Cavitation erosion – a possible cause of the mass.pdf - Karpaty - dawid1051

Acta Geologica Polonica, Vo l. 57 (2007), No. 2, pp. 305-323

Cavitation erosion – a possible cause of the mass

loss within thrust zones in the Tatra Mts., Poland

EDYTA JUREWICZ 1 , BOLES¸AW GIRE¡ 2 & JANUSZ STELLER 2

1 Faculty of Geology, University of Warsaw, Al. ˚wirki i Wigury 93, PL-02-089 Warsaw, Poland.

E-mail: edyta.jurewicz@uw.edu.pl

2 The Szewalski Institute of Fluid Flow Machinery , Polish Academy of Sciences , ul. Fiszera 14 ,

PL - 80-952 Gdaƒsk, Poland. E-mails : giren@imp.gda.pl , steller@imp.gda.pl

ABSTRACT:

J UREWICZ , E., G IRE¡ , B. & S TELLER , J. 2007. Cavitation erosion – a possible cause of the mass loss within

thrust zones in the Tatra Mts., Poland. Acta Geologica Polonica , 57 (3), 305-323. Warszawa.

In the Tatra Mts., thrust-napping and shearing were multi-stage re-activated processes. Their cyclic char-

acter was determined by increases and decreases in pore fluid pressure. During each cycle, new parts of

the rheologically heterogeneous wall-rock were selectively destroyed due to hydraulic fracturing, brec-

ciation and mylonitization, and moved out as a solution and/or suspension. As a result of these process-

es, including pressure solution, considerable mass loss could have taken place. All these processes took

place under the considerable influence of fluids. In this paper we consider the possible contribution of

cavitation erosion to mass loss processes. Displacement along an uneven thrust surface could create

chambers filled with fluid and sudden falls in local pressure promoting the inception of cavitation.

Cavitation damage, mainly mechanical in nature, could act synergistically with slurry abrasion and pres-

sure solution processes.

Our work is of a hypothetical character. To prove the possibility of cavitation erosion within shear

zones in the Tatra Mts. we conducted an experiment to show the low resistance of rock samples to cavi-

tation erosion. We also discuss and characterize the conditions essential to induce cavitation within thrust

zones at the base of nappes.

Key words: Cavitation erosion, Hydrotectonic pump, Fault slurry, Shear zone,

Tatra Mts.

INTRODUCTION

(S IBSON 1996). Fluids could be released to the shear

zone due to fracturing of country rocks. Other

processes, such as the dehydration of gypsum, could

also be a source of water. Fluidised rocks at the

base of nappes act as a low-viscosity tectonic lubri-

cant consisting of a suspension comprising salt solu-

The presence of fluids plays a key role during

shearing processes. A shear zone could be a path-

way of fluid migration, with the direction of fluid

flow governed by the maximum hydraulic gradient

306

EDYTA JUREWICZ & al .

tions, dissolved gases, rock fragments and matrix.

Fluids may cause a change in the predominant

mechanism of deformation due to hydrolytic weak-

ening or crack-seal-slip (P ETIT & al. 1999). Gases

can be released from solution as a result of a tem-

porary decrease in pressure or an increase in tem-

perature.

In the upper crust, fluid pressure is typically

normal and equal to hydrostatic pressure. H UBBERT

& R UBEY (1959) and S ECOR (1965) indicated the

importance of pore-fluid pressure in the mechanics

of sliding of large rock masses, especially in the case

of low-angle overthrusts. According to S IBSON

(2004), high fluid overpressures are easier to sus-

tain in compressional regimes that also allow the

highest amplitude fluid-pressure cycling.

In the Tatra Mts., nappe structures and nappe-

thrust surfaces are unique because of their geomet-

ric irregularity. Many investigations questioned the

cause of these geometric peculiarities. Earlier

works indicated that the morphology of the shear

zone was conditioned by anisotropy of the sedi-

mentary rocks and that the geometry of nappe

structures was connected with large-scale mass-loss

processes (J UREWICZ 2003, 2006; J UREWICZ &

S ¸ ABY 2004).

Shear zones in the Tatra Mts. connected with

Alpine nappe-thrusting processes cut layers of

sedimentary rocks as well as the crystalline core.

