Multiple shortening took place during the Mesozoic in the periphery of the North China block (NCB), a major tectonic grain of East Asia, but no consensus exists as to the timing, intensity, and causes of individual events. This work focuses on the first phase shortening in Jurassic, which represents the initiation of Yanshanian orogeny, by investigating the structural evolution of the Yanshan belt in the middle segment of the northern NCB. The first-phase shortening used to be regarded as a weak compressive event for it is recorded by parallel or low-angle unconformities between Middle and Upper Jurassic strata in some places. Our recent field investigations, however, identified some pronounced angular unconformities separating the Upper Jurassic strata from the underlying highly folded units in the northern Yanshan belt. The new observations demonstrate that the first-phase shortening was actually a strong contractional event. The co-existence of parallel and angular unconformities in the southern and northern Yanshan belt can be explained by different thrusting processes. It is argued that lateral variations of initial basement dips must have played a crucial role in controlling structural evolution of the overlying pre-Middle Jurassic sedimentary succession. It is shown that the first-phase thrusting and folding system was thin-skinned and predominantly verge northwestward. Deformational intensity also weakened toward the north and northwest. The first-phase shortening is constrained to occur in a short time span from 170 to 165 Ma based on new radiometric ages of strata above and below the unconformity, and presumably took place simultaneously in the northern NCB. The first-phase shortening was likely driven by horizontal tectonic push from the southeast. It is suggested that low-angle subduction of the paleo-Pacific plate might be responsible for the Middle Jurassic orogeny in the northern NCB and other parts of East Asia.

华北地块（NCB）的外围中生代发生了多次缩短，这是东亚的主要构造构造，但就个别事件的发生时间，强度和成因没有共识。这项工作的重点是通过研究北卡罗莱纳州北部中段燕山带的构造演化，来研究侏罗纪的第一阶段缩短，这代表了燕山造山运动的开始。由于某些地方中侏罗统地层与上侏罗统地层之间存在平行或低角度不整合，因此记录了第一阶段的缩短，因为它被认为是微弱的压缩事件。然而，我们最近的实地调查发现，燕山北部北部地区上侏罗统地层与下伏的高度折叠单元之间存在明显的角不整合。新的观察结果表明，第一阶段缩短实际上是一个强烈的收缩事件。燕山南部和北部平行和角度不整合面的共存可以通过不同的逆冲过程来解释。有人认为，初始基底倾角的横向变化必定在控制上中侏罗世前沉积沉积演替的结构演化中起着至关重要的作用。结果表明，第一阶段的推覆和折叠系统是薄皮的，主要向西北倾斜。变形强度也向北和西北方向减弱。基于不整合上方和下方的新的辐射年龄，第一阶段缩短被限制在170至165 Ma的短时间内发生，大概是在北部NCB同时发生的。第一阶段的缩短可能是由于东南部的水平构造推动。有人认为，古太平洋板块的低角度俯冲作用可能是造成北NCB和东亚其他地区中侏罗世造山运动的原因。