High-temperature embrittlement of extensional Alpine mylonite zones in the midcrustal ductile-brittle transition
High-temperature embrittlement of extensional Alpine mylonite zones in the midcrustal ductile-brittle transition
复制标题
中地壳韧脆转变中伸展高山糜棱岩带的高温脆化
DOI：
--
复制
发表时间：
2001
期刊：
影响因子：
--
通讯作者：
T. Wawrzyniec
中科院分区：
文献类型：
--
作者：
G. Axen;J. Selverstone;T. Wawrzyniec
两个大位移伸展阿尔卑斯剪切带的构造和流体包裹体分析表明，脆化发生在异常高温下，表明除了逐渐变化的温度和压力外，其他因素对韧性-脆性转变起主要控制作用。Brenner和Simplon糜棱岩带内的大多数岩石，包括丰富的弱片岩，在T=450°-575°C和P=400-750 Mpa时，由于分布的断裂、破裂和脆-韧性剪切而脆性破坏，即使在强烈的正片麻岩中，也应该是塑性或半脆性流动而不是脆性变形。脆化是由瞬变(？)较高的流体压力和局部弯曲应变而不是由温度或压力降低。尽管底部继续受到高温剥蚀，剪切带脆性部分的糜棱岩化作用永久停止。然而，糜棱岩化作用显然在剪切带结构最高的∼50m处继续进行，那里没有或很少有脆性结构。在这些晚期、薄的糜棱岩带和糜棱岩化结束的更坚固、更深的部分之间形成了强度对比。这种对比可能既反映了晚期糜棱岩带的减弱，也反映了深部脆性部分的加强，尽管前者最终可能由于剪切带变薄而应变速率增加而增加了差应力。剪切带可能在T≈450°C和P≈400 Mpa(∼15 km)的作用下演化为离散的摩擦断裂。
Structural and fluid inclusion analyses of two large-displacement extensional Alpine shear zones show that embrittlement occurred at anomalously high temperatures and indicate that factors other than gradually changing temperature and pressure can exert primary control on the ductile-brittle transition. Most rocks within the Brenner and Simplon mylonite zones, including abundant weak schists, failed brittlely by distributed faulting, fracturing, and brittle-ductile shearing at T = 450°–575°C and P = 400–750 MPa, conditions in which plastic or semibrittle flow rather than brittle deformation is expected, even in strong orthogneiss. Embrittlement was caused by transiently(?) high fluid pressure and local bending strain rather than by temperature or pressure decrease. Mylonitization shut off permanently in the embrittled parts of the shear zones despite continued high-T denudation of the footwalls. However, mylonitization apparently did continue in the structurally highest ∼50 m of the shear zones where brittle structures are absent or rare. A strength contrast evolved between these late, thin mylonite zones and the stronger, deeper parts where mylonitization ended. This contrast probably reflects both weakening of the late mylonite zones and strengthening of the deeper embrittled parts, although differential stress may have ultimately increased in the former due to strain rate increase as the shear zones thinned. The shear zones probably evolved to discrete frictional faults by T ≈ 450°C and P ≈ 400 MPa (∼15 km).