Writing multiput files

The following program I wrote to calculate mean velocity value of 104 files and write all the average values in one single file.

PROGRAM velo
IMPLICIT NONE

INTEGER::n,seqnum,ierr
!DOUBLE PRECISION::avg
DOUBLE PRECISION,DIMENSION(1048576)::vdata
DOUBLE PRECISION::avg
!vdata = velocity data, nvdata = normalized velocity data
CHARACTER (LEN=12)::seqfilein

n = 1048576									
ierr = 0														
seqfilein = 'caseXXXX.txt'					

DO seqnum = 1,104						
WRITE(seqfilein(5:8),'(I4.4)') seqnum		
WRITE(*,*) seqfilein 
OPEN (UNIT=10,FILE=seqfilein, STATUS='OLD', ACTION='READ', IOSTAT=ierr) 

CALL READDATA(vdata,n,seqfilein)
!CALL NORMALIZEDVDATA(vdata,n,nvdata,seqfilein)
CALL MEAN(vdata,avg,n,seqfilein)

ENDDO
STOP
END 

!&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
SUBROUTINE READDATA(y,n,seqfname)
	IMPLICIT NONE
	INTEGER::i,ierr
    REAL::xdummy
	INTEGER,INTENT(IN)::n 
	DOUBLE PRECISION,INTENT(OUT),DIMENSION(n)::y
	CHARACTER(LEN=12),INTENT(IN)::seqfname
    
	ierr = 0
	OPEN (UNIT=3, FILE=seqfname, STATUS='OLD', ACTION='READ', IOSTAT=ierr) 
		DO i = 1, n
			READ (3,*,IOSTAT=ierr) xdummy, y(i)  
		END DO
	CLOSE (UNIT = 3)
    
	RETURN
END SUBROUTINE
			


!$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
SUBROUTINE MEAN(vdata,avg,n,seqfileout)
	IMPLICIT NONE
	DOUBLE PRECISION :: sum1
	INTEGER, INTENT(IN) :: n
	DOUBLE PRECISION, INTENT(IN), DIMENSION(n) :: vdata
	DOUBLE PRECISION, INTENT(OUT) :: avg
    CHARACTER(LEN=12)::seqfout   
    CHARACTER(LEN=12),INTENT(IN)::seqfileout 
	INTEGER::i, ierr
	ierr = 0
	seqfout = seqfileout(1:8)								

	sum1 = 0

	DO i = 1, n
	sum1 = sum1 + vdata(i)
	END DO

	avg = sum1/REAL(n)

OPEN (UNIT=13, FILE='mvelocity.txt', STATUS='REPLACE', ACTION='WRITE', IOSTAT=ierr)

WRITE(13,*) 'mean velocity of ', trim(seqfout), avg

CLOSE (UNIT=13)

RETURN
END SUBROUTINE

But the program is updating the most recent value only and thereby the output file has only the last mean value instead of desired 104 values. I am confused if I have to use DO loop and how to apply it around WRITE command. Pls advise.

Many thanks!

OPEN (UNIT=13, FILE='mvelocity.txt', STATUS='REPLACE', POSITION='APPEND', ACTION='WRITE', IOSTAT=ierr)) 

I tried this which is appending from the last input lines. But I am trying to replace the entire file and then put the values followed by the most recent one without deleting the old one. Any further suggestion? Many thanks!

Many thanks for your kind suggestion!