美国Paradox盆地盐构造新生代的拉伸活化

美国科罗拉多高原Paradox盆地中的裂口盐墙和盐背斜,主要是在古生界和中生界沉积物的差异负载作用下发育起来的。盐构造顶部的平缓褶皱,通常被认为是晚白垩纪至古新世拉腊米挤压运动的产物,而其脊顶地堑与山谷,则被认为是挤压后的松弛拉伸及盐溶作用所造成的。基于野外调查、物理模拟以及现代盐构造学理论,研究认为这些平缓褶皱和脊顶地堑,主要是新生代始新世至渐新世北北东向区域拉伸作用的产物。因为岩盐较其围岩软弱,拉伸形变主要集中在早先形成的盐墙、盐背斜及其较薄的顶板中,使盐构造发生活化并形成新的拉伸构造。顶板中的拉伸构造主要包括正断层、地堑、断层滑移(断滑)褶皱以及滚动褶皱。由于区域拉伸方向斜交盐构造走向,断层分布多呈羽列状。在母盐层较厚地区,拉伸使盐刺穿在正断层之下次动上升;而在母盐层较薄地区,拉伸使盐刺穿下降。在初始盐刺穿较高地区,盐构造的侧翼下降;而在初始盐刺穿较低地区,盐构造整体下降。拉伸使盐刺穿变宽,脊顶地堑下沉,形成拉伸型下降盐刺穿特征性的尖角盐体构造。局部地区,盐体从破碎的顶板间溢出,覆盖了下沉的脊顶地堑。

Cenozoic Extensional Rejuvenation of Salt Structures in the Paradox Basin, Utah and Colorado, USA

The famous breached salt anticlines and walls in the northeastern Paradox Basin have previously been considered to be preexisting diapirs amplified by late Cretaceous to Paleocene Laramide regional compression.Similarly,the spectacular arrays of normal faults along the valley sides have been attributed to dissolution of underlying salt.We present evidence that both folding and normal faulting were a response to NE-directed regional extension during late Eocene to Oligocene,which reactivated the preexisting NW-trending salt diapirs.Salt diapirs,which grew mainly by sedimentary differential loading during late Paleozoic and Mesozoic times,localized the Cenozoic extension within the weak diapirs and their thin roofs,resulting in normal faults above the diapir crests.Faults are en echelon because extension was oblique to the trend of the preexisting salt structures.Extension created fault-slip and rollover anticlines separated by listric normal faults.Where the source layer was thick,diapirs reactively rose below the normal faults.Elsewhere,where the source layer was depleted,diapirs subsided as they widened during regional extension. The flanks of tall diapirs subsided,whereas low diapirs entirely subsided.Crestal grabens sank into the widening,subsiding diapirs,creating cusps of salt.Salt locally extruded over extensionally foundered grabens.