ABSTRACT

The central Sierra Madre Oriental in Mexico developed primarily in the Late Cretaceous to early Palaeogene as a thin-skinned fold belt above a décollement hosted in Jurassic evaporite. In some regions shortening transitioned to sub-décollement structural levels, resulting in thick-skinned uplifts such as the Potosí uplift in Nuevo León. Thick-skinned deformation in the Potosí uplift involved folding of sub-décollement strata into an NNW-trending anticlinorium, cleavage development, thrust and conjugate strike-slip faulting, and extension fractures associated with barite mineralization. These structures consistently record subhorizontal shortening, directed ~52–65° in the southern uplift, ~69–72° in the northern uplift. Palaeocene to mid-Eocene zircon (U-Th)/He cooling dates record the timing of exhumation associated with thick-skinned uplift and suggest a continuation of shortening rather than a separate tectonic event. Zircon (U-Th)/He dates across the southern Potosí uplift and the Aramberri uplift, ~50 km to the south, range from ~66–53 Ma, whereas dates in the northern part of the Potosí uplift range from ~49–44 Ma. We attribute the transition to thick-skinned shortening to the mechanical strengthening of a planar décollement as rheologically weak evaporite was evacuated beneath synclinal keels of detachment folds. Along-strike differences in timing of exhumation and shortening directions may relate to differences in mechanical stratigraphy. Thicker intervals of evaporite in the northern uplift allowed thin-skinned shortening to continue while the southern uplift transitioned to thick-skinned shortening as the evaporite décollement was exhausted. As a result, stress-strain trajectories in the northern uplift refracted clockwise during continued deformation. Our findings provide new insight into the structural evolution of the Potosí uplift and may provide a framework for studying other thick-skinned uplifts in the orogen, and more generally orogenic belts that record a transition in deformation styles during progressive shortening.

摘要

墨西哥中部的西马德雷山脉东方山脉主要在白垩纪晚期至古近纪早期发育，是侏罗纪蒸发岩上沉积的褶皱带上的薄皮褶皱带。在某些地区，起酥油过渡到了下扇形构造水平，导致了隆隆的隆起，如新莱昂州的波托西隆起。波托西隆起的厚皮变形涉及将亚下折屑岩地层折叠成一个向北倾斜的趋势的抗气候层，分裂发育，逆冲和共轭走滑断层，以及与重晶石矿化有关的伸展裂缝。这些结构始终记录着水平下缩短，南部隆起指向〜52–65°，北部隆起指向〜69–72°。古新世至中新世锆石（U-Th）/ He冷却日期记录了与厚皮隆起有关的发掘时间，并表明持续时间的缩短而不是单独的构造事件。锆石（U-Th）/他的日期跨越南部波托西隆起和Aramberri隆升，向南〜50 km，范围从〜66–53 Ma，而在波托西隆起北部的日期范围为〜49–44嘛。我们将过渡归因于厚皮缩短，是由于流变学上较弱的蒸发岩被疏散到了褶皱的斜向龙骨下面，导致了平面折弯的机械强化。掘进时间和缩短方向的沿走向差异可能与机械地层学差异有关。北部隆起上较厚的蒸发岩层使得薄皮的起酥油持续，而南部隆起随着蒸发岩脱层的耗尽而转变为厚皮的起酥油。结果，北部隆起的应力-应变轨迹在连续变形期间顺时针折射。我们的发现为波托斯隆起的结构演化提供了新的见解，并可能为研究造山带中其他厚皮隆起以及更一般的造山带提供框架，这些造山带记录了渐进缩短过程中变形样式的转变。