Distribution of short-period geomagnetic variations related to tectonics in Central Europe

T om (V o lu m e) X X X V — 1965 Z e sz y t (F a sc ic u le ) 1 K r a k ó w 1965

W ŁADYSŁAW POŻARYSKI, ZDZISŁAW MAŁKOW SKI, JERZY JANKOW SKI

ROZKŁAD KRÓTKOOKRESOWYCH ZMIAN

POLA MAGNETYCZNEGO ZIEMI W EUROPIE ŚRODKOWEJ W POW IĄZANIU Z TEKTONIKĄ

(1 f ig .)

Distribution of short— period geomagnetic variations related to tectonics in Central Europe

(1 Fig.)

STR ESZC ZEN IE

R óżnice w przebiegach zm ian pola m agnetycznego obserw ow ane w ró żn y ch p u n k ta c h n a pow ierzchni Z iem i w iążą się z rozkładem p rzew o d n ictw a w podłożu. W oparciu o p race trzech au to ró w S c h m u - c k e r U. (1959), W i e s e H. (1963), J a n k o w s k i J. (1964) zestaw iono c h a ra k te ry s ty k i ro zk ład u zm ian k ró tk o o k reso w y ch ziem skiego pola m a ­ gnetycznego o czasie trw a n ia od k ilk u do k ilk u d ziesięciu m in u t d la części E u ro p y środkow ej. D ane dośw iadczalne po u jed n o licen iu ostateczn ej fo r­

m y ich opraco w an ia przed staw io n o w postaci w e k to ró w c na m ap ie (fig. 1). W edług jakościow ej in te rp re ta c ji geofizycznej, linie p rz e ry w a n e w y k reślo n e na m apie w in n y odpow iadać strefo m dużych d y slo k acji te k to ­ nicznych. L inie te, ja k w idać z m apy, dzielą om aw ianą część E u ro p y na

trzy obszary A , B i C. W dalszym ciągu p racy zestaw iono dane geofizyczne z geologicznym i.

N a obszarze A k ie ru n k i w ek to ró w są zgodne z k ie ru n k a m i s tr u k tu r w ary scy jsk ich , a niezgodne z p o stw ary scy jsk ą te k to n ik ą tego obszaru, k tó ra m u sia ła słabo przefo rm o w ać podłoże.

S tre fa gran iczn a obszarów A i B p o k ry w a się z północną gran icą kaledonidów , a obszar B je st m asą oporow ą ty c h gór. D la s tr u k tu r po- w ary sc y jsk ic h obszar B odpow iada ściśle c e n tra ln e j, n a jsiln ie j obniżonej części b asen u północnoniem ieckiego. O bszar C leży w obrębie p la tfo rm y w sch o d n io eu ro p ejsk iej i odznacza się zgodnym i n a ogół k ie ru n k a m i w e k ­ torów z te k to n ik ą p o k ry w y m ezozoicznej. D owodzi to zgodności k sz ta łto ­ w an ia się tektonicznego całej p o k ry w y osadow ej. P ew n e niezgodności tłu m aczą się istn ien iem w podłożu górotw oru, praw d o p o d o b n ie k aled o ń - skiego w ieku, k tó ry ciągnie się od Bydgoszczy przez S ta rg a rd , R ugię, M an aż do J u tla n d ii środkow ej, znacząc się ciągiem anom alii m a g n e ty c z ­ n ych składow ej Z. O bszar B należy w ięc z in te rp re to w a ć jako szty w n y m asyw śródgórski kaledonidów .

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7 R o c z n ik P T G t. X X X V z. 1

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R esearch on th e n a tu r a l oscillations of th e geom agnetic field h as show n th e possib ility of th e ir u tiliz a tio n fo r in v estig atio n of th e d eep geological stru c tu re s. In th e follow ing are given some re s u lts o b tain ed w ith one of th e re le v a n t m eth o d s now u n d e r d ev elo p m en t w hich consists in utilizin g th e tim e -v a ria tio n s of th e th re e m ag n etic field com ponents o b serv ed at th e E a rth ’s surface. S h o rt-p e rio d v a ria tio n s are called h e re those w hose d u ra tio n is of th e o rd e r of sev eral to som e ten s of m in u tes.

