GEOLOGICAL SURVEY OF FINLAND Nuclear Waste Di sposal Research

Report YST - 57

Paavo Vuorel a, Aimo Kui vamaki and Markku Paananen

NEOTECTONIC BEDROCK MOVEMENTS

A p r e l i m i n a r y survey o f t h e Pasmajarvi f a u l t

Study f i nanced by t h e M i n i s t r y o f Trade and I ndus t r y , Department of Energy, and p a r t l y by t h e Nord ic l i a i s o n committee f o r atomic energy, NKA/KAV-330

Espoo, October 1987

Paavo Vuorela, Aimo Kuivamaki and Markku Paananen, 1987. Neotectonic bedrock movements. A p r e l i m i n a r y survey o f t h e Pasmajarvi f a u l t . Geologica l S~ l r vey o f F i n l and, lVucl ear Waste D i sposal Research, Report 57, 15 pages, 8 f i gu res .

ABSTRACT

Present knowledge o f t h e u p l i f t process and t h e p r e c i s e l e v e l l i n g and se i smi c da ta i n F i n l and i ndi ca te bedrock niovements i n agreement w i t h t h e assumption t h a t a1 1 bedrock movements f o l 1 ow o l d f r a c t u r e zones and t h a t t h e bedrock i s composed o f s t a b l e b locks t h a t s tay mos t l y unbroken i n c r u s t a l movements. The s l i g h t p r o b a b i l i t y t h a t new f r a c t u r i n g , f o r i ns tance a f t e r an i c e age, cou ld t ake p lace i n unexpected p a r t s o f t h e s o l i d bedrock has l e d t o i n t e n s i v e s tud ies . A1 1 poss ib le pos tg lac i a1 f a u l t scarps i n F i n l a n d have been recorded. I n no r the rn F in land and Sweden t h e l o c a l p o s t g l a c i a l f a u l t scarps, t h e Lans ja rv i f a u l t scarp (Sweden) and t h e Pasmajarvi f a u l t scarp (F in land ) a re be ing s tud ied.

The Pasmajarvi f a u l t scarp i s p a r t o f a b recc ia ted 20 metre t h i c k f a u l t zone d ipp ing 45 degrees southeast. A zone o f f r a c t u r e d bedrock severa l hundreds of metres wide was i n d i c a t e d by seismic measurements. The 20 metre t h i c k b recc ia ted zone i t s e l f i s c l e a r l y r e l a t e d t o a s l ingram anomaly and a wet c l a y gouge w i t h h igher e l e c t r i c c o n d u c t i v i t y was found a t t h e bottom o f t h e zone.

The f a u l t p lane was i n t e r s e c t e d w i t h two diamond d r i 11 ho les 40 and about 80 metres long. The zone was c l e a r l y i n d i c a t e d i n water consumption t e s t s . Measuring poi n t s were e s t a b l i shed on bo th s ides of t h e f a u l t t o observe poss ib le f u t u r e v e r t i c a l and h o r i z o n t a l movements.

The Pasmajarvi f a u l t seems t o be a regenerated o l d f r a c t u r e zone which was a c t i v e i n p o s t g l a c i a l t ime, probably 8,000 years ago.

Paavo Vuorel a Aimo Kui vamaki Markku Paananen Geologica l Survey o f F i n l and SF-021 50 ESP00 F i n l and

ISBN 951 -690-297-9 ISSlV 0783-3555

CONTENTS

Abst rac t

I n t r o d u c t i o n ................................................... 5

Pos tg lac ia l movements .......................................... 5

The Pasmajarvi f a u l t . ....................................... 10

P re l im ina ry r e s u l t s ............................................ 14

References .................................................... 15

INTRODUCTION

Es t ima t i ons o f t h e r e g u l a r i t y and magnitude o f bedrock movements

d u r i n g coming thousands o r hundreds o f thousands o f years a re based

on s t u d i e s o f present-day bedrock movements and i n d i c a t i o n s l e f t by

movements t h a t took p lace a f t e r t h e l a s t I c e Age. The p o s s i b l e

ex i s tence of new o r p o s t g l a c i a1 f r a c t u r i n g i n t h e bedrock has been

s tud ied .

POSTGLACIAL MOVEMENTS

The v e r t i c a l component o f t h e c r u s t a l movements i n t h e Fennoscandian

S h i e l d i s q u i t e w e l l known be ing r e l a t e d t o t h e p o s t g l a c i a l

l a n d - u p l i f t process. I n d i c a t i o n s e x i s t t h a t , j u s t a f t e r t h e m e l t i n g

o f t h e c o n t i n e n t a l i ce , t h e bedrock movements were v i o l e n t b u t l a t e r

s e t t l e d down t o t h e present u p l i f t r a t e o f a t most 1 cm pe r year .

The r a t e o f t h e present u p l i f t i s a l s o r e l a t e d t o t h e movement o f

t h e E a r t h ' s c r u s t as a whole. The observable movement between

d i f f e r e n t bedrock b locks amounts t o o n l y f r a c t i o n s o f m i l l i m e t e r s

annua l l y . The e n t i r e u p l i f t d u r i n g t h e p o s t g l a c i a l t i m e has been a t

most 700 meters, and t h e f a u l t movement have been about 10 per cen t

o f t h i s t o t a l .

The present-day c r u s t a l movements can be used as analogs f o r

mode l l i qg t h e dynamics t h a t have been and i n t h e f u t u r e w i l l be

a c t i v e a t t h e m e l t i n g s tage o f c o n t i n e n t a l i c e . S t a t i s t i c a l l y , t h e

t ime b e f o r e t h e nex t a c t i v e m e l t i n g s tage w i l l be about 100 000

years . The g loba l c l i m a t e w i l l s l o w l y coo l down f o r about 100 000

years, and t hen t h e m e l t i n g s tage o f t h e c o n t i n e n t a l i c e w i l l be

shor te r , as i n d i c a t e d i n F ig . 1.

t h e f a u l t scarp. I n t h e midd le p a r t o f t h e scarp, f o u r p i l l a r s were

s e t up w i t h i n eyeshot o f each o the r f o r t h e measurement o f

h o r i z o n t a l movements. Measuring accuracy w i l l be a t l e a s t 0.5 mm

h o r i z o n t a l l y and 0.2 mm v e r t i c a l l y . Plans have been made t o work ou t

some e x t r a measurement l i n e s over t h e main o l d f r a c t u r e zones

s t r i k i n g NW-SE.

PRELIM1 NARY RESULTS

The main ques t ion r e l a t i n g t o t h e f r a c t u r i n g i n p r e v i o u s l y unbroken

rocks was n o t encountered w i t h i n t h e Pasmajarvi f a u l t scarp. The

f a u l t b r e c c i a was compact ly r e c r y s t a l l i z e d i n many p laces and t h e

f r a c t u r e minera ls must c e r t a i n l y i n d i c a t e q u i t e h i g h ages. The

f r a c t u r e zone p o s s i b l y underwent movement i n p o s t g l a c i a l t ime, bu t

even t h a t i s d i f f i c u l t t o see i n t h e d r i l l i n g cores. The c l e a r e s t

i n d i c a t i o n s should appear i n t h e f i e l d and on s te reo photos.

The b r e c c i a t e d zone had a h i g h water c o n d u c t i v i t y and t h e d i p was

about 45". Whether t h e d i p p i n g angle becomes sma l l e r o r a

subhor izon ta l f a u l t e x i s t s deeper down a re ques t ions t h a t ought t o

be s tud ied .

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