Isla San Pedro Nolasco is located in the central-eastern part of the Gulf of California, formed by late Miocene (ca. 9–11 Ma) hypabyssal rocks, between the Sonora coast and oblique-divergent Pacific-North America plate boundary. Continental blocks exposed as islands in the Gulf of California host key geological evidence for understanding the evolution of the plate boundary. We present a detailed rock magnetic study of 31 independently oriented paleomagnetic cores from late Miocene dikes collected from a southeastern bay of the island. Opaque microscopy and rock magnetic properties suggest that magnetic mineralogy is mainly Ti-poor titanomagnetite or magnetite with limited maghemite and rarely titanohematite. Magnetization has two components. The primary and more stable is of medium to high temperature (300–560 °C) or high fields (up to 100 mT). The primary thermal remanent magnetization (TRM) and the oxide mineral textures observed suggest that cores from these dikes provide high reliability for paleomagnetic analysis. The mean paleomagnetic direction is as follows: Dec = 354.86°, Inc = 46.2°, α95 = 5.2, k = 44. Comparison of these results to the 10 Ma paleomagnetic pole for North America indicates that Isla San Pedro Nolasco has experienced no vertical-axis rotation but has displaced ~ 100 km to the northwest, similar to independent estimates for fault offset along with this structural system from the northern Gulf of California and regional tectonic reconstructions. These results, supported by detailed geological studies, indicate that the island was detached from mainland Mexico by transtensional faulting in the late Miocene Gulf of California shear zone that also formed the adjacent Yaqui pull-apart basin.

Isla San Pedro Nolasco 位于加利福尼亚湾的中东部，由晚中新世（约 9-11 Ma）浅层岩石形成，位于索诺拉海岸和斜发散的太平洋-北美板块边界之间。在加利福尼亚湾以岛屿形式暴露的大陆块为了解板块边界的演化提供了重要的地质证据。我们对从该岛东南部海湾收集的中新世晚期岩脉中的 31 个独立定向的古地磁核心进行了详细的岩石磁性研究。不透明的显微镜和岩石磁性表明磁性矿物主要是贫钛的钛磁铁矿或磁铁矿，磁赤铁矿有限，钛赤铁矿很少。磁化有两个组成部分。主要且更稳定的是中高温（300–560 °C）或高场（高达 100 mT）。观察到的初级热剩磁 (TRM) 和氧化物矿物结构表明，这些岩脉的岩心为古地磁分析提供了高可靠性。平均古地磁方向如下：Dec = 354.86°，Inc = 46.2°，α95 = 5.2，k = 44。这些结果与北美 10 Ma 古地磁极的比较表明，Isla San Pedro Nolasco 没有经历过垂直-轴旋转，但已向西北偏移约 100 公里，类似于对断层偏移的独立估计以及来自加利福尼亚湾北部和区域构造重建的结构系统。这些结果得到详细地质研究的支持，