Slip and Dilation Tendency Analysis of the Salt Wells Geothermal Area

Critically stressed fault segments have a relatively high likelihood of acting as fluid flow conduits (Sibson, 1994). As such, the tendency of a fault segment to slip (slip tendency; Ts; Morris et al., 1996) or to dilate (dilation tendency; Td; Ferrill et al., 1999) provides an indication of which faults or fault segments within a geothermal system are critically stressed and therefore likely to transmit geothermal fluids. The slip tendency of a surface is defined by the ratio of shear stress to normal stress on that surface: Ts = T / on (Morris et al., 1996). Dilation tendency is defined by the stress acting normal to a given surface: Td = (o1-on) / (o1-o3) (Ferrill et al., 1999). Slip and dilation were calculated using 3DStress (Southwest Research Institute). Slip and dilation tendency are both unitless ratios of the resolved stresses applied to the fault plane by ambient stress conditions. Values range from a maximum of 1, a fault plane ideally oriented to slip or dilate under ambient stress conditions to zero, a fault plane with no potential to slip or dilate. Slip and dilation tendency values were calculated for each fault in the focus study areas at, McGinness Hills, Neal Hot Springs, Patua, Salt Wells, San Emidio, and Tuscarora on fault traces. As dip is not well constrained or unknown for many faults mapped in within these we made these calculations using the dip for each fault that would yield the maximum slip tendency or dilation tendency. As such, these results should be viewed as maximum tendency of each fault to slip or dilate. The resulting along-fault and fault-to-fault variation in slip or dilation potential is a proxy for along fault and fault-to-fault variation in fluid flow conduit potential. Stress Magnitudes and directions Stress field variation within each focus area was approximated based on regional published data and the world stress database (Hickman et al., 2000; Hickman et al., 1998 Robertson-Tait et al., 2004; Hickman and Davatzes, 2010; Davatzes and Hickman, 2006; Blake and Davatzes 2011; Blake and Davatzes, 2012; Moeck et al., 2010; Moos and Ronne, 2010 and Reinecker et al., 2005) as well as local stress information if applicable. For faults within these focus systems we applied either a normal faulting stress regime where the vertical stress (sv) is larger than the maximum horizontal stress (shmax) which is larger than the minimum horizontal stress (sv>shmax>shmin) or strike-slip faulting stress regime where the maximum horizontal stress (shmax) is larger than the vertical stress (sv) which is larger than the minimum horizontal stress (shmax >sv>shmin) depending on the general tectonic province of the system. Based on visual inspection of the limited stress magnitude data in the Great Basin we used magnitudes such that shmin/shmax = .527 and shmin/sv= .46, which are consistent with complete and partial stress field determinations from Desert Peak, Coso, the Fallon area and Dixie valley (Hickman et al., 2000; Hickman et al., 1998 Robertson-Tait et al., 2004; Hickman and Davatzes, 2011; Davatzes and Hickman, 2006; Blake and Davatzes 2011; Blake and Davatzes, 2012). Slip and dilation tendency for the Salt Wells geothermal field was calculated based on the faults mapped in the Bunejug Mountains quadrangle (Hinz et al., 2011). The Salt Wells area lies in the Basin and Range Province (N. Hinz personal comm.) As such we applied a normal faulting stress regime to the Salt Wells area faults, with a minimum horizontal stress direction oriented 105, based on inspection of local and regional stress determinations. Under these stress conditions north-northeast striking, steeply dipping fault segments have the highest dilation tendency, while north-northeast striking 60 degrees dipping fault segments have the highest tendency to slip. Several such faults intersect in high density in the core of the accommodation zone in the Bunejug Mountains and local to the Salt Wells geothermal .

处于临界应力状态的断层段，作为流体运移通道的概率相对较高（Sibson, 1994）。据此，断层段发生滑动的倾向（滑动倾向，slip tendency; Ts; Morris等，1996）或扩容的倾向（扩容倾向，dilation tendency; Td; Ferrill等，1999），可用于指示地热系统中哪些断层或断层段处于临界应力状态，进而具备输送地热流体的潜力。
某一表面的滑动倾向由该表面上的剪应力与正应力的比值定义：Ts = τ / σₙ（Morris等，1996）。扩容倾向则由作用于给定表面的正应力关系定义：Td = (σ₁ - σₙ) / (σ₁ - σ₃)（Ferrill等，1999）。
滑动与扩容参数通过3DStress（西南研究院，Southwest Research Institute）计算得到。滑动倾向与扩容倾向均为无量纲比值，反映了围限应力条件下施加于断层面的分解应力。其取值范围介于0至1之间：取值为1时，代表断层面在围限应力条件下理论上最易于发生滑动或扩容；取值为0时，则代表断层面无滑动或扩容潜力。
我们针对研究重点区域——McGinness Hills、Neal Hot Springs、Patua、Salt Wells、San Emidio及Tuscarora——的断层迹线，计算了各断层的滑动与扩容倾向值。由于上述区域内多数已填绘的断层其倾角约束不足或未知，我们采用可使滑动倾向或扩容倾向达到最大值的倾角完成了上述计算。因此，本研究结果可视作各断层滑动或扩容的最大倾向。
沿断层及断层间的滑动或扩容潜力变化，可作为沿断层及断层间流体运移通道潜力变化的替代指标。
应力大小与方向
各重点区域内的应力场变化，基于已发表的区域数据、全球应力数据库（Hickman等，2000; Hickman等，1998; Robertson-Tait等，2004; Hickman & Davatzes, 2010; Davatzes & Hickman, 2006; Blake & Davatzes, 2011; Blake & Davatzes, 2012; Moeck等，2010; Moos & Ronne, 2010; Reinecker等，2005）以及适用的局部应力信息进行近似估算。
针对上述重点系统内的断层，我们根据区域的大地构造背景，分别采用两种应力体制：①正断层应力体制，即垂向应力（σv）大于最大水平主应力（Shmax），且最大水平主应力大于最小水平主应力（σv > Shmax > Shmin）；②走滑断层应力体制，即最大水平主应力大于垂向应力，且垂向应力大于最小水平主应力（Shmax > σv > Shmin）。
基于对大盆地（Great Basin）有限应力大小数据的目视分析，我们采用的应力比值为Shmin/Shmax = 0.527、Shmin/σv = 0.46，该比值与Desert Peak、Coso、Fallon地区及Dixie Valley的完整和部分应力场测定结果一致（Hickman等，2000; Hickman等，1998; Robertson-Tait等，2004; Hickman & Davatzes, 2011; Davatzes & Hickman, 2006; Blake & Davatzes, 2011; Blake & Davatzes, 2012）。
Salt Wells地热田的滑动与扩容倾向，基于Bunejug Mountains幅地质图中填绘的断层进行计算（Hinz等，2011）。Salt Wells区域位于盆地与山脉省（N. Hinz 私人通信），因此我们对该区域的断层采用正断层应力体制，基于局部与区域应力测定结果，将最小水平主应力方向设为105°。
在该应力条件下，北东-北北东走向的高倾角断层段具有最高的扩容倾向，而北东-北北东走向、倾角为60°的断层段则具有最高的滑动倾向。在Bunejug Mountains调节带核心区以及Salt Wells地热田附近区域，多条此类断层以高密度相互交汇。