The Baja California peninsula forms the western margin of the Gulf of California (GC) rift system, which is an active tectonic setting manifested by seismicity and numerous geothermal sites. The present study examines the geochemistry of thermal fluids (major ions and gas species, δ18O-δD, δ13C, 3He/4He) from the southern tip of the peninsula (Los Cabos Block, LCB). Sampling was mostly focused on the coastal thermal manifestations in the towns of Buenavista and El Sargento, but other sites further inland are also included for broadening the scope of the study. The main objectives include: (i) characterize water-rock interactions and other processes controlling the fluid composition, (ii) constrain solute geothermometry estimates through multi-step geochemical modeling, and (iii) discuss the fluid origins in terms of tectonics and regional shear-wave velocity anomalies in the upper lithosphere. The geothermal systems in the area have a tectonic origin and result from fluid circulation along regional faults that penetrate the upper crust through the granitic basement. Mixing between the thermal fluids and seawater at the coast is clearly illustrated through major ions and δ18O-δD relationships. Reconstruction of the pre-mixing chemical compositions indicates a low-salinity fluid at Buenavista (Cl = 104–109 mg L−1) and a saline fluid at El Sargento (Cl = 7169 mg L−1). The geochemical modeling allows us to validate these endmember compositions and to address a common issue when using solute geothermometry, which is the uncertainty on the state of equilibrium of the thermal fluid with respect to wall-rock minerals. The corresponding results reveal contrasting thermal regimes at depths (Buenavista: 101–122 °C, El Sargento: 212–220 °C), likely associated with differences in geothermal gradient and fluid circulation depth. Our gas samples have among the lowest He isotopic ratios (0.07–0.95 Ra) in the Baja California peninsula, indicating that the origin of helium is mainly crustal. Shear wave tomography demonstrates that the study area is associated with two low velocity regions, one in the crust and the other one in the upper mantle. Our results favor the hypothesis that the heating of fluids is produced by lower-crustal flow driven by strong topographic gradients along the rifted margin. This study provides new insights into the origin of the thermal anomalies in the LCB and the adjacent GC rift system.

中文翻译：

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下加利福尼亚半岛南部（墨西哥）沿海地热系统的地球化学：流体起源、水岩相互作用和构造


下加利福尼亚半岛形成了加利福尼亚湾 (GC) 裂谷系统的西缘，该系统是一个活跃的构造环境，表现为地震活动和众多地热点。本研究研究了半岛南端（洛斯卡沃斯区块，LCB）热流体的地球化学（主要离子和气体种类、δ18O-δD、δ13C、3He/4He）。采样主要集中在布埃纳维斯塔和埃尔萨根托镇的沿海热表现，但为了扩大研究范围，还包括更内陆的其他地点。主要目标包括：（i）表征水-岩石相互作用和控制流体成分的其他过程，（ii）通过多步地球化学建模约束溶质地温测量估计，以及（iii）从构造和区域剪切角度讨论流体起源-上层岩石圈的波速异常。该地区的地热系统具有构造起源，是沿区域断层的流体循环产生的，区域断层穿过花岗岩基底穿透上地壳。通过主要离子和 δ18O-δD 关系清楚地说明了热流体和海岸海水之间的混合。预混合化学成分的重建表明布埃纳维斯塔 (Cl = 104–109 mg L−1) 为低盐度流体，El Sargento (Cl = 7169 mg L−1) 为含盐流体。地球化学模型使我们能够验证这些端元成分，并解决使用溶质地温测量时的常见问题，即热流体相对于围岩矿物的平衡状态的不确定性。 相应的结果揭示了不同深度的热状况（布埃纳维斯塔：101-122°C，埃尔萨根托：212-220°C），可能与地温梯度和流体循环深度的差异有关。我们的气体样本具有下加利福尼亚半岛最低的 He 同位素比（0.07–0.95 Ra），这表明氦的来源主要是地壳。剪切波断层扫描表明，该研究区域与两个低速区域相关，一个位于地壳，另一个位于上地幔。我们的结果支持这样的假设：流体的加热是由沿裂谷边缘的强地形梯度驱动的下地壳流产生的。这项研究为 LCB 和邻近的 GC 裂谷系统中热异常的起源提供了新的见解。