Clay mineral formation and fabric development in the DFDP-1B borehole, central Alpine Fault, New Zealand

Figures & data

Figure 1 A, Location map showing the Australia–Pacific plate boundary through New Zealand. B, Map showing the Alpine Fault and other major tectonic features in the South Island. The star marks the DFDP-1 drill site (coordinates 43°17′5′′S, 170°24′22′′E).

Figure 2 A, Sample locations in core from run 59, section 1 and run 69, section 2. B, Photos and description of selected rocks. Note that only three samples are shown here, but four samples are analysed in total in core run 69/2. Picture width c. 15 cm. C, X-ray diffraction patterns of clay size fraction of the samples, showing the occurrence of smectite in two principal slip zones (PSZ). Dark grey lines: air-dried analysis; light grey lines: ethylene glycolated. D, Changes of peak width measured in FWHM (full width at half maximum) in glycolated state. Note the similar thicknesses of chlorite and illite/smectite and broad FWHM in the slip zones.

Table 1 X-ray diffraction data of chlorite and illite, with distinct peak positions (001/002 d-value), intensities (without background), peak-widths (FWHM) and 2θ angles.

Sample (DFDP-1B; core run/section/cm)	Depths (m)	Rock	Mineral	d-value (d)		Intensity	FWHM (glycolated)	2θ
59/1/0.12	c. 128.3	Fault gouge	Chlorite	d(001)	n.a.	n.a.	n.a.	n.a.
				d(002)	n.a.	n.a.	n.a.	n.a.
PSZ-1			Illite	d(001)	10.13	221	0.37	8.72
				d(002)	4.99	196	0.192	17.76
			Smectite	d(001)	17.6	1059	0.8	5.7
69/2/17–22	c. 143.5	Cataclasite	Chlorite	d(001)	14.29	2352	0.194	6.18
				d(002)	7.11	7857	0.187	12.44
			Illite	d(001)	10.02	6219	0.212	8.82
				d(002)	4.99	1652	0.195	17.74
69/2/40–41	c. 143.7	Cataclasite	Chlorite	d(001)	14.2	1067	0.2	6.22
				d(002)	7.09	3623	0.181	12.48
			Illite	d(001)	9.98	2316	0.229	8.86
				d(002)	4.99	509	0.231	17.76
69/2/46–66	c. 144	Fault gouge	Chlorite	d(001)	14.04	1163	0.426	6.2
				d(002)	7.09	2618	0.21	12.48
PSZ-2			Illite	d(001)	10.05	2389	0.25	8.8
				d(002)	4.99	592	0.18	17.76
			Smectite	d(001)	17.6	740	n.a.	5.7
69/2/76–79	c. 144.3	Breccia	Chlorite	d(001)	14.06	743	0.156	6.28
				d(002)	7.06	2042	0.167	12.52
			Illite	d(001)	9.97	5898	0.101	8.9
				d(002)	4.98	967	0.124	17.82

Figure 3 Matches of calculated WILDFIRE patterns (grey line) and XRD standard mixtures with polytype patterns for cataclasite and fault gouge from the Alpine Fault (black line), allowing quantification of metamorphic illite/muscovite and authigenic illite polytypes.

Figure 4 Secondary electron images showing clay minerals in two principal slip zones (PSZ) of Unit 5, the hanging wall and the footwall. A, Hanging-wall illite forming in a matrix of feldspar, quartz and chlorite (sample 69/2/40–41). B, Footwall illite growth as pore fillings (sample 69/2/76–79). C, PSZ-1 Ca-rich clay (smectite) growing on a feldspar surface. D, PSZ-2 smectite minerals occurring within a mixture of illite and chlorite.

Figure 5 X-ray texture goniometry of samples from two PSZs and cataclasites, showing higher fabric intensities of illite in the cataclasite (multiple of random distribution m.r.d. = 3.3–3.7) than in slip zones (m.r.d. < 2.3).