T his m e th o d is based on th e follow ing concept:

T he o u te r sources of geom agnetic field v a ria tio n s a re a lte rn a tin g c u r­

re n ts in hig h lay ers of th e atm o sp h ere producing in d u ced c u rre n ts in E a rth . T he e x p e rim e n ta l d ata in d icate th a t, in re s p e c t to sh o rt-p e rio d d istu rb an ces, those o u te r sources pro d u ce on th e E a r th ’s su rface in firs t ap p ro x im atio n only v ariatio n s in th e h o rizo n tal com ponents. If th e co n d u ctiv ity d is trib u tio n in the E a rth d epends on d e p th only (e.g. in a h o rizo n tal sy stem of hom ogeneous h o rizo n tal p a ra lle l layers), th e n the induced c u rre n ts w ill produce on th e E a r th ’s su rfa c e only h o riz o n ta l com ponents too.

If th e d is trib u tio n of electric co n d u ctiv ity in th e g ro u n d is d is tu rb e d in com parison w ith a h o rizo n tal p a ra lle l la y e r system , th e in d u ced c u rre n t lines w ill co n c e n tra te n o tab ly along th e v e rtic a l in te rfa c e s of com plexes w ith d iffe re n t valu es of specific re sistiv ity . S tro n g d ifferen ces in concen­

tra tio n of c u rre n t lines cause d istin ct v arian ce in m ag n etic fields, m e asu rab le a t th e E a r th ’s surface. T h ere occurs in th is case in p a rtic u la r an anom alous v a ria tio n in th e v e rtic a l com ponent Z of th e m ag n etic field, th e c h a ra c te ristic su rface d istrib u tio n re g u la rity of th o se v a ria tio n s

•yielding th e e x p e rim e n ta l d ata on th is m ethod.

T he changes in th e v e rtic a l com ponent d ep en d on m a g n itu d e and d irectio n of th e v a ria tio n s in th e in d u ced field. T his re la tio n m ay, in firs t ap p ro x im atio n , be considered as lin ear:

AZ(t) = a • AD(t) -!- b • AH(t)

w h ere AZ(t) denotes th e tim e -v a ria tio n in th e v e rtic a l com ponent AH(t) an d AD(l) th e tim e -v a ria tio n s of th e h o rizo n tal com ponents in th e directio n of m ag n etic N o rth an d East.

The coefficients a an d b are p a ra m e te rs c h aracterizin g th e o b serv atio n p o in t fo r th e selected v a ria tio n period. To e v e ry p o in t can th u s be a ttrib u te d a p a ir of n u m b e rs co m p u ted fro m e x p e rim e n ta l d ata. T his p air can be co n v en ien tly c h a ra c te riz e d by a v ecto r c of a p p ro p ria te d irectio n an d length.

Oh the m ap fig. 1 a re p lo tte d th e v ecto rs c fo r a p a r t of C en tral Europe, ta k e n from stu d ies by S c h m u c k e r K. (1959), W i e s e H.

(1963), J a n k o w s k i J. (1964). T his m a te ria l is n o t q u ite hom ogeneous since so m ew h at d iffe re n t m eth o d s w ere used by th e a u th o rs in elab o ­ ra tio n of th e ir data. T he vectors a ( | a | = | c | ) com puted fo r G.D.R. by

W i e s e (1963) w ere tu rn e d b y 90ü fo r th e pu rp o se of unificatio n . The vectors c fo r P o lan d w ere ta k e n w ith o u t change fro m p a p e r J a n - k o w s k i (1964). T he v ecto rs c fo r G.F.R. w ere p lo tte d fro m m a te ria l given by S c h m u c k e r (1959). T his m a te ria l h ad a lre a d y p rev io u sly been w o rk ed upon an d m a th e m a tic a lly sm oothed by th e ir a u th o r to a co n sid erab le e x te n t (the directio n s of th e v ecto rs c w ere d e te rm in e d

g

c — ---— , w h ere z t an d h t den o te th e in n e r p a r t of th e d istu rb a n c e s Z

1 ~ f- hi

a n d H, w hich are com puted in th e q u o ted paper). T he v ecto rs com puted in th is m a n n e r d iffe r m a rk e d ly from those co m p u ted w ith th e o th e r m en tio n ed m ethods, being e.g. s h o rte r th a n those ob tain ed w ith W i e s e ’s m ethod. T his fa c t h o w ev er can be ex p lain ed on th e g ro u n d s of g en eral co n sid eratio n s an d th e basic e x p e rim e n ta l facts form in o u r opinion an a d e q u a te basis fo r c e rta in g en eral conclusions.

