Introduction

Summary

Magnetic properties of fault rocks

Geological background and samples

Fluid-driven processes in fault zones are of importance because they not only play an important role on chemical and mineralogical composition, and textural evolution of fault zones, but also have a major impact on mechanics of faulting. After the 2008 Mw 7.9 Wenchuan earthquake in Sichuan, China, two shallow holes were dilled and penetrated into the Yingxiu-Beichuan fault (YBF), which is one of the four major faults of the Longmenshan thrust belt, and was the major ruptured fault zone during the earthquake. Different fault rocks including fault breccia, cataclasite and fault gouge were cored. Magnetic measurements showed that the three gouge zones with different colors have different magnetic properties. Diverse mineral assemblages and chemical compositions have also been identified in them. Here we report the detailed magnetic properties of these fault rocks and their relationships with mineral assemblages and chemical compositions. In an effort to understand the possible magnetic mineral alterations associated with the fault fluid, and explore the potential magnetic records of fault fluid flow.

Magnetic properties of fault rocks from the active Longmenshan fault zone and their association with hydrothermal fluid activity Tao Yang1,2 (yangtao@cea-igp.ac.cn), Xiaosong Yang3, Qingbao Duan3

1. Inst. Geophys.,China Earthquake Adm., Beijing, China; 2. Dept. Earth Sci., Utrecht Univ., The Netherlands; 3. Inst. Geol., China Earthquake Adm., Beijing, China

Acknowledgements

Chlorite occurs throughout almost the entire depth of fault zone (Fig. 7), suggest ing widely occurrence of hydrothermal activity related to fault activity. Close relationships are observed between both Xlf and Xhf with clay minerals and chlorite (Fig. 9) that is product of hydrothermal activities, as well as immobile oxides (e.g., TiO2 and FeOT) (Fig. 10) , and mass loss values (Fig. 11) . These observations suggest that magnetic properties can be used as proxy of fluid-rock interaction and associated clay formation (in particular chlorite).

This study was supported by the National Natural Science Foundation of China (No. 41204062), the Wenchuan Earthquake Fault Scientific Drilling Program (WFSD 0009), and the State Key Laboratory of Earthquake Dynamics, Institute of Geology, CEA, under grant LED2010A03.

EGU2015-4706

Fig. 1. Geological map of the Longmenshan fault zone with t h e c o s e i s m i c s u r f a c e r u p t u r e s o f t h e 2 0 0 8 Wenchuan E.Q. (adapted from Li et al., 2014). The drilling site (Jinhe exposure) is located at the middle section of the YBF. The hanging wall protol i th consists of the Neoproterozoic Pengguan complex with an age of 859~699 Ma, whereas the f o o t w a l l i s P a l e o z o i c carbonate.

Fig. 2. (a) Photograph of Jinhe exposure. A fault scarp with ~3.4 m of vertical displacement was produced by the 2008 Wenchuan E.Q.. The principal slip surface cuts through the granitic layers.   (b) Sketch of (a) showing the occurrence of a fault zone. Outline and lithological profile of (c) the primary borehole (LMFD-3A) and (d) the secondary hole (LMFD-3B).

Fig. 3. Fault rock distribution in the fracture zone from 41.8 m to 51.4 m in the hole LMFD-3B. I t penetrates granite (5-24 m), altered fractured granite (24-35 m), fault breccia (41.8-42.5 m), fault gouge (42-49 m), fault breccia (48.6-49.7 m), and cataclasites (49.7-51.4 m). Fault gouge is divided into three types according to their colors. Type I (YGG) is yellow-green in color and appears as thin layer (~10 cm) at depth of 42 m; Type II (BG) is black in color and occurs from 42.5 to 47.2 m; type III (GGG) is grey-green in color and distr ibuted between 47.3-49.05 m. Hysteresis loops (Fig. 5) and temperature-dependent magnetization (Fig. 6) of

selected samples. Magnetite is identified as the dominant magnetic carrier in host rock. The extremely low proportion of ferrimagnetic phases in all fault rocks confirm that magnetic properties are essentially controlled by the paramagnetic fraction. Moreover, iron sulfide (e.g., pyrite) is also identified in all fault gouge zones.

Fig. 4. Downhole magnetic parameters of core samples from the hole LMFD-3B. The granitic host rocks have variable χlf with a highest average value, followed by that of fault breccia, protocataclasite and cataclasite in the hanging wall. Within gouge zones, YGG zone has the highest χlf, followed by GGG zone, and BG zone displays the lowest average χlf. With respect to the host rock, fault rocks (fault breccia, protocataclasite, cataclasite and fault gouges) have much higher χhf.

Fig. 7. Host rock is composed predominantly of quartz and feldspar. Fault breccia and cataclasite have enriched abundance of quartz and clay minerals, but depletion of feldspar. Within gouge zones, YGG zone is composed of the most enrichment of clay minerals (mainly, Chlorite). BG zone has the lowest clay minerals (mainly I/S, illite and chlorite, no smectite), and feldspar is absent. GGG zone has heterogeneous calcite and feldspar, with medium content of clay minerals (mainly chlorite and minor smectite, with occasional presence of kaolinite). Fig. 8. C-type elements (e.g., MgO, P2O5, TiO2, MnO, Fe2O3, FeO) are enriched in fault gouge zone. Isocon analysis reveals quite different mass-losses have occurred in fault breccia (61.5%), cataclasite (10.7% in the hanging wall, 35.9% in the footwall), YGG zone (91.5%), BG zone (64.8%), and GGG zone (90.2%), respectively.

v  Depletion of magnetic minerals from granitic protolith, through protocataclasite to cataclasite, may imply that cataclastic rocks in the drill cores developed and matured by fluid-assisted cataclastic processes, which (partially) altered and/or removed of magnetite in protolith.

v  Close relationships between high-field magnetic susceptibility and abundance of clay minerals, chlorite in particular, immobile oxides, as well as isocon mass loss values were observed, indicating that hydrothermal fluid flow associated clay mineral reactions may control the Fe partitioning.

v  Mineral magnetic properties of fault rocks could be expected as indications of fluid-rock interaction and associated clay formation.

v  The exact mechanism for evolution of Fe-bearing minerals and alteration/transformation of magnetic mineralogy with hydrothermal activity in a fault zone needs to be further investigated.

Mineral assemblages and chemical compositions

Magnetic properties and hydrothermal activity