Only within shear zones originating in the sedi-

mentary cover of the Tatra Mts. could processes

of intense brecciation, mylonitization and pres-

Fig. 1. A – Main tectonic units of the Tatra Mts. and sampling points (after B AC -M OSZASZWILI & al. 1979). B – Tectonic contact of two

units (Giewont and Czerwone Wierchy) of the Tatric Nappe with clearly visible geometrical irregularity of the thrust surface; SW slope

of Giewont Mt. (geological interpretation after G ÑSIENICA -S ZOSTAK 1973)

CAVITATION EROSION

307

sure solution leading to mass-loss be observed

(J UREWICZ 2006). The surfaces of thrusts are usu-

ally corrugated and rough. From tectonically

deformed rocks uniquely preserved in dilation fis-

sures, we were able to reconstruct the shearing

processes resulting from repeated fault nucle-

ation by reactivation of pre-existing mechanical

discontinuities due to cyclic changes in pore-fluid

pressure (J UREWICZ 2003). The petrophysical

properties of the rocks and associated liquid spots

play an important role in the shearing processes.

In comparison with the sedimentary cover, the

main component of the crystalline core, a grani-

toid, shows low porosity and low solubility; the

shear surfaces dip typically at a low-angle and are

smooth and planar. This difference in petrophys-

ical properties results in the lack of comparable

processes within the granitoid core and in the sed-

imentary cover (J UREWICZ 2006).

The significant extent of mass loss associated

with thrust-napping surfaces within the Tatra Mts.

is difficult to explain by damage resulting from

fracturing and pressure solution processes alone.

We try to show below that cavitation erosion

could have been equally responsible for the wall

rock mass loss along the thrusts in the Tatra Mts.

as the above processes. Cavitation is a phenome-

non commonly associated with hydraulic equip-

ment; it is well known to mechanical and civil

engineers as a powerful erosive mechanism lead-

ing to severe damage to hydraulic equipment and

civil engineering structures (K NAPP & al. 1970,

G ALPERIN & al. 1977, K ENN 1983). Practical

implementations of cavitation include drilling

rocks in boreholes and cutting rocks in quarries.

Even if most geologists know cavitation, this

knowledge is of little use in explaining tectonic

processes. The difficulty is that we are looking for

something that does not exist now: the mass lost

and the inferred process or mechanism resulting

in the mass loss, i.e. cavitation erosion. Within the

shear zones there is no preserved record of cavi-

tation bubbles, but we can indicate the possibility

of the occurrence of cavitation during thrusting in

the Tatra Mts. and we are able to prove the low

resistance to cavitation of rocks from shear zones

and their vicinity. The authors are aware that, at

this stage in the investigation, their idea is only “a

working hypothesis”, and that further work is

required to substantiate it.

GEOLOGICAL SETTING

The Tatra Mountains are composed of a crys-

talline core, overlain by a Mesozoic sedimentary

cover and the Tatric, Kríˇna and Choˇ nappes,

which comprise many minor tectonic units (Text-

figs 1A, B). They represent the most northern mas-

sif of the Inner Carpathians. Nappe-folding pro-

ceeded from the south, gradually incorporating

increasingly northerly located sedimentary zones.

Thrusting of the Choˇ onto the Kríˇna Nappe

started after the Early Albian in the south, and

after the Early Cenomanian in the north (R AKÚS &

M ARSCHALKO 1997). The nappe-thrusting in the

Tatric took place during the Cenomanian and

Early Turonian (M I ˇ IK & al. 1985). As a final stage,

the autochthonous sedimentary cover of the crys-

talline core underwent folding. At the base of the

thrust nappes Middle Triassic dolomites and rocks

known as Rauhwacke or Zellendolomite usually

appeared (K OTA ¡ SKI 1956, P LA ˇ IENKA & S OTÁK

1996, J UREWICZ 2003). According to W ARREN

(1999), evaporite-lubricated protoliths of

Rauhwacke acted as detachment horizons during

thrusting and folding.