W hen m ak in g d eductions re g a rd in g th e n a tu re of co n d u ctiv ity d is tri­

b u tio n in th e s u b s tra tu m w e have to b e a r in m in d th e follow ing principles:

1. V ecto r c h as n e a r-z e ro le n g th p ro v id ed th a t:

a) th e sy stem of g ro u n d la y e rs w ith d iffe re n t electric co n d u cti­

v ity is fo rm ed b y h o rizo n tal hom ogeneous layers. In th is case n e a r-b y points (up to som e te n s of k ilo m eters distance) should have too v ecto rs c of n ear-zero len g th ;

b) th e observ atio n p o in t is located a p p ro x im a te ly above th e s tru c tu re dislocated stro n g ly in re sp ec t to th e sy ste m of h o ri­

zo n tal la y e rs w ith hom ogeneous e le c trica l co n d u ctiv ity . In th is case th e n e a r p o in ts a t b o th sides of th e dislocation w ill be ch a ra c te riz e d by in v erse d irectio n of v ecto rs c. In th e id eal case of tw od im en sio n al co n d u ctiv ity d is trib u tio n th e d ifferen ce in v ecto r d irectio n s should be 180°, th e d irectio n s c ru n n in g p a ra lle l to th e dislocation.

The d is trib u tio n of v ecto r c len g th s along th e pro file v e rtic a l to th e p a th of co nduction an o m aly m ay fo rm an in d ic a to r re g a rd in g th e disloca­

tion g eo m etry (asym m etric d is trib u tio n e.g. a fa u lt, sy m m e tric d is tri­

b u tio n e.g. a syncline).

2. T he cause of anom alous d is trib u tio n of sh o rt-p e rio d m ag n etic v a ria tio n s on th e E a r th ’s su rface is to be fo u n d in a la y e r w hose th ick n ess is re la te d to th e p eriod of th e m ag n etic oscillations. We observe h e re th e sk in -e ffe c t p h enom enon ow ing to w h ich a t v e ry sh o rt-p e rio d s th e c u rre n t flo w is re s tric te d to an e x tre m e ly th in o u te r la y e r of th e conductor, w h ile a t lo n g er periods it p e n e tra te s deeper. P e n e tra tio n d e p th d ep en d s on th e specific re s istiv ity of th e s u b s tra tu m an d th e d istu rb a n c e period. F o r th e o b serv ed periods of sev eral to some te n s of m in u te s th e th ick n ess of th e la y e r m ay a tta in fro m a b o u t a dozen to ov er one h u n d re d k ilo m eters.

On m ap fig. 1 is p lo tte d also th e course of th e sectors of th e line c = 0. T h eir location w as d e te rm in e d b y in te rp o la tio n b etw e e n a rro w s of d istin c tly d ifferin g directions.

W ith o u t p reju d ice to fu tu re , m ore d etailed , in te rp re ta tio n s of th e problem , w e incline to th e opinion th a t those lines corresp o n d to th e dislocation axes of com plexes w ith d iffe re n t re sistiv ity . C om parison of th e p ath s of line c = 0 w ith av ailab le tecto n ic d a ta m ig h t be of g re a t in te re st, th o u g h u n fo rtu n a te ly geologic d a ta cover in m o st cases only n e a r-s u rfa c e la y e rs of re la tiv e ly sm all dep th , exceeding ra re ly a few kilo m eters.

To fa c ilita te such com parisons, th e discussed E u ro p ean space w as d iv id ed in th re e areas A, B, C, (comp. fig. 1), ta k in g as th e ir b o u n d aries long p a rts of th e lines c = 0.

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T u rn in g now to geological in te rp re ta tio n w e are ta k in g as sta rtin g p o int th e division in th re e se p a ra te areas.

A r e a A. T he v ecto r d irectio n s on th is a re a are g e n e ra l in accordance w ith th e tre n d s of V ariscian folding, as w as observed fo r G e rm a n y by U. S c h m u c k e r (1959), an d H. W i e s e (1956), (1962), (1963). W est of th e D resd en m erid ian , th e y are fo rm in g an arc cu rv ed co n v ex ely to th e n o rth , w h ile e a st of th e m e rid ia n th e y tu r n slig h tly to w a rd s th e n o rth , fo rm in g in th e en v iro n s of B erlin an d D resd en a b u lg e p o in tin g to th e south. In th e w e ste rn p a r t of a re a A th e convergence of tre n d s is based on d ire c t observ atio n s of th e tre n d s of V ariscian an d C aledonian folds in exposed old m assifs (H. G e r t n e r 1950, E . S t i 11 e 1951). F o r th e e a s te rn a rea p a r t no d ire c t observ atio n s are h o w ev er available. We con­

clude h e re fro m in d ire c t d a ta on a p a ra lle l course of th e V ariscian folds on th e a re a b etw e e n th e H arz an d Ś w ięto k rzy sk ie Mts. (H. S t i 11 e 1951, R. L a u t e r b a c h 1953, W. P o ż a r y s k i 1957). A n ew fa c t w o u ld be h e re th e slig h t d ev iatio n of th e ir tre n d to ENE. I t h as to be n o ted th a t th e P o st-V ariscian tecto n ic changes on area A, w hose folding tre n d s d iffe r fro m C aledonian an d V ariscian tren d s, did n o t fin d a n y reflex io n in th e v ecto r p ictu re, w hich in d icates th a t th e ir effect on tra n sfo rm a tio n of th e d e e p e r la y e rs of th e E a rth 's c ru s t w as n o t su ffic ie n tly strong.

