The tectonic history of Adelaide’s scarp-forming faults

A series of linear to arcuate fault scarps separate the Mount Lofty Ranges from the Cenozoic St Vincent and Murray basins of South Australia. Their tectonic, sedimentary and geomorphic evolution is traced from the oldest rock record through to present-day seismicity. The scarps are the latest manifestation of repeated compressive reactivation of ancient, deep-seated crustal faults and fractures whenever the stress field was of appropriate orientation. Formation of the basins and uplift of the ranges resulted from the same processes of repeated compressive reactivation. Continental crust was intensely fractured during three episodes of Neoproterozoic–Cambrian rifting that led to the formation of the Adelaide Geosyncline and break-up of Rodinia. Neoproterozoic eastward-dipping, listric extensional faults provided accommodation space for deposition of the Burra Group. Sediments of the Umberatana and Wilpena groups were deposited under mainly sag-phase conditions. In the early Cambrian, new extensional faults formed the deeply subsident Kanmantoo Trough. Cambrian rift faults swung from east–west on Kangaroo Island through northeasterly on Fleurieu Peninsula to north–south in the easten Mount Lofty Ranges, cutting across the older meridional rifts. These two sets of extensional faults were reactivated as basement-rooted thrusts in the ensuing Delamerian Orogeny. The Willunga Fault originated as a Cambrian rift fault and was reactivated in the Delamerian Orogeny as a thrust dipping southeast under a regional basement-cored antiform on southern Fleurieu Peninsula. Much of southern Australia, including the eroded remnants of the Delamerian highlands, was covered by a continental ice sheet in the Carboniferous–Permian. The preferential preservation of glacial sediments on Fleurieu Peninsula may have resulted from extensional reactivation of the Willunga Fault, possibly in the early Mesozoic. Fleurieu Peninsula was then warped into an open, southwest-plunging antiform, spatially coincident with the much higher amplitude Delamerian antiform. Glacial sediments were eroded from uplifted (up-plunge) areas before formation of a ‘summit surface’ across deeply weathered bedrock and preserved glacial sediments in the later Mesozoic. This surface was covered with fluvial to lacustrine sediments in the middle Eocene. Neotectonic movements under a renewed compressive regime commenced with reactivation of the Willunga Fault, restricting subsequent Eocene to Miocene sedimentation to the St Vincent Basin. The Willunga scarp was onlapped in the Oligocene–Miocene concomitant with continuing uplift and formation of a hanging-wall antiform. In the late Cenozoic, repeated faulting and mild folding, angular unconformities, ferruginisation and proximal coarse sedimentation took place on various faults at different times until the late Pleistocene.

一系列线性至弧形的断层崖将洛夫蒂岭（Mount Lofty Ranges）与南澳大利亚州的新生代（Cenozoic）圣文森特盆地和默里盆地（St Vincent and Murray basins）分隔开来。研究追溯了这些断层崖从最古老的岩石记录直至现今地震活动性（seismicity）的构造、沉积与地貌演化过程。这些断层崖是古深部地壳断层与裂隙（crustal faults and fractures）在应力场（stress field）方向适宜时，反复遭受挤压活化的最新表现。盆地的形成与山脉的抬升均源自同一类反复挤压活化的作用过程。大陆地壳（continental crust）在三次新元古代-寒武纪（Neoproterozoic–Cambrian）裂谷作用（rifting）期间发生强烈断裂，这一过程促成了阿德莱德地向斜（Adelaide Geosyncline）的形成以及罗迪尼亚超大陆（Rodinia）的裂解。新元古代东倾铲状正断层（listric extensional faults）为巴拉组（Burra Group）的沉积提供了可容空间（accommodation space）。翁巴拉塔纳群和威尔皮纳群（Umberatana and Wilpena groups）的沉积物主要沉积于坳陷期（sag-phase）环境。寒武纪（Cambrian）早期，新生的正断层形成了深陷的坎曼图槽（Kanmantoo Trough）。寒武纪裂谷断层的走向从袋鼠岛（Kangaroo Island）的东西向，经弗勒里厄半岛（Fleurieu Peninsula）的北东向，延伸至洛夫蒂岭东部的南北向，切割了早期的南北向裂谷。这两套正断层在随后的德拉梅里亚造山运动（Delamerian Orogeny）中被活化成为基底逆冲断层（basement-rooted thrusts）。威隆加断层（Willunga Fault）最初为寒武纪裂谷断层，在德拉梅里亚造山运动中被活化为南东倾逆冲断层，覆于弗勒里厄半岛南部的区域基底背斜（basement-cored antiform）之下。石炭纪-二叠纪（Carboniferous–Permian）期间，包括德拉梅里亚高地侵蚀残余在内的澳大利亚南部大部分区域被大陆冰盖（continental ice sheet）覆盖。弗勒里厄半岛上冰川沉积物（glacial sediments）的优先保存可能源自威隆加断层的伸展活化作用，该作用可能发生于早中生代（early Mesozoic）。此后弗勒里厄半岛被弯曲形成一个开阔的、向南西倾伏的背斜（southwest-plunging antiform），其空间位置与振幅大得多的德拉梅里亚背斜重合。冰川沉积物从抬升区域被侵蚀，随后在晚中生代形成了覆盖于深度风化基岩（weathered bedrock）之上的夷平面（summit surface），并保存了冰川沉积物。该夷平面在中始新世（middle Eocene）被河流-湖泊沉积物（fluvial to lacustrine sediments）覆盖。在更新的挤压应力体制（compressive regime）下，新构造运动（Neotectonic movements）始于威隆加断层的活化，这限制了随后始新世至中新世的沉积物仅沉积于圣文森特盆地。渐新世-中新世（Oligocene–Miocene）期间，威隆加断层崖被超覆沉积，伴随持续的抬升与上盘背斜（hanging-wall antiform）的形成。晚新生代（late Cenozoic）时期，不同断层在不同时段经历了反复断裂作用、轻微褶皱、角度不整合（angular unconformities）、铁质化作用（ferruginisation）以及近源粗碎屑沉积，直至晚更新世（late Pleistocene）。