Demurtas, Matteo

Di Toro, Giulio

Fondriest, Michele

Smith, Steven

Spagnuolo, Elena

English

Steven Smith

Matteo Demurtas

Elena Spagnuolo

Michele Fondriest

Giulio Di Toro

Archivio istituzionale della ricerca - Università di Padova

Coseismic	origin	of	foliated	cataclasites	and	preservation	potential	during	the	seismic	cycle		Steven	Smith1,	Matteo	Demurtas2,	Elena	Spagnuolo3,	Michele	Fondriest4,	Giulio	Di	Toro2,3,4		1.	Geology	Department,	University	of	Otago,	Dunedin,	New	Zealand	2.	Dipartimento	di	Geoscienze,	Università	degli	Studi	di	Padova,	Padova,	Italy	3.	Istituto	Nazionale	di	Geofisica	e	Vulcanologia	(INGV),	Rome,	Italy	4.	School	of	Earth,	Atmospheric	and	Environmental	Sciences,	University	of	Manchester,	UK			 Foliated	gouges	and	cataclasites	are	most	commonly	interpreted	as	the	result	of	aseismic	faulting	in	the	brittle	upper	crust.	However,	the	occurrence	of	foliated	cataclasites	associated	with	possible	 indicators	of	earthquake	ruptures	 (mirror-like	slip	surfaces	with	 truncated	clasts,	 in-situ	pulverized	rocks,	etc.)	suggests	that	some	examples	may	have	a	coseismic	origin.			 Here	we	present	the	results	of	friction	experiments	and	microstructural	analysis	performed	on	 mixtures	 (50/50wt%)	 of	 calcite-dolomite	 gouges	 to	 ascertain	 the	 conditions	 of	 foliation	formation.	The	gouges	were	sheared	for	40	cm	 in	a	rotary	shear	apparatus	 (SHIVA,	 INGV-Rome)	under	constant	normal	stress	of	17.5	MPa	and	slip	rates	of	30	µm/s	to	1	m/s.		 In	 room-humidity	 conditions,	 a	 striking	 foliated	 fabric	 was	 formed	 only	 at	 V	 =	 1	 m/s,	associated	 with	 strain	 localization	 and	 evidence	 of	 thermal	 decomposition	 and	 crystal-plastic	deformation	 in	 a	 slipping	 zone	 less	 than	 a	 few	micrometres-thick.	 Instead,	 in	 water-dampened	conditions,	no	foliation	was	formed	at	any	slip	rate	and	strain	localized	within	an	ultrafine	(grain	size	<<	1	µm)	fluidized	layer	whose	thickness	decreased	with	increasing	velocity.	To	investigate	the	preservation	 potential	 of	 these	 microstructures	 during	 the	 seismic	 cycle,	 we	 also	 conducted	experiments	that	stepped	from	slow	(30	µm/s	for	10	cm	slip)	to	high	(1	m/s	for	30	cm	slip)	velocity	and	vice-versa.	 In	 the	30	µm/s	 to	1	m/s	experiment,	 in	 room-humidity	conditions,	characteristic	microstructures	of	 both	 slip	 velocities	were	preserved	and	 the	overall	 fabric	 strongly	 resembles	that	found	in	natural	foliated	cataclasites	from	the	active	Vado	di	Corno	Fault	Zone,	Italian	Central	Apennines.	 In	 the	1	m/s	 to	30	µm/s	experiment,	performed	under	water-dampened	conditions,	evenly	spaced	gouge	injection	veins	departing	from	the	localized	slip	surface	formed	during	the	30	µm/s	slip	event.		 Our	experiments	suggest	that	foliations	defined	by	compositional	banding	and/or	grain	size	variations	in	gouge	and	cataclasite	can	form	during	coseismic	sliding	(V	≥	1	m/s)	in	dry	conditions	and	 that	 foliation	 is	 likely	 to	be	preserved	during	 the	seismic	cycle.	Recognition	of	 such	 foliated	cataclasites	in	the	geological	record	would	provide	a	marker	of	seismic	rupture	and	also	a	window	in	to	the	mechanics	of	strain	localization	in	brittle	fault	rocks.	

Coseismic origin of foliated cataclasites and preservation potential during the seismic cycle

Steven, Smith

Elena, Spagnuolo

Padua Research Archive

https://www.research.unipd.it/bitstream/11577/3254473/1/Smith_etal_AGU2016.pdf

Coseismic origin of foliated cataclasites and preservation potential during the seismic cycle

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