A r e a B. T he b o u n d a ry zone of areas A an d B coincides w ith th e n o rth e rn b o u n d a ry of th e C aledonian O rogen. Its location sho u ld th e r e ­ fore be e x p ected in th e region of th e fo red eep s of those m o u n tain s, w hich is in good a g re e m e n t w ith geophysical in te rp re ta tio n . A rea B is a fo rela n d of th e Paleozoic O rogen. A sim ilar in te rp re ta tio n of th e m ain tectonic fe a tu re s of th is region w as given b y H. G e r t n e r (1960 fig. 1 n on fig. 2) an d A. B o g d a n o v (1962).

As re g a rd s P o st-V ariscian folding, th e area B corresponds closely to th e c e n tra l p a r t of th e N o rth G erm an se d im e n ta ry basin (O ld en b u rg -S ch les- w ig — zone, F. T r u s h e i m 1957) w hich is se p a ra te d fro m th e so u th e rn b asin p a r t b y a fla t fle x u re in d ic a te d on th e base of th e G e rm a n and P o lish P erm ian . T his area, n o t folded fro m th e C am b rian on, sh ould have in its d e p th s h o riz o n ta l stra ta , w hich w ould co rresp o n d to th e sh o rt, or near-zero, v ecto r len g th s of th e area. D istin ct d istu rb an ces of th e v ecto r p ic tu re a p p e a r only to th e so u th w e st of Rostock, in th e geologically E ast E lbian M assif (K o 1 b e 1 1963), w hose tectonics a re m o re com plex th a n those of th e ad jo in in g p a rts of a re a B. T he b o u n d a ry b e tw e e n areas A an d B divides th e E ast E lb ian M assif in a n o rth e rn and a so u th e rn p a r t w hich d iffe r in re sp e c t to tectonics of th e P e rm ia n an d y o u n g er layers.

A r e a C. I t lies in th e ran g e of th e E ast E u ro p ean P la tfo rm . The w hole area d iffers fro m th e tw o o th e rs in th a t th e v ecto r directio n s ag ree in g e n e ra l w ith th e tectonic tre n d s in th e m esozoic cover. T his proves co n fo rm ity of th e tectonic fram e w o rk in th e w hole se d im e n ta ry m an tle, ap p earin g w ith special d istin ctn ess east of th e line K oszalin — Bydgoszcz — Łódź. T he b o u n d a ry b e tw e e n a re a s A a n d C ru n s on th e secto r P iła — Bydgoszcz — Łódź n e a r th e M id d le-P o lish A n ticlin o riu m an d th e M ogilno — Łódź S ynclinorium , coinciding p ro b a b ly w ith it. The b o u n d ary line on th e secto r lies th e re fo re on th e D an ish -P o lish G ec- syncline.

A n im p o rta n t phenom enon, occurring w ith special d istin ctn ess in th e Łódź region, is th e oblique ap p ro ach fro m th e w e st of Paleozoic folds to th e above m en tio n ed b o u n d a ry line an d th e ir d isap p earan ce on it,

70

3 ₇ 8

z e r u ; 2 — p u n k t p o m i a r o w y o w a r t o ś c i c = 0 , 5 ; 3 — g r a n i c e o b s z a r ó w w y z n a c z o n e n a p o d s t a w i e r o z k ł a d u ś r e d n i c h w e k t o ­ r ó w c ; 4 — g r a n i c e a n t y k l i n o r i u m ś r o d k o - w o - p o l s k i e g o ; 5 — w a r s t w y k r e d o w e ; 6 — w a r s t w y j u r y i t r i a s u ; 7 — w a r s t w y g ó r ­ n e g o p e r m u ( c e c h s z t y n ) ; 8 — p o d ł o ż e k r y ­ s t a l i c z n e .

