. Longitudinal profile of gully and the rhythm of mountain uplifting
Based on the surveyings of longitudinal profile along 3 gullies on the north piedmont of Wutai Mountain and 4 gullies on the north piedmont of Xizhou Mountain, it is found that the riverbed consists of serial rapid reaches with steep slope and slow reaches with gentle slope. The rapid reach is composed of a series of dense knickpoints, while on the slow reach the knickppoints have small height and large interval. The extinction point of terrace corresponds to the slope-break of riverbed, from which downstream there is narrow rapid reach and upstream there is broad slow reach. Because the episodic uplifting of fault-block plays a leading role in the incision of transverse river, the above features in the river profile are difficult to explain by differential erosion due to lithological differences. Instead they should be the result of episodic fault slippings. The rapid reachs were formed in active tectonic periods, the knickpoints resulted from many times of large-displacement surface faultings at short recurrence intervals; while the slow reachs were formed in stable tectonic periods when the displacement and frequency of faulting were low.
If the retreating speeds of the knickpoints between two neighboring terraces were uniform, then the duration time of active and quiet tectonic periods can be estimated from the ages of terraces and the lengths of rapid and slow reachs. In this way the uplifting rhythm of Wutai fault-block mountain in the recent 120Ma can be described as follows. The rapid uplifting periods are 1200 to 1056ka, 600 to 522ka, 130 to 99ka, and 20 to Oka; the rest times are quiet periods. The uplifting rhythm of Xizhou fault-block mountain in the recent 120Ma can be described as follows. The rapid uplifting periods are 1200 to 1063ka, 600 to 486ka, 130 to 106ka, and 20 to 0ka; the rest times are quiet periods. The results from two regions are comparable but with some differences, which are considered due to insufficient statistic samples and local lithological and drainage differences. With the increase of surveying gullies and statistic samples, the local factors can be further suppressed. The statistics indicates that the duration of an active period is much shorter than the consecutive stable period, being about l/2.4～1/18.8 of the previous stable period and 1/2.0～1/6.7 of the following stable period. Moreover, the longer the stable period lasted, the longer the following active period was.
Nearby the extinction point of terrace the river may sometimes have a transition zone of gradient, i.e., the rapid reach and slow reach form an S-shaped slope break zone, which means that the change of tectonic mode may have a transition period. But not all cases are like this, further verification is needed.
Comparing to the result of paleo-earthquake studies for the recent 20ka, the number of surface-rupture earthquakes is not consistent with the number of knickpoints below terrace T2, the latter is generally less than the former. The reason should be further investigated.
On the premise that fault-block uplifting plays a leading role in the formation of river terraces, the durations of tectonic stages can be estimated by dating the terraces and analyzing the longitudinal profile of the river, thus improving the accuracy of age of tectonic stages. This is the third major progress achieved by the dissertation.
6. Mechanism discussion of episodic block uplifting
Two possible mechanisms are responsible for the episodic block uplifting around Xinding basin—regional tectonism and climate changes. Regional crust pulling-apart induce sinking of basin, multiperiodic activity of boundary faults and uplifting of mountain. Cooling and greater variability of Quaternary climate cause intense periodical erosion and isostasy rebound of the mountain, which is a factor which cannot be neglected either in the episodic uplifting of block mountains.
The heights of pediment series indicate that the long term average uplifting rate of the block mountains around Xinding basin changed little since late Pliocene, but the uplifting frequency increased distinctly since the last 1.2Ma, and the uplifting has a tendency of shorter duration, shorter recurrent time and bigger uplifting rate, which cannot be ascribed to pure regional tectonism. On the other hand, quick uplifting stages did not accord completely with the fluctuation of climate, which favor that fluctuation of climate is not the dominant agent. However three of four uplifting stage started from early interglacial, and two are completely in interglacial stage, which suggest some temporal relation between uplifts and climate changes.
The greater variability of climate since 1.2Ma ago enhance the interglacial exhumation of mountain and the accumulation of basin, intensifying the isostasy rebound and favoring the episodic uplift of mountains, which maybe the reason why the mode of mountain uplifting changed. The erosion of mountain in the switch from glacial to interglacial is the strongest, so the isostasy is the most prominent, which may be the reason why mountain uplifts mostly start at early interglacial.
In a word, some combination of regional tectonism and climate changes has been responsible for episodic uplifting of block mountain in Quaternary. Crust isostasy rebound induced by fluctuation of climate has been intensifying since Quaternary, and periodically acting on crustal dynamic process, which may be the key mechanism for more frequent uplift of mountain, shorter duration and higher rate of uplift since Quaternary.
Rules of regional tectonism and climate changes in episodic uplift of block mountains since Quaternary has been discussed preliminarily. This is the fourth major progress achieved by the dissertation.
7. Responses of earthquakes to tectonic episodes in late Quaternary
The Xizhou Mountain north piedmont fault was not active between 100ka and 22ka BP, no surface-faulting event was found; on the other hand, the vertical displacement since 22ka amounts to 16～18m, 6～7 faulting events have taken place. The active period of earthquakes accords to the most recent active tectonic stage, while the time of seismic quiescence consists with the last stable tectonic stage.
Since the last 22ka, the earthquakes were clustering in the forepart and the terminal. but relative sparse in the middle stage. The magnitudes of earthquakes waned in the early stage, then largened in the late stage. The Wutai north piedmont fault underwent a quiet period 6 to 7 ka before, then entered a active period of earthquakes until now. The longitudinal profiles of transverse gullies indicates that since the last 20ka the two fault have experienced three inferior tectonic stages near equally long which are in turn active, quiet and active. Paleoearthquakes show that switch from the last stable to the recent active tectonic stage was abrupt and intense.
The responses of earthquakes to tectonic episodes in late Quaternary have been explored initially, which helps to unveil the mechanism of earthquake clustering of fault segment in long time span. This is the fifth major progress achieved by the dissertation.
引文
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