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在构造上必须把被动大陆边缘看作是活动的;它们经厉了变薄、沉降,以及引张等过程,这些形式影响着上覆沉积层,也为上覆沉积盖层的类型所影响。在中大西洋和特提斯洋张开期间沉积在南特提斯大陆边缘上的中生代和早第三纪的碳酸盐。对沉降作用作了很好的记录。由于极其广大的地区在逾150百万年的时间里持续沉降,而且在此期间除了正断层作用外,显然再无其它的构造活动,故这种沉降很可能是地壳变薄的一种均衡反应。采用适当密度进行的均衡研究表明,可用下述公式求得某特定时间里的地壳厚度 t’c:t’c=t_C-4.6t’_W,-1.8t’S式中,t_C 为原始地壳厚度,t’_W、t’_S分别为水深和沉积厚度。根据亚平宁的地层层系计算,意大利陆壳在深海盆地下变薄了40％左右,在碳酸盐台地下则为20％。我们认为,这种变薄也许是由Bott 1971年提出的机制引起的地壳引张造成的。根据这种机制,即便地壳处于均衡平衡的状态,在不同厚度地壳间的接触部位也应存在不平衡的横向压力。南特提斯边缘的引张和变薄影响到宽达数百公里的地带,我们认为这是发生于侏罗纪大洋裂谷作用之前的三迭纪陆内裂谷作用的影响所致。在陆内裂谷作用时期,形成了许多个纽瓦克(Newark)型的盆地,与它们有关的地壳不连续则可能为 Bott的横向流机制提供了一系列场所。亚平宁地壳的厚度目前至少是正常的,这表明它又重新增厚过;这种增厚可能发生在第三纪碰撞造山作用时期,Helwig 1976年提出的东阿尔卑斯造山运动亦发生在此时期。Helwig 增厚模式的一个重要方面是,它排除了在碰撞造山过程中有巨大体积的轻浮陆壳俯冲的必要性。
Passive continental margins must be considered structurally active; they undergo thinning, subsidence, and extension processes that affect the overlying sediments and the type of overlying caprock. Mesozoic and early Tertiary carbonate deposits on the margin of the southern Tethys continental margin were deposited during the opening of the Middle Atlantic and the Tethys Ocean. The settlement has been well documented. Since the vast majority of the region continues to settle over more than 150 million years and there is clearly no other tectonic activity other than the normal fault during this period, it is likely that this subsidence is an equilibrium response to crustal thinning . Equilibrium studies using appropriate densities show that crustal thickness t’c for a given time can be determined using the following formula: t’c = t_C-4.6t’_W, -1.8t where t_C is the thickness of the original crust, t ’_W, t’_S, respectively, the depth of water and sediment thickness. According to the Appenining stratigraphic system, the continental crust in Italy has thinned by about 40% below the deep-sea basin and 20% below the carbonate platform. We think this thinning may be caused by the crustal extension caused by the mechanism proposed by Bott 1971. According to this mechanism, even if the crust is in a state of equilibrium equilibrium, there should be unbalanced lateral pressure at the contact points between crusts of different thicknesses. The extension and thinning of the margin of South Tethys affected hundreds of kilometers of land, which we believe is due to the Triassic intracontinental rifting that occurred before the Jurassic ocean rifting. During the intracontinental rifting period, many Newark-type basins were formed, and the discontinuous crust associated with them may provide a series of places for the lateral flow mechanism of Bott. The thickness of the apennine crust is now at least normal, suggesting that it has re-thickened; this thickening may have occurred during the Tertiary collision orogeny and Helwig’s 1976 Eastern Alps orogeny also occurred during this period. An important aspect of the Helwig thickening model is that it precludes the need for a massive volcanic crust subduction during a collision orogeny.