P r z e k r o j e g e o l o g i c z n e o p r a c o w a ł W . P o - ż a r y s k i p r z y w s p ó ł p r a c y M . J a s k o - w i a k o p i e r a j ą c s i ę g ł ó w n i e n a d a n y c h s e j s m i c z n y c h . O z n a c z e n i a l i t e r o w e l u b l i c z ­

b o w e p u n k t ó w p o m i a r o w y c h n a m a p i e s ą w z i ę t e z p o d a n y c h w t e k ś c i e ź r ó d e ł b i b l i o ­ g r a f i c z n y c h

D i s t r i b u t i o n o f m e a n v e c t o r c v a l u e s i n P o l a n d , G . D . R . a n d G . F . R . 1 — m e a s u ­ r i n g p o i n t w i t h n e a r - z e r o v a l u e o f c ; 2 — m e a s u r i n g p o i n t w i t h v a l u e c = 0 , 5 ; 3 — a r e a b o u n d a r i e s d e t e r m i n e d f r o m m e a n v e c t o r s c d i s t r i b u t i o n ; 4 — b o u n d a r i e s o f t h e M i d d l e - P o l i s h A n t i c l i n o r i u m ; 5 — C r e t a c e o u s ; 6 — J u r a s s i c a n d T r i a s s i c ; 7 — U p p e r P e r m i a n ( Z e c h s t e i n ) ; 8 — m e - t a m o r p h i c a n d i g n e o u s r o c k s o f b a s e m e n t .

T h e g e o l o g i c a l c r o s s - s e c t i o n s w e r e w o r ­ k e d o u t b y W . P o z a r y s k i w i t h t h e a s s i s t a n c e o f M i s s M . J a s k o w i a k , m a i n l y o n b a s i s o f s e i s m o l o g i c a l d a t a . T h e n u m b e r s o r l e t t e r s y m b o l s o f m e a ­ s u r i n g p o i n t s u s e d a b o v e a r e t a k e n f r o m t h e q u o t e d b i b l i o g r a p h i c s o u r c e s

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w h ereas so u th of P io trk ó w those folds hav e passed th e A n ticlin o riu m b o u n d a ry in e a ste rn d irection, as w e know from geologic an d g ra v im e tric data.

N o rth w e st of Bydgoszcz th e v ecto r d irections do n o t closely agree w ith th e A n ticlin o riu m ax is b u t coincide w ith o ld er d irectio n s visible in th e mesozoic cover an d crossing th e axis a t an angle. F o r th e n o rth e rn p a r t of a re a C w e can tak e in to account th e an aly sis of th e m ap of m ag n etic anom alies of th e v e rtic a l com ponent ,,Z” . It a p p e a rs fro m th e m ap th a t in th e w e ste rn p ro lo n g atio n of th e P e rib a ltic S yneclise occur a n u m b e r of m ag n etic anom alies w ith p a ra lle l direction. O ne of th e m ru n s fro m Bydgoszcz th ro u g h th e P iła to S ta ro g a rd w h e re it tu r n s n o rth w e s t an d goes over in to th e an o m aly of U sedom Islan d an d n o rth e rn R ügen.

A long th e so u th w e st b o rd e r of th is se t of anom alies ru n s th e b o u n d a ry se p a ra tin g areas B an d C. I t seem s p ro b ab le th a t th is se t e x te n d s to th e anom alies of th e Isle of M an, so u th e rn Z eeland, F y n an d m iddle Ju tla n d , fo rm in g a trace of a C aledonian O rogen b e lt su rro u n d in g th e B altic S hield (R. Z w e r g e r 1948), w hich agrees w ith boring d a ta fro m N o rth R ügen (H. K ö l b e l 1963) an d Z eelan d (G. L a r s e n , A. B u c h 1960).

T his h y p o th esis fin d s sig n ifican t co n firm atio n in th e v ecto r d irectio n s on th e isles of U sedom an d Rügen.

The area B w ould th u s fo rm th e fo relan d of th e C aledonian O rogen ru n n in g th ro u g h R ügen.

The above com parison of geophysical an d geological d a ta p o in ts to in te re stin g in te rre la tio n s, in v itin g th e ir geological in te rp re ta tio n . The geologic aspects p re sen te d above are h o w ev er still in a d e q u a te fo r elab o ­ ra tio n of a geophysical in te rp re ta tio n .

Geological In stitu te Institute of G eophysics

and D ep a rtm en t of G eology of the Polish A c a d e m y

W a rsa w U n iv e r s ity of Sciences, W a rsa w

WYKAZ LITERATURY REFERENCES

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G a e r t n e r H. "R. (1950), Erwägungen über präperm ische G ebirgszusam m enhänge in der Um gebung und in Untergrund von Norddeutschland. Geol. Jb. 64,

pp. 123— 157.

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