Abstract

Construction of a 3D geological model for the eastern Lachlan Orogen provided a new reference model for a combined magnetic and gravity model of the Macquarie Arc, an intra-oceanic arc accreted to Gondwana during the Late Ordovician to early Silurian Benambran Orogeny. Geologically constrained, iterative 2.5 D forward modelling represented the entire thickness of the crust over a west-to-east profile extending for 230 km, approximately coinciding with seismic reflection profiles 99AGSL1 and 99AGSL2. The output model, referred to here as the Forbes model, confirms west-dipping thrust stacks of the Junee-Narromine and Molong volcanic belts of the Macquarie Arc, in packages that also comprise underlying arc basement, overlying Ordovician turbidites, and overlying Silurian–Devonian volcanic and sedimentary units. Ordovician rocks in the Parkes Fault Zone are distributed in wedges within a flower-fault structure. These structural elements are broadly similar to an earlier interpretation of the seismic profiles and corresponding potential-field models but differ in the nature of the substrate to the Macquarie Arc. The arc substrate is best fitted with physical properties broadly that of an intermediate calc-alkaline rock, rather than mid-ocean ridge basalt (MORB) as previously inferred. Highly magnetic metamafic rocks of the Jindalee Group appear to occupy relatively narrow fault-bounded slivers and may correspond to mafic volcanic rocks in the forearc rather than fragments of MORB basement. The thick intermediate-composition crustal profile required by the Forbes model cannot distinguish between an Izu-Bonin-like thickened arc or a compound of an Ordovician arc developed on a pre-existing Cambrian arc, but the simple MORB substrate previously assumed for the Macquarie Arc can be excluded, as can obduction of the Macquarie Arc as a nappe over the Wagga Belt.

1. KEY POINTS

2. A 3D model for the Lachlan Orogen provides geological constraints on joint magnetic and gravity model across the accreted intra-oceanic Macquarie Arc.

3. The substrate of the arc is best fitted with magnetic susceptibility and density typical of an intermediate composition, in contrast with previous interpretations that the substrate preserves an oceanic crust basement.

4. Western and central belts of the Macquarie Arc are stacked by west-dipping thrusts, inconsistent with emplacement as an allochthonous nappe.

摘要

拉克兰造山带东部 3D 地质模型的构建为麦格理弧的磁力和重力组合模型提供了新的参考模型，麦格理弧是在晚奥陶世至志留纪 Benambran 造山运动期间形成于冈瓦纳的洋内弧。受地质约束的迭代 2.5 D 正演模拟代表了从西向东延伸 230 公里的剖面上地壳的整个厚度，与地震反射剖面 99AGSL1 和 99AGSL2 大致一致。输出模型，这里称为福布斯模型，证实了麦格理弧的 Junee-Narromine 和 Molong 火山带的西倾逆冲叠层，其中还包括下伏弧基底、上覆奥陶纪浊积岩和上覆志留纪-泥盆纪火山和沉积单元。帕克斯断裂带奥陶系岩石呈楔形分布，呈花断层结构。这些结构元素与地震剖面和相应势场模型的早期解释大体相似，但与麦格理弧的基底性质不同。弧形基底最适合广泛地与中间钙碱性岩石的物理特性相匹配，而不是如先前推断的洋中脊玄武岩（MORB）。Jindalee 群的高磁性超镁铁岩似乎占据了相对狭窄的断层边界，可能对应于前弧中的镁铁质火山岩，而不是 MORB 基底的碎片。

1. 关键点

2. 拉克兰造山带的 3D 模型为跨洋内麦格理弧的联合磁力和重力模型提供了地质约束。

3. 弧的基底最适合具有典型的中间成分的磁化率和密度，与之前的解释相反，基底保留了大洋地壳基底。

4. 麦格理弧的西部和中部带由向西倾斜的冲断层堆积而成，与作为外地推覆的就位不一致。