Pressure and temperature during thrusting

Processes of tectonic transport probably took

place in underwater conditions at full saturation of

rocks with seawater. Studies of fluid inclusions in

synkinematic quartz on slickensided low-angle fault

surfaces from the granitoid core (connected with

nappe-thrusting) proved that they originated at a

pressure of 145-170 MPa and a temperature of 212-

254°C (J UREWICZ & K OZ ¸ OWSKI 2003). Nappe-

thrusting processes of the Tatric Nappe took place

at a depth about 2 km nearer the surface (6-7 km

for the upper part of granitoid rocks minus the

thickness of autochthonous High-Tatric sedimenta-

ry cover c. 1.1 to 2.4 km by K OTA ¡ SKI 1959) and

hence temperatures and pressures could have been

lower; however temperatures determined from

chlorite and feldspar thermometers obtained from

the shear zone within the Tatric Nappe varied in the

range 300-350°C (J UREWICZ & S ¸ ABY 2004).

In a very similar situation, much higher temper-

atures (213-471°C) and pressures (20-540 MPa)

were obtained by M ILOVSKY ∂& al. (2003) from

investigation of fluid inclusions in the basal catacla-

sites of the Murá ˇ Nappe belonging to the

308

EDYTA JUREWICZ & al .

Silicicum cover nappe system (southern part of the

Central Western Carpathians). The wide range of

pressure values was interpreted by those authors as

a reflection of locally supralithostatic overpres-

sures. Such large amplitude fluid pressure fluctua-

tions determined on the basis of fluid inclusion

studies have also been noted by R OBERTS & al.

(1996) on fault-fracture veins associated with steep

reverse faults.

zones are diverse and complicated (Text-fig. 3). On

the Sto∏y Hill, the contact between the Tatric and

Kríˇna nappes is locally stylolitic (B AC -

M OSZASZWILI & al. 1981). The shear fissure is

closed. Triassic dolomites of the Kríˇna Nappe and

Urgonian limestones of the Tatric Nappe stick

closely to each other, without any damage zone or

cementing material, as if glued together (B AC -

M OSZASZWILI & al. 1981, J AROSZEWSKI 1982,

J UREWICZ 2003, J UREWICZ & S ¸ ABY 2004). In some

places a number of isolated megaclasts of Urgonian

limestones, belonging to the Tatric Nappe, and tec-

tonically incorporated into the Triassic dolomite of

the Kríˇna nappe, were noted in the hanging wall.

There is no tectonic deformation connected with

this incorporation. The thrust surface lacks the

slickensides that would enable kinematic analysis of

tectonic transport and reconstruction of the stress

field. Rocks in the vicinity of the stylolitic contact

show no evidence of strong deformation, with the

Character of the thrust zones

In many cases, rocks within the shear zones and

at the bases of the nappes do not show any features

which could indicate such high temperatures.

There is no evidence of metamorphism in the wall-

rocks. The surfaces of the thrusts are not planar.

Their complex course is clearly visible in the cross-

section (Text-fig. 1B) and directly in the field (Text-

fig. 2A). Structures appearing within the thrust

Fig. 2. A – Complicated course of the thrust zone at the base of the Czerwone Wierchy unit (Kozi Grzbiet Ridge). B – Tectonic con-

tact between Urgonian limestone of the Tatric Nappe (light) and Anisian dolomite of the Kríˇna Nappe (dark); Sto∏y Hill. C – Typical

slickensided fault plane, mineralized with quartz and epidote, originated within granitoid core of the Tatra Mts. D – Fragment of the

granitoid rock with fault plane coated with synkinematically-grown quartz and epidote

CAVITATION EROSION

309

exception of pressure solution within the limestone

and of hydraulic fractures within the dolomite

(Text-fig. 2B). This is connected with the radically

different mechanical behaviour of the limestone

and dolomite, which makes the dolomite less sus-

ceptible to dissolution and more prone to brittle

disintegration (P ASSCHIER & T ROUW 1996,

K ENNEDY & L OGAN 1997). To sum up, there is a

lack of evidence that this is the base of the nappe or

that this is the shear zone.

