Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California
Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California
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加利福尼亚州最东南部加维兰山地区 Orocopia 片岩及相关岩石的发掘历史
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发表时间：
2002
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通讯作者：
D. Sherrod
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作者：
C. Jacobson;M. Grove;Matthew M. Stamp;A. Vučić;F. Oyarzabal;G. Haxel;R. Tosdal;D. Sherrod
位于加州最东南部的加维兰丘陵地区，在一个变质后的东西向延长穹隆中暴露出三种明显不同的结晶岩包。结构上最深的是相对高压的、优地斜的Orocopia片岩，它位于低角度的巧克力山断层之下。在片岩之上的是源自中生代科迪勒拉岩浆弧中低地壳水平的片麻岩。反过来，片麻岩又被加图纳断层与温特黑文组的低级变质沉积岩和变质火山岩分开。巧克力山断层最初被认为是一个西南倾斜的俯冲逆冲断层，沿着它，一个外来的大陆长条被缝合到北美。然而，最近的工作者提出，这是一个晚期断层，负责奥罗科皮亚片岩的折返，它对埋葬历史没有任何限制。后一种解释得到了片岩中两种不同结构组构的支持，一种可能与俯冲有关，一种可能与折返有关。较古老的组构保存在远离巧克力山断层的片岩中，并与进变质作用至最低角闪岩相期间形成的NNE-SSW向线理相关。较年轻的组构在巧克力山断层100 m的结构范围内最好，其特征是离散的剪切带、绿片岩相退积和东西向线理。在巧克力山和加图纳断层以及温特黑文组附近的片麻岩中也出现了类似方向的线理。这个雅各布森C.E.格罗夫，M.，斯坦普，M. M.，Vucic，A.，Oyarzabal，F.R.，哈塞尔，G.B.，Tosdal，R.M.，和谢罗德，D.R.，2002年，加州东南部加维兰山地区奥罗莫皮亚片岩和相关岩石的发掘历史，位于A.巴斯，编辑，对美国西南部地壳演化的贡献：博尔德，科罗拉多，美国地质学会特别论文365，第129-154页。* 电子邮件：www.example.com cejac@iastate.edu Jacobson等人130的观察，结合对露头模式的解释，表明加图纳断层（以前被认为是中新世时期的陡峭浅层断层）是一个低角度结构，可容纳类似于巧克力山断层记录的相对高温变形。两个故障可能同时处于活动状态。然而，由于加图纳断层表现出更强烈的早期糜棱岩组构和更大的结构切除比巧克力山断层脆性叠加，它被认为是最近更活跃和更重要的两个在挖掘的片岩和覆盖片麻岩浅地壳水平。热历史的基础上的Ar/Ar分析的角闪石，白云母，黑云母，钾长石和以前的磷灰石裂变径迹测量结果揭示了一个两阶段的折返历史，我们涉及到滑沿着巧克力山和加图纳断层，分别。60 ~ 44 Ma为快速冷却的初始阶段。相对于片麻岩（59-64 Ma），片岩（52-57 Ma）的角闪石记录的Ar/Ar年龄更年轻、更不一致，证实了两个单元沿着巧克力山断层峰后变质并置，其方式与正断层一致。Orocopia片岩和片麻岩之间的白云母Ar/Ar年龄的巧合意味着，这种并列发生48 - 2 Ma，并表明巧克力山断层是一个Laramide年龄结构。45 ~ 31 Ma的黑云母年龄表明，在最初的折返阶段之后，片岩和片麻岩在350 ℃时长期驻留在中地壳中。钾长石记录的第二个时期的快速折返28至24马，我们与加图纳断层运动的脆性阶段。这第二阶段的折返被认为是反映了早期阶段的中第三纪伸展事件是广泛的东南部加州和亚利桑那州西南部。巧克力山断层的局部破坏，并置结构更深的片岩对片麻岩在加维兰山东端可能也发生在这个时候。
The Gavilan Hills area of southeasternmost California exposes three distinctly different crystalline rock packages in a postmetamorphic, E–W elongated dome. Structurally deepest is the relatively high-pressure, eugeoclinal Orocopia Schist, which underlies the low-angle Chocolate Mountains fault. Above the schist are gneisses derived from midto lower-crustal levels of the Mesozoic Cordilleran magmatic arc. The gneisses, in turn, are separated by the Gatuna fault from low-grade metasedimentary and metavolcanic rocks of the Winterhaven Formation. The Chocolate Mountains fault was originally thought to be a SW-dipping subduction thrust along which an exotic continental sliver was sutured to North America. Recent workers, however, have proposed that it is a late fault responsible for exhumation of the Orocopia Schist and that it places no constraints on burial history. The latter interpretation is supported by the presence in the schist of two distinct structural fabrics, an older one presumably related to underthrusting, and a younger one attributed to exhumation. The older fabric is preserved in schist away from the Chocolate Mountains fault and is associated with a NNE–SSW-trending lineation that formed during prograde metamorphism to lowermost amphibolite facies. The younger fabric is best developed within 100 m structurally of the Chocolate Mountains fault and is characterized by discrete shear zones, greenschist-facies retrogression, and E–W-trending lineations. Lineations with similar orientation also occur in gneiss adjacent to both the Chocolate Mountains and Gatuna faults and in the Winterhaven Formation. This Jacobson, C.E., Grove, M., Stamp, M. M., Vucic, A., Oyarzabal, F.R., Haxel, G.B., Tosdal, R.M., and Sherrod, D.R., 2002, Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United States: Boulder, Colorado, Geological Society of America Special Paper 365, p. 129–154. *E-mail: cejac@iastate.edu C.E. Jacobson et al. 130 observation, combined with interpretation of outcrop patterns, suggests that the Gatuna fault, which was previously considered a steep, shallow-level fault of Miocene age, is a low-angle structure that accommodated relatively high-temperature deformation similar to that recorded by the Chocolate Mountains fault. Both faults may have been synchronously active. However, because the Gatuna fault exhibits more intense brittle overprinting of early mylonitic fabrics and greater structural excision than the Chocolate Mountains fault, it is indicated to have been more recently active and the more important of the two in exhuming the schist and overriding gneiss to shallow crustal levels. Thermal history results based upon Ar/Ar analysis of hornblende, muscovite, biotite, and K-feldspar and previous apatite fission track measurements reveal a twostage exhumation history that we relate to slip along the Chocolate Mountains and Gatuna faults, respectively. An initial phase of rapid cooling occurred from 60 Ma to 44 Ma. Younger and more discordant Ar/Ar ages recorded by hornblendes from the schist (52–57 Ma) relative to the gneiss (59–64 Ma) confirm postpeak metamorphic juxtaposition of the two units along the Chocolate Mountains fault in a manner consistent with normal faulting. Coincidence of muscovite Ar/Ar ages between the Orocopia Schist and gneiss implies that this juxtaposition occurred by 48 2 Ma and indicate that the Chocolate Mountains fault is a Laramide-age structure. Biotite ages ranging from 45 to 31 Ma reveal that the initial exhumation phase was followed by protracted residence of the schist and gneiss in the middle crust at 350 C. Kfeldspars record a second period of rapid exhumation from 28 to 24 Ma, which we correlate with the brittle phase of movement on the Gatuna fault. This second phase of exhumation is considered to reflect an early stage of the middle Tertiary extensional event that is widespread in southeastern California and southwestern Arizona. Localized disruption of the Chocolate Mountains fault that juxtaposed structurally deeper schist against gneiss at the eastern end of the Gavilan Hills probably also occurred at this time.