Abstract Over forty new Nd isotope determinations are presented for granitoid orthogneisses from the Chalk River area, on the border between Ontario and Quebec in the SW Grenville Province. TDM model ages provide evidence for a tectonic window through the allochthonous belt into the Grenvillian parautochthon in the Chalk River area. This tectonic window constrains the location of the main ramp of the Allochthon Boundary Thrust southeast of the Chalk River area. However, the structure of the tectonic window has been complicated by down-faulting on the Ontario side of the Ottawa River, due to reactivation of late Precambrian normal faulting associated with the Ottawa–Bonnechere graben. Thus, on the Ontario side of the border, the surface expression of the tectonic window is represented by a strongly sheared tectonic duplex that underlies the main allochthon. However, the lower section of a 1 km-deep borehole drilled by Canadian Nuclear Laboratories has TDM model ages indicating penetration into the underlying parautochthon. The new data, combined with previous Nd isotope mapping and Lithoprobe seismic reflection results, point to a relatively constant thickness for the exhumed Muskoka allochthon. By showing that this exhumation occurred immediately below the Central Metasedimentary Belt boundary, this evidence strongly supports a ramp-flat thrusting model for the SW Grenville province. This model greatly reduces the likelihood of major crustal exhumation to the NW of Parry Sound domain, thus removing the theoretical basis for pervasive southeast directed orogenic collapse across the Ontario gneiss belt.

中文翻译：

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安大略乔克河附近构造窗口的 Nd 同位素映射：对大规模 Grenvillian 构造的影响

摘要 对来自西南格伦维尔省安大略省和魁北克省边界的乔克河地区的花岗岩类正片麻岩提供了四十多种新的 Nd 同位素测定。TDM 模型年龄为通过外地带进入乔克河地区的 Grenvillian 准原生地的构造窗口提供了证据。这个构造窗口限制了乔克河地区东南部的 Allochthon 边界冲断层的主要斜坡的位置。然而，由于与渥太华-博内切尔地堑相关的晚前寒武纪正断层的重新激活，渥太华河安大略一侧的向下断层使构造窗口的结构变得复杂。因此，在边境的安大略一侧，构造窗口的表面表现是由一个强烈剪切的构造双相体代表，它位于主要的异地生物之下。然而，加拿大核实验室钻探的 1 公里深钻孔的下部具有 TDM 模型年龄，表明渗透到下面的原地生物中。新数据与之前的 Nd 同位素映射和 Lithoprobe 地震反射结果相结合，表明挖掘出的 Muskoka 异地生物具有相对恒定的厚度。通过表明这种挖掘发生在中央变质沉积带边界正下方，这一证据有力地支持了 SW Grenville 省的斜坡平坦逆冲模型。该模型大大降低了主要地壳剥脱到 Parry Sound 域 NW 的可能性，