A different situation occurs e.g. at the base of

the Giewont Unit or in the Zadnie Kamienne shear

zone (J UREWICZ 2003, J UREWICZ & S ¸ ABY 2004),

where typical dynamometamorphic structures such

as foliation, stretching lineation and veining, associ-

ated with pressure solution, strain shadow and neo-

formed minerals, can be found (Text-figs 3D-F). In

some cases (e.g. in the base of the Czerwone

Wierchy Unit), these structures show signs of fold-

ing (Text-fig. 3F). Different characters of the shear

zones in the Tatra Mts. are inseperably linked to the

anisotropy of the rocks and the concomitant

anisotropy of the shear surfaces. Such variability of

tectonic structures appeared in the sedimentary

cover only. The main cause for the lack of a com-

parable structures within the granitoid core is the

planar character of the fault surfaces (Text-figs 2C,

D) and tightened fissures preventing fluid migra-

tion (J UREWICZ 2006). The above-mentioned fluid

inclusion investigation revealed rather narrow

ranges of pressure and temperature values, which

indicate rather uniform conditions of quartz forma-

tion and deformation. This is connected with the

relatively isotropic character of the granites in com-

parison to the lithologically diverse sedimentary

rocks, and with the fact that in many cases the acti-

vation took place only once. Due to the low solubil-

ity and low porosity of granite, as well as the lack of

sources of fluids, hydrotectonic phenomena and

associated cavitation erosion could not appear in

the crystalline core of the Tatra Mts.

various ways. The multistage character of rocks

deformations are connected with multiply activa-

tion of tectonic displacement depended on cyclical

changes in pore fluid pressure. The build-up of

pore fluid pressure leads to a decrease in stress to

an effective value (H UBBERT & R UBEY 1959) and to

rupturing by faulting associated with hydraulic frac-

turing. A model of fault development in carbonate

rocks based on repeated crack-seal-slip cycles was

described by P ETIT & al. (1999) and R ENARD & al.

(2000). Processes of such a kind, in which fluid

pressure plays a key role, were termed hydrotec-

tonic phenomena by J AROSZEWSKI (1982) and K OPF

(1982, 2003). In the Tatra Mts., the cyclical charac-

ter of nappe movement was described by J UREWICZ

(2003).

Newly-formed shear zones associated with a

fault-related fracture net provide a drainage path

for fluid migration (S IBSON 1996, 2004; G UDMUND -

SSON & al . 2001). In the case of the Tatra Mts., the

source of fluids could have been pores (which

appeared due to solution of salt crystals and filled

with meteoric water) or fluids could have originat-

ed from, for example, the dehydration of gypsum,

occurring in the so-called Anisian “cellular

dolomites” (K ASI ¡ SKI 1981) and in sediments of

Rauhwacke type (P LA ˇ IENKA & S OTÁK 1996,

M ILOVSKY ∂& al. 2003). It cannot be excluded that

the shear zone was a passageway for hydrothermal

solutions, this being supported by the presence of

hydrothermal feldspar crystallizing at a tempera-

ture of ~350°C (J UREWICZ & S ¸ ABY 2004). Fluids

and rock fragments filling the shear zone formed a

suspension circulating along the hydraulic gradient

(S IBSON 1996).

The presence of such a suspension, described by

K OPF (2003) as a “fault slurry”, causes a decrease in

friction, with the angle of internal friction decreas-

ing to values close to zero (J UREWICZ 2003).

According to B RODSKY & K ANAMORI (2001), the

lubrication effect of the fluid in all fluid-filled faults

facilitates tectonic transport and reduces the fric-

tion stress by as much as 30% relative to the hydro-

static value or 50% to the dry rock friction. M ARON

(2004) stated that friction at the interface between

the fault walls may additionally fall dramatically

due to high rates of slip, e.g. seismic in nature.

After displacement, the shear zone was gradu-

ally veined, cemented and immobilised. During this

time, ductile folding took place of material filling

the shear zone, comprising brecciated, mylonitized

Mechanism of tectonic transport and shearing-

related processes

The character and mechanism of deformations

within the shear zone at the base of the Tatric

nappes could be established from the investigation

of dilation fissures associated with the thrust sur-

face (J UREWICZ 2003, J UREWICZ & S ¸ ABY 2004).

The preserved rocks are tectonically